This research was supported by Siena College and the Center for Undergraduate Research and Creative Activity. We thank Dr. Thomas Hughes and Dr. Kristopher Kolonko for helpful conversations, Ms. Allycia Barbera for early stage work on this methodology, and the Siena College Stewart&#39;s Advanced Instrumentation and Technology (SAInT) Center for instrumentation resources.

Caution: Consult Safety Data Sheets (SDSs) prior to the use of the chemicals in this procedure. Use appropriate personal protective equipment (PPE) including splash goggles, lab coat, and nitrile or butyl gloves as many of the reagents and solvents are corrosive or flammable. Carry out all reactions in a fume hood. It is unnecessary to dry glassware or to use a nitrogen atmosphere for this protocol.
1. Carbodiimide Coupling Reaction for Primary Alcohols
<ol>
	<li>In a 50 mL round bottom flask, combine (E)-cinnamic acid (151 mg, 1.02 mmol, 1.2 equiv), DMAP (312 mg, 2.55&#160;mmol, 3 equiv), and EDC (244 mg, 1.28 mmol, 1.5 equiv). Add acetonitrile (15 mL) and 3-methoxybenzyl alcohol (98 &#956;L, 0.85 mmol, 1 equiv) to the mixture along with a stir bar. 
	CAUTION: Acetonitrile is a flammable solvent.</li>
	<li>Clamp the flask in a 40 &#176;C water bath and stir the reaction. 
	NOTE: If the reaction involves an aromatic alcohol, monitor the reaction for the loss of the alcohol via thin-layer chromatography (TLC) using 1:3 ethyl acetate/hexane. The reaction is complete when the alcohol spot is no longer visible on the TLC plate by irradiation with a UV lamp.</li>
</ol>
2. Extraction Workup
<ol>
	<li>Once the reaction is complete as indicated by TLC or after 45 min, remove the acetonitrile under reduced pressure using a rotary evaporator to obtain a crude solid. 
	NOTE: Please see additional resources for the information regarding the use of a rotary evaporator24,25.</li>
	<li>To the residue, add diethyl ether (20 mL) and 1 M HCl (20 mL). Swirl the flask to dissolve the residue into the solvent layers. 
	CAUTION: Diethyl ether is a highly flammable solvent. 
	NOTE: To decrease the solvent hazard, ethyl acetate can be used in place of the diethyl ether; however, there is a greater potential for emulsion formation during the extraction and wash steps.</li>
	<li>Pour the solution into a separatory funnel. Rinse the evaporating flask with additional diethyl ether (5 mL) and add the rinse to the separatory funnel.</li>
	<li>Gently shake the separatory funnel to extract the product into the ether layer, venting periodically. Allow the layers to separate, and then remove the aqueous layer by draining it out from the bottom of the funnel into an Erlenmeyer flask or beaker. 
	NOTE: Please see additional resources for the information regarding extractions and the use of a separatory funnel24,25.</li>
</ol>
3. Washing Procedure
<ol>
	<li>To the organic layer remaining in the separatory funnel, add 1 M HCl (20 mL) and gently shake the separatory flask, venting periodically. Allow the layers to separate, and then remove the aqueous layer by draining it out from the bottom of the funnel into an Erlenmeyer flask or beaker.</li>
	<li>Repeat the washing procedure with saturated sodium bicarbonate solution (2 x&#160;20 mL) and then with saturated sodium chloride solution (20 mL).</li>
	<li>Pour the organic layer out from the top of the separatory funnel into a clean Erlenmeyer flask, dry the layer with magnesium sulfate, and gravity filter the solution through filter paper into a massed evaporation flask. 
	NOTE: Please see additional resources for information regarding extractions and the use of magnesium sulfate as a drying agent24,25.</li>
	<li>Remove the diethyl ether solvent under reduced pressure using a rotary evaporator.</li>
	<li>Analyze a sample of the product by 1H and 13C NMR spectroscopy in CDCl3 and by mass spectrometry. 
	NOTE: Please see additional resources for the information regarding the preparation of samples for NMR analysis24,25.</li>
</ol>
4. Carbodiimide Coupling Reaction for Secondary and Electron-deficient Alcohols
<ol>
	<li>In a 50 mL round bottom flask, combine (E)-cinnamic acid (151 mg, 1.02 mmol, 1.2 equiv), DMAP (312 mg, 2.55&#160;mmol, 3 equiv), and EDC (244 mg, 1.28 mmol, 1.5 equiv). Add acetonitrile (15 mL) and diphenylmethanol (157 mg, 0.85 mmol, 1 equiv) to the mixture along with a stir bar. 
	CAUTION: Acetonitrile is a flammable solvent.</li>
	<li>Clamp the flask and stir the reaction at room temperature for 24 h. Insert an air condenser into the flask neck to minimize solvent evaporation.</li>
	<li>Follow the extraction workup and washing procedure described in Steps 2-3 above.</li>
</ol>
5. Carbodiimide Coupling Reaction for Long-Chain or Hydrophobic Carboxylic Acids
<ol>
	<li>In a 50 mL round bottom flask, combine decanoic acid (146 mg, 0.85 mmol, 1 equiv), DMAP (312 mg, 2.55&#160;mmol, 3 equiv), and EDC (244 mg, 1.28 mmol, 1.5 equiv). Add acetonitrile (15 mL) and diphenylmethanol (157 mg, 0.85 mmol, 1 equiv) to the mixture along with a stir bar. 
	CAUTION: Acetonitrile is a flammable solvent.</li>
	<li>Clamp the flask and stir the reaction at room temperature for 24 h. Insert an air condenser into the flask neck to minimize solvent evaporation. If a primary alcohol is used, stir the reaction in a water bath at 40 &#176;C for 1 h.</li>
	<li>Follow the extraction workup and washing procedure described in Steps 2-3 above.</li>
</ol>

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<li>Corker, E. C., Mentzel, U. V., Mielby, J., Riisager, A., Fehrmann, R. <a target="_blank" href="http://pubs.rsc.org/en/content/articlehtml/2013/gc/c3gc36513a">An alternative pathway for production of acetonitrile: ruthenium catalysed aerobic dehydrogenation of ethylamine.</a> Green Chemistry. 15 (4), 928-933 (2013).</li>
<li>McConvey, I. F., Woods, D., Lewis, M., Gan, Q., Nancarrow, P. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/op2003503">The Importance of Acetonitrile in the Pharmaceutical Industry and Opportunities for its Recovery from Waste.</a> Organic Process Research & Development. 16 (4), 612-624 (2012).</li>
<li>Jad, Y. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Peptide+synthesis+beyond+DMF%3A+THF+and+ACN+as+excellent+and+friendlier+alternatives%5BTitle%5D)+AND+%22Organic+%26+Biomolecular+Chemistry%22%5BJournal%5D)">Peptide synthesis beyond DMF: THF and ACN as excellent and friendlier alternatives.</a> Organic & Biomolecular Chemistry. 13 (8), 2393-2398 (2015).</li>
<li>Williams, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Quantitative+method+for+the+profiling+of+the+endocannabinoid+metabolome+by+LC-atmospheric+pressure+chemical+ionization-MS%5BTitle%5D)+AND+%22Analytical+Chemistry%22%5BJournal%5D)">Quantitative method for the profiling of the endocannabinoid metabolome by LC-atmospheric pressure chemical ionization-MS.</a> Analytical Chemistry. 79 (15), 5582-5593 (2007).</li>
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<li>Schwartz, E., et al. <a target="_blank" href="https://onlinelibrary.wiley.com/doi/abs/10.1002/pola.23477">Water soluble azido polyisocyanopeptides as functional β-sheet mimics.</a> Journal of Polymer Science Part A: Polymer Chemistry. 47 (16), 4150-4164 (2009).</li>
<li>Hangauer, M. J., Bertozzi, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+FRET-Based+Fluorogenic+Phosphine+for+Live-Cell+Imaging+with+the+Staudinger+Ligation%5BTitle%5D)+AND+%22Angewandte+Chemie+International+Edition%22%5BJournal%5D)">A FRET-Based Fluorogenic Phosphine for Live-Cell Imaging with the Staudinger Ligation.</a> Angewandte Chemie International Edition. 47 (13), 2394-2397 (2008).</li>
<li>Hsieh, P. -W., Chen, W. -Y., Aljuffali, I., Chen, C. -C., Fang, J. -Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Co-Drug+Strategy+for+Promoting+Skin+Targeting+and+Minimizing+the+Transdermal+Diffusion+of+Hydroquinone+and+Tranexamic+Acid%5BTitle%5D)+AND+%22Current+Medicinal+Chemistry%22%5BJournal%5D)">Co-Drug Strategy for Promoting Skin Targeting and Minimizing the Transdermal Diffusion of Hydroquinone and Tranexamic Acid.</a> Current Medicinal Chemistry. 20 (32), 4080-4092 (2013).</li>
<li>Moretto, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+Rigid+Helical+Peptide+Axle+for+a+%5B2%5DRotaxane+Molecular+Machine%5BTitle%5D)+AND+%22Angewandte+Chemie+International+Edition%22%5BJournal%5D)">A Rigid Helical Peptide Axle for a [2]Rotaxane Molecular Machine.</a> Angewandte Chemie International Edition. 48 (47), 8986-8989 (2009).</li>
<li>Hanessian, S., McNaughton-Smith, G. <a target="_blank" href="https://www-sciencedirect-com.remotexs.ntu.edu.sg/science/article/pii/S0960894X96002752">A versatile synthesis of a β-turn peptidomimetic scaffold: An approach towards a designed model antagonist of the tachykinin NK-2 receptor.</a> Bioorganic & Medicinal Chemistry Letters. 6 (13), 1567-1572 (1996).</li>
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The authors have nothing to disclose.

The methodology presented here was developed to minimize the hazards from solvent associated with a traditional Steglich esterification by using a greener solvent system and by reducing the need for column chromatography8,9. Comparable reaction yields and rates can be achieved with the use of acetonitrile in place of dry chlorinated solvents or DMF22.
Several key steps enable the efficient purification of product without the need for chromatography. After the reaction, the acetonitrile is first removed by rotary evaporation. The removal of solvent is essential, as acetonitrile is miscible with water and will affect the partitioning of reaction components during the extraction and wash steps. Basic impurities, including DMAP, EDC, and urea byproducts, are then removed with the acid wash steps. Any residual carboxylic acid is removed during the basic wash steps. Thus, all reagents and impurities can be removed, leaving ester in the organic layer. Subsequent drying and solvent removal led to high yields of pure ester products.
Adjustments of the protocol were needed to obtain high yields of ester product for the use of secondary alcohols or very hydrophobic carboxylic acids. The rate of the reaction for secondary or electron deficient alcohols is slower than that of primary alcohols, so it is necessary to either increase the reaction temperature (60 &#176;C) or to run the reaction for longer time periods at room temperature. In addition, we found that excess carboxylic acid cannot be used if the acid is insoluble in the saturated sodium bicarbonate wash solution. For long chain carboxylic acids, such as decanoic acid, the reaction mixture should have a 1:1 ratio of alcohol to carboxylic acid reagents to avoid a carboxylic acid impurity in the final product.
Various carboxylic acids and alcohol partners can be used in the formation of esters, shown here and in previous work22. However, esters of tertiary alcohols have not been isolated with the current methodology. As the ability to couple carboxylic acids to sterically hindered tertiary alcohols is a common application of the Steglich esterification23, the inability to obtain esters with tertiary alcohols is a limitation of the current methodology. We are pursuing NMR kinetics studies to investigate the mechanism and constraints of this reaction both in acetonitrile-d3 and chloroform-d. In the future, we hope to adapt the method to enable the synthesis of esters with tertiary alcohols.
In summary, this work describes a greener Steglich esterification protocol that can be utilized for the synthesis of esters of various carboxylic acids with primary, secondary, benzylic and allylic alcohols and phenols. The methodology provides a less hazardous alternative to a common esterification reaction.

Ester compounds are widely used for the applications such as flavor compounds, pharmaceuticals, cosmetics, and materials. Commonly, the use of carbodiimide coupling reagents is used to facilitate an ester formation from a carboxylic acid and an alcohol1. For example, in the Steglich esterification, dicyclohexylcarbodiimide (DCC) is reacted with a carboxylic acid in the presence of 4-dimethylaminopyridine (DMAP) to form an activated acid derivative, generally in a chlorinated solvent system or dimethylformamide (DMF)2,3,4. The activated acid derivative then undergoes a nucleophilic acyl substitution with an alcohol to form the ester product, which is usually purified via chromatography. The Steglich esterification enables mild coupling of large, complex carboxylic acids and alcohols, including sterically hindered secondary and tertiary alcohols2,5,6. The goal of this work is to modify the standard Steglich esterification protocol to provide a greener synthetic option for this common esterification reaction.
One important aspect in the design of new synthetic methodology is to seek to minimize the use and formation of hazardous substances. The Twelve Principles of Green Chemistry7 can be used to provide a guideline for creating safer syntheses. Some of these include the prevention of waste generation (Principle 1) and the use of safer solvents (Principle 5). In particular, solvents account for 80-90% of the non-aqueous mass of the materials in pharmaceutical manufacturing8. Thus, modifying a protocol to use a less hazardous solvent can make a large impact on the greenness of an organic reaction.
Steglich esterification reactions often use anhydrous chlorinated solvent systems or DMF; however, these solvents are of concern for both the environment and human health. Dichloromethane (CH2Cl2) and chloroform (CHCl3) are probable human carcinogens, and DMF has reproductive toxicity concerns9. In addition, CH2Cl2 is ozone depleting10. Thus, a less hazardous solvent for the Steglich esterification would be of great utility. While there are not yet green replacements for polar aprotic solvents, acetonitrile is recommended as a greener replacement for CH2Cl2, CHCl3, and DMF9. Acetonitrile is currently produced as a byproduct in acrylonitrile manufacturing; however, a green synthesis of acetonitrile from biomass on an academic scale has been reported11, and potential options for the reuse and recovery from waste streams are being investigated12. Acetonitrile has previously been used as a greener solvent alternative for carbodiimide coupling reactions in solid-phase peptide synthesis to form amide linkages13. The use of acetonitrile as a solvent system for Steglich esterifications has been demonstrated14,15,16,17,18,19,20,21; however, these methods have not focused on the green aspect of the solvent and also employ additional purification via column chromatography.
Reducing the need for column chromatography as a purification step also minimizes hazardous solvent waste8. In addition to using a less hazardous reaction solvent, the methodology enables the isolation of highly pure product without the need for chromatography. The traditionally used dicyclohexylcarbodiimide (DCC) coupling reagent is substituted with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC). The basic amine functional group on this reagent enables the reaction byproducts and any residual reagents to be removed via acidic and basic wash steps.
The protocol presented herein can be used with a variety of acid and alcohol partners (Figure 1). It was used to synthesize a small library of cinnamyl ester derivatives using primary, secondary, benzyl, and allyl alcohols and phenols22. Additionally, the rate of the esterification reaction in acetonitrile is comparable to that in the chlorinated and DMF solvent systems, without a need to dry or distill the acetonitrile prior to the reaction22. Esters synthesized from tertiary alcohols have not been isolated, which is currently a limitation of the methodology compared to the traditional Steglich esterification in chlorinated solvent23. In addition, other acid-labile groups could be affected by the acid wash steps, potentially necessitating column chromatography for purification after acetonitrile removal. Despite these limitations, the reaction is a facile and general method for the synthesis of esters in high yields using a range of both alcohol and carboxylic acid components. The use of a greener solvent system and high purity without the need for chromatography steps make this protocol an attractive alternative to a traditional Steglich esterification.
<img alt="Chemical reaction diagram showing acetic acid esterification with acetonitrile; acids and alcohols." class="xfigimg" src="/files/ftp_upload/58803/58803fig1.jpg" data-altupdatedat="1747911167000" data-alt="Figure 1"> 
Figure 1. General reaction scheme. The general scheme for the reaction involves the coupling of a carboxylic acid and an alcohol, which is facilitated using a carbodiimide coupling reagent (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, or EDC) and 4-dimethylaminopyridine (DMAP) in acetonitrile. To demonstrate the reaction breadth, esters were formed using various acids (1-5) with either a primary (6) or secondary (7) alcohol. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58803/58803fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>&lt;em&gt;trans&lt;/em&gt; -cinnamic acid</td><td>Acros Organics</td><td>158571000</td><td></td></tr><tr><td>butyric acid</td><td>Sigma-Aldrich</td><td>B103500</td><td>Caution: corrosive</td></tr><tr><td>hexanoic acid</td><td>Sigma-Aldrich</td><td>153745-100G</td><td>Caution: corrosive</td></tr><tr><td>decanoic acid</td><td>Sigma-Aldrich</td><td>21409-5G</td><td>Caution: corrosive</td></tr><tr><td>phenylacetic acid</td><td>Sigma-Aldrich</td><td>P16621-5G</td><td></td></tr><tr><td>3-methoxybenzyl alcohol</td><td>Sigma-Aldrich</td><td>M11006-25G</td><td></td></tr><tr><td>diphenylmethanol</td><td>Acros Organics</td><td>105391000</td><td>Benzhydrol</td></tr><tr><td>chloroform-&lt;em&gt;d&lt;/em&gt;</td><td>Acros Organics</td><td>166260250</td><td>99.8% with 1% v/v tetramethylsilane, Caution: toxic</td></tr><tr><td>hexane</td><td>BDH Chemicals</td><td>BDH1129-4LP</td><td>Caution: flammable</td></tr><tr><td>ethyl acetate</td><td>Sigma-Aldrich</td><td>650528</td><td>Caution: flammable</td></tr><tr><td>diethyl ether</td><td>Fisher Scientific</td><td>E138-500</td><td>Caution: flammable</td></tr><tr><td>acetonitrile</td><td>Fisher Scientific</td><td>A21-1</td><td>ACS Certified, &gt;99.5%, Caution: flammable</td></tr><tr><td>4-dimethylaminopyridine</td><td>Acros Organics</td><td>148270250</td><td>Caution: toxic</td></tr><tr><td>magnesium sulfate</td><td>Fisher Scientific</td><td>M65-3</td><td></td></tr><tr><td>hydrochloric acid, 1 M</td><td>Fisher Scientific</td><td>S848-4</td><td>Caution: corrosive</td></tr><tr><td>sodium chloride</td><td>BDH Chemicals</td><td>BDH8014</td><td></td></tr><tr><td>sodium bicarbonate</td><td>Fisher Scientific</td><td>S25533B</td><td></td></tr><tr><td>1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride</td><td>Chem-Impex</td><td>00050</td><td>Caution: skin and eye irritant</td></tr><tr><td>thin layer chromatography plates</td><td>EMD Millipore</td><td>1055540001</td><td>aluminum backed sheets</td></tr><tr><td>Note: All commercially available reagents and solvents were used as received without further purification.</td><td></td><td></td><td></td></tr></tbody></table>

synthesis of esters via a greener steglich esterification in acetonitrile

The Steglich esterification is a widely-used reaction for the synthesis of esters from carboxylic acids and alcohols. While efficient and mild, the reaction is commonly performed using chlorinated or amide solvent systems, which are hazardous to human health and the environment. Our methodology utilizes acetonitrile as a greener and less hazardous solvent system. This protocol exhibits rates and yields that are comparable to traditional solvent systems and employs an extraction and wash sequence that eliminates the need for the purification of the ester product via column chromatography. This general method can be used to couple a variety of carboxylic acids with 1&#176; and 2&#176; aliphatic alcohols, benzylic and allylic alcohols, and phenols to obtain pure esters in high yields. The goal of the protocol detailed here is to provide a greener alternative to a common esterification reaction, which could serve useful for ester synthesis in both academic and industrial applications.

Using the modified Steglich esterification in acetonitrile followed by an acid-base extraction workup, 3-methoxybenzyl cinnamate (8) was obtained as a light-yellow oil (205 mg, 90% yield) without the need for column chromatography. 1H and 13C NMR spectra are presented in Figure 2 to confirm the structure and to indicate purity.
Compounds 9-17 were synthesized using a similar protocol (Figure 3) with yields of 77-90%. All compounds were analyzed by 1H and 13C NMR spectroscopy and high-resolution mass spectrometry (HRMS) and found to be of similar purity to 3-methoxybenzyl cinnamate by NMR analysis. Tabulated data for compounds 8-17 is reported in Table 1.
Slight changes to the general protocol for primary alcohols were made to obtain optimal yields and purity for compounds 12-17. Secondary alcohol reactions were run for 24 h at room temperature to allow the reaction to go to completion22. For the decanoic acid reactions, using 1.2 equivalents of carboxylic acid to 1 equivalent of alcohol for both primary and secondary alcohols yielded esters with a decanoic acid impurity (Figure 4). The long-chain acid is not soluble in the basic aqueous wash layers and remains in the organic layer. Other hydrophobic acids could behave similarly. This issue was solved by used a 1:1 molar ratio of decanoic acid to alcohol, which yielded pure ester products. A slightly longer reaction time (60 min) was required for the primary alcohol reaction to go to completion for the 1:1 molar ratio reaction.
<img alt="NMR spectroscopy results; (E)-3-Methoxybenzyl cinnamate; proton and carbon spectra; structural analysis." class="xfigimg" src="/files/ftp_upload/58803/58803fig2.jpg" data-altupdatedat="1747911167000" data-alt="Figure 2"> 
Figure 2. 1H and 13C NMR spectra for 3-methoxybenzyl cinnamate (8). The 1H NMR spectrum (A) and 13C NMR spectrum (B) of 3-methoxybenzyl cinnamate are shown with the product structure. Corresponding assignments are denoted on each spectrum and were confirmed using 1H-1H COSY, 1H-13C HSQC, and 1H-13C HMBC 2D NMR experiments. Spectra were obtained after solvent removal; no additional purification steps were used. The purity of this compound is representative of all reactions tested. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58803/58803fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Chemical structure diagram of compound yields, showing molecular formulas with percent outcomes." class="xfigimg" src="/files/ftp_upload/58803/58803fig3.jpg" data-altupdatedat="1747911167000" data-alt="Figure 3"> 
Figure 3. Ester structures synthesized using the methodology. Five acids (1-5, Figure 1) were combined with either a primary or secondary alcohol (6 and 7, respectively, Figure 1). The ester structures (8-17) are shown along with the percent yield for the reaction. Reactions were monitored for loss of the alcohol by TLC (1:3 ethyl acetate/hexane). Primary alcohol reactions were run at 40 °C in a water bath for 45 min for esters 8-11 and 60 min for ester 12. Secondary alcohol reactions were run for 24 h at room temperature. For decanoic acid reactions (12 and 17), 1 molar equivalent of carboxylic acid to alcohol was used instead of 1.2 equivalents. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58803/58803fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Proton NMR spectra comparison, diagrams show peak zoom-in for structural analysis, chemical shift." class="xfigimg" src="/files/ftp_upload/58803/58803fig4.jpg" data-altupdatedat="1747911167000" data-alt="Figure 4"> 
Figure 4. 1H NMR spectra for diphenylmethyl decanoate (17) using 1:1.2 and 1:1 molar equivalents of alcohol to carboxylic acid. (A) Decanoic acid (1.2 equiv, top, or 1 equiv, bottom) was reacted with diphenylmethanol (1 equiv), EDC (1.5 equiv), and DMAP (3 equiv) in acetonitrile. The reactions were stirred at room temperature for 24 h and then the ester was isolated via the extraction and wash protocol. Residual decanoic acid remains in the product when the carboxylic acid is used in excess, as it is not soluble in the basic aqueous layer. The signal at 2.35 ppm shown in the inset indicates residual carboxylic acid in the product sample. (B) The use of a 1:1 ratio of carboxylic acid to alcohol enables a clean isolation of the ester, indicated by the loss of the signal at 2.35 ppm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58803/58803fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Compound</td>
			<td>Rf (1:3 EtOAc/hex); appearance</td>
			<td>1H NMR (500 MHz, CDCl3)</td>
			<td>13C NMR (126 MHz, CDCl3)</td>
			<td>HRMS</td>
		</tr>
		<tr>
			<td>3-methoxybenzyl cinnamate (8)</td>
			<td>3-methoxybenzyl alcohol Rf = 0.27; Product Rf = 0.61; light yellow oil</td>
			<td>δ 7.77 (d, J = 16.0 Hz, 1H), 7.60 – 7.50 (m, 2H), 7.49 – 7.36 (m, 3H), 7.33 (t, J = 7.8 Hz, 2H), 7.03 (ddd, J = 7.4, 1.5, 0.8 Hz, 1H), 7.02 – 6.97 (m, 1H), 6.91 (ddd, J = 8.3, 2.6, 1.0 Hz, 1H), 6.53 (d, J = 16.0 Hz, 1H), 5.26 (s, 2H), 3.86 (s, 3H)</td>
			<td>δ 166.8, 159.8, 145.2, 137.6, 134.4, 130.4, 129.7, 128.9, 128.1, 120.4, 117.9, 113.8, 113.7, 66.2, 55.3</td>
			<td>ESI calcd for C17H16O3 (M+Na)+ 291.0992, found 291.0993</td>
		</tr>
		<tr>
			<td>3-methoxybenzyl phenylacetate (9)</td>
			<td>Rf = 0.57; 
			light yellow oil</td>
			<td>δ 7.35 – 7.27 (m, 5H), 7.25 (t, J = 8.6 Hz, 1H), 6.89 (ddd, J = 7.4, 1.5, 0.8 Hz, 1H), 6.85 (ddd, J = 8.3, 2.6, 1.0 Hz, 1H), 6.83 – 6.81 (m, 1H), 5.11 (s, 2H), 3.77 (s, 3H), 3.68 (s, 2H)</td>
			<td>δ 171.3, 159.8, 137.4, 133.9, 129.6, 129.3, 128.6, 127.1, 120.2, 113.9, 113.3, 66.4, 55.2, 41.4</td>
			<td>ESI calcd for C16H16O3 (M+Na)+ 279.0992, found 279.0990</td>
		</tr>
		<tr>
			<td>3-methoxybenzyl butyrate (10)</td>
			<td>Rf = 0.68; 
			colorless oil</td>
			<td>δ 7.27 (t, J = 7.7 Hz, 1H), 6.93 (ddd, J = 7.5, 1.6, 0.8 Hz, 1H), 6.90 – 6.88 (m, 1H), 6.86 (ddd, J = 8.2, 2.6, 1.0 Hz, 1H), 5.09 (s, 2H), 3.81 (s, 3H), 2.35 (t, J = 7.4 Hz, 2H), 1.68 (h, J = 7.4 Hz, 2H), 0.95 (t, J = 7.4 Hz, 3H).</td>
			<td>δ 173.5, 159.8, 137.7, 129.6, 120.3, 113.7, 113.6, 65.9, 55.2, 36.2, 18.5, 13.7</td>
			<td>ESI calcd for C12H16O3 (M+Na)+ 231.0992, found 231.0991</td>
		</tr>
		<tr>
			<td>3-methoxybenzyl hexanoate (11)</td>
			<td>Rf = 0.74; 
			colorless oil</td>
			<td>δ 7.27 (d, J = 7.7 Hz, 1H), 6.93 (ddd, J = 7.5, 1.6, 0.6 Hz, 1H), 6.90 – 6.88 (m, 1H), 6.85 (ddd, J = 8.2, 2.6, 0.9 Hz, 1H), 5.09 (s, 2H), 3.81 (s, 3H), 2.35 (t, J = 7.5 Hz, 2H), 1.74 – 1.56 (m, 2H), 1.39 – 1.25 (m, 4H), 0.89 (t, J = 7.1 Hz, 3H)</td>
			<td>δ 173.6, 159.8, 137.7, 129.6, 120.3, 113.7, 113.6, 65.9, 55.2, 34.3, 31.3, 24.7, 22.3, 13.9</td>
			<td>ESI calcd for C14H20O3 (M+Na)+ 259.1305, found 259.1304</td>
		</tr>
		<tr>
			<td>3-methoxybenzyl decanoate (12)</td>
			<td>Rf = 0.71; 
			colorless oil</td>
			<td>δ 7.27 (t, J = 7.9 Hz, 1H), 6.93 (ddd, J = 7.5, 1.6, 0.6 Hz, 1H), 6.90 – 6.87 (m, 1H), 6.88 (ddd, J = 8.3, 2.5, 0.6 Hz, 1H), 5.09 (s, 2H), 3.80 (s, 3H), 2.35 (t, J = 7.6 Hz, 2H), 1.76 – 1.52 (m, 2H), 1.42 – 1.12 (m, 12H), 0.88 (t, J = 7.0 Hz, 3H)</td>
			<td>δ 173.7, 159.8, 137.7, 129.6, 120.3, 113.7, 113.6, 65.9, 55.2, 34.3, 31.9, 29.4, 29.3, 29.2, 25.0, 22.7, 14.1</td>
			<td>ESI calcd for C18H28O3 (M+Na)+ 315.1931, found 315.1931</td>
		</tr>
		<tr>
			<td>diphenylmethyl cinnamate (13)</td>
			<td>diphenylmethanol Rf = 0.47; 
			Product Rf = 0.66; white solid</td>
			<td>δ 7.79 (d, J = 16.0 Hz, 1H), 7.60 – 7.54 (m, 2H), 7.46 – 7.36 (m, 11H), 7.35 – 7.30 (m, 2H), 7.05 (s, 1H), 6.60 (d, J = 16.0 Hz, 1H)</td>
			<td>δ 166.0, 145.4, 140.3, 134.4, 130.4, 128.9, 128.5, 128.2, 127.9, 127.2, 118.0, 77.0</td>
			<td>ESI calcd for C22H18O2 (M+Na)+ 337.1199, found 337.1191</td>
		</tr>
		<tr>
			<td>diphenylmethyl phenylacetate (14)</td>
			<td>Rf = 0.66; 
			light yellow oil</td>
			<td>δ 7.35 – 7.19 (m, 15H), 6.87 (s, 1H), 3.72 (s, 2H)</td>
			<td>δ 170.4, 140.1, 133.8, 129.4, 128.6, 128.5, 127.9, 127.1, 127.0, 77.2, 41.7</td>
			<td>ESI calcd for C21H18O2 (M+Na)+ 325.1199, found 325.1201</td>
		</tr>
		<tr>
			<td>diphenylmethyl butyrate (15)</td>
			<td>Rf = 0.72; 
			light yellow oil</td>
			<td>δ 7.37 – 7.30 (m, 10H), 7.29 – 7.25 (m, 2H), 6.89 (s, 1H), 2.40 (t, J = 7.5 Hz, 2H), 1.69 (h, J = 7.4 Hz, 2H), 0.93 (t, J = 7.4 Hz, 3H)</td>
			<td>δ 172.6, 140.4, 128.5, 127.8, 127.1, 76.6, 36.5, 18.5, 13.7</td>
			<td>ESI calcd for C17H18O2 (M+Na)+ 277.1199, found 279.1197</td>
		</tr>
		<tr>
			<td>diphenylmethyl hexanoate (16)</td>
			<td>Rf = 0.76; 
			light yellow oil</td>
			<td>δ 7.36 – 7.29 (m, 8H), 7.29 – 7.24 (m, 2H), 6.89 (s, 1H), 2.41 (t, J = 7.5 Hz, 2H), 1.72 – 1.60 (m, 2H), 1.36 – 1.21 (m, 4H), 0.87 (t, J = 7.0 Hz, 3H)</td>
			<td>δ 172.8, 140.4, 128.5, 127.8, 127.1, 76.6, 34.6, 31.3, 24.6, 22.3, 13.9</td>
			<td>ESI calcd for C19H22O2 (M+Na)+ 305.1512, found 305.1509</td>
		</tr>
		<tr>
			<td>diphenylmethyl decanoate (17)</td>
			<td>Rf = 0.76; 
			light yellow oil</td>
			<td>δ 7.35 – 7.29 (m, 8H), 7.29 – 7.23 (m, 2H), 6.89 (s, 1H), 2.41 (t, J = 7.5 Hz, 2H), 1.71 – 1.59 (m, 2H), 1.33 – 1.18 (m, 12H), 0.87 (t, J = 7.0 Hz, 3H)</td>
			<td>δ 172.8, 140.4, 128.5, 127.9, 127.1, 76.6, 34.6, 31.9, 29.4, 29.3, 29.1, 25.0, 22.7, 14.1</td>
			<td>ESI calcd for C23H30O2 (M+Na)+ 361.2138, found 361.2150</td>
		</tr>
	</tbody>
</table>
Table 1. Tabulated data for compounds 8-17. Chemical shifts (δ) are reported in ppm, and coupling constants (J) are reported in hertz (Hz). Signals are reported as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), or combinations of the above. HRMS data are reported as m/z.

Watch this Scientific Journal Video about Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile at JoVE.com

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

A modified Steglich esterification reaction was used to synthesize a small library of ester derivatives with primary and secondary alcohols. The methodology uses a non-halogenated and greener solvent, acetonitrile, and enables product isolation in high yields without the need for chromatographic purification.

The Steglich esterification is a widely-used reaction for the synthesis of esters from carboxylic acids and alcohols. While efficient and mild, the ...

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Carboxylic Acid Derivatives

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35.4K Views.  Siena College. A modified Steglich esterification reaction was used to synthesize a small library of ester derivatives with primary and secondary alcohols. The methodology uses a non-halogenated and greener solvent, acetonitrile, and enables product isolation in high yields without the need for chromatographic purification.

Video: Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Esterification

<h4>Esters</h4>
The structure of an ester is a carbonyl with an alkyl or aryl group (R) on one side, and an oxygen bound to another alkyl or aryl group (R&#8217;) on the other side, represented by the general formula: RCOOR&#8217;. Esters can be commonly derived by an esterification reaction between a carboxylic acid and an alcohol. The hydroxyl group of the carboxylic acid is replaced by an alkyl or aryl group. Simple esters, which have low molecular weight and small R and R&#8217; functional groups, have smells and flavors of fruits and flowers.
The simplest carboxylic acid is formic acid (HCOOH), with the simplest alcohol being methanol. Thus, the simplest ester formed from the esterification of these two molecules is methyl formate (HCOOCH3). Methyl formate is considered a derivative of formic acid because the OH group is replaced by the OCH3 group. The structural differences between methyl formate and formic acid dictate the difference in their properties. This is because the properties of the molecule depend heavily on the R and R&#8217; groups.
Esters that are found in fruits and flowers are known as simple esters due to the low complexity of their structure in comparison to other esters. The alkyl and aryl groups found on these esters greatly influences the fragrance they emit; thus, the addition of a carbon or a different functional group can impact the scent produced by the ester. For example, propyl acetate (C5H10O2) has the scent of pears. However, butyl acetate (C6H12O2), which has only one more carbon in its chain, smells like apples.
Long-chain esters, meaning those with long alkyl chains, are called fatty acids. Fatty acids are important biological compounds that are essential for certain biological processes. Fatty acids are also a biological form of energy storage. When three fatty acids react with glycerol, they form a triglyceride &#8212; the most common form of fatty acids. Triglycerides have a very high molecular weight and can be highly branched. Their function and properties are dependent on the length and types of alkyl chains incorporated into their structure. One of the most common industrial uses for triglycerides is in the production of soaps and detergents.
<h4>Esterification Reaction</h4>
Esters are formed from an esterification reaction, with simple esters being formed through Fisher esterification. This reaction converts a carboxylic acid and alcohol into an ester with water as a by-product. Fisher esterification is a reversible reaction that proceeds very slowly. An acid catalyst, typically in the form of sulfuric acid, is added to increase the rate of the reaction while also acting as a dehydrating agent.
The mechanism of action for a Fisher esterification begins by the carbonyl oxygen attacking the sulfuric acid and deprotonating its OH group. This leads to a positively charged carbonyl oxygen. To better stabilize the positive charge, one electron pair of the carbonyl double bond is pushed to the carbonyl oxygen while simultaneously creating an electron-poor carbon center. Next, the hydroxy group of the alcohol acts as a nucleophile and attacks the electron-poor carbon &#8212; the electrophile &#8212; forming an intermediate. Then, the positively-charged hydroxyl group from the alcohol is deprotonated by the hydroxyl group of the carboxylic acid, stabilizing the oxygen atom of the alcohol. In the last step, the carbonyl oxygen is deprotonated by the conjugated base, and the free electron pair is moved towards the carbonyl center while the protonated carboxyl group leaves as a water molecule.
Since the esterification reaction is reversible, a 1:1 mixture of the carboxylic acid and the alcohol will reach equilibrium with about 70% yield of the ester. However, as with any equilibrium process, the reaction can be driven in one direction by changing the concentration, pressure, temperature, or volume of the reagents. This change causes the reaction to adjust to a new equilibrium to counteract the change, which is the basis of Le Chatelier&#8217;s principle.
When the concentration of one of the reactants is increased, the equilibrium shifts in the direction that will decrease its concentration. Thus, by increasing the concentration of one reactant, the equilibrium produces more of the product, thereby resulting in a higher yield of the ester product.
One common method utilized in organic chemistry labs is to provide an excess of one of the reagents, typically the alcohol. As mentioned earlier, Fisher esterification can utilize sulfuric acid as a catalyst. However, one of the other functions of sulfuric acid is to act as a dehydrating agent, sequestering water molecules away from the reaction. Applying Le Chatelier&#8217;s principle demonstrates that as one of the products is removed &#8212; in this case, water &#8212; the reaction is pushed to making more product. This increases the yield of the ester formed.
<h4>Reference</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.</li>
</ol>

Esterification: Synthesis of Ester by Fischer Esterification - Concept

Esters
The structure of an ester is a carbonyl with an alkyl or aryl group (R) on one side, and an oxygen bound to another alkyl or aryl group (R&#8217;) ...

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Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

In organic chemistry ionic liquids (ILs) have emerged as safe and recyclable reaction solvents. In the presence of a base ILs can be deprotonated to form catalytically active N-Heterocyclic Carbenes (NHCs). Here we have used ILs as precatalysts in the addition of &#945;,&#946;-unsaturated aldehydes to chalcones to form 1,6-ketoesters, incorporating an anti-diphenyl moiety in a highly stereoselective fashion. The reaction has a broad substrate scope and several functional groups and heteroaromatics can be integrated into the ketoester backbone in generally good yields with maintained stereoselectivity. The reaction protocol is robust and scalable. The starting materials are inexpensive and the products can be obtained after simple filtration, avoiding solvent-demanding chromatography. Furthermore, the IL can be recycled up to 5 times without any loss of reactivity. Moreover, the 1,6-ketoester end product is a potent gelator in several hydrocarbon based solvents. The method enables rapid access to and evaluation of a new class of low molecular weight gelators (LMWGs) from recyclable and inexpensive starting materials.

Ionic liquids (ILs) mediate fast, simple and cheap access to 1,6-ketoesters in high diastereoselectivities and good yields. The reaction protocol is robust and the 1,6-ketoesters can be obtained in gram scale after a simple filtration protocol. Moreover, the 1,6-ketoesters are potent gelators in hydrocarbon solvents.

In organic chemistry ionic liquids (ILs) have emerged as safe and recyclable reaction solvents. In the presence of a base ILs can be deprotonated to form ...

Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high atomic and step economy. However, this functionality-directed cross-coupling reaction has not been developed, as there are still limited directing groups in practical use. In particular, a stoichiometric amount of oxidant is usually required, producing a large amount of waste. Due to our interest in novel 1,3-butadiene synthesis, we describe the ruthenium-catalyzed olefination of electron-deficient alkenes using allyl acetate and without external oxidant. The reaction of 2-phenyl acrylamide and allyl acetate was chosen as a model reaction, and the desired diene product was obtained in 80% isolated yield with good stereoselectivity (Z,E/Z,Z = 88:12) under optimal conditions: [Ru(p-cymene) Cl2]2 (3 mol %) and AgSbF6 (20 mol %) in DCE at 110 &#186;C for 16 h. With the optimized catalytic conditions in hand, representative &#945;- and/or &#946;-substituted acrylamides were investigated, and all reacted smoothly, regardless of aliphatic or aromatic groups. Also, differently N-substituted acrylamides have proven to be good substrates. Moreover, we examined the reactivity of different allyl derivatives, suggesting that the chelation of acetate oxygen to the metal is crucial for the catalytic process. Deuterium-labeled experiments were also conducted to investigate the reaction mechanism. Only Z-selective H/D exchanges on acrylamide were observed, indicating a reversible cyclometalation event. In addition, a kinetic isotope effect (KIE) of 3.2 was observed in the intermolecular isotopic study, suggesting that the olefinic C-H metalation step is probably involved in the rate-determining step.

Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

The ruthenium-catalyzed olefination of electron-deficient alkenes with allyl acetate is described here. By using aminocarbonyl as a directing group, this external oxidant-free protocol has high efficiency and good stereo- and regioselectivity, opening a novel synthetic route to (Z,E)-butadiene skeletons.

Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high ...

Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Overview

The Fischer esterification reaction was developed by the German chemist Emil Fischer in 1895. It is a condensation reaction between carboxylic acids and alcohols in an acidic medium to give esters and water.
<img alt="Esterification reaction diagram: acetic acid + methanol ⇌ methyl acetate + water, catalyzed by HCl." src="/files/ftp_upload/12348/12348_PT_Figure_1.svg" style="text-align: center; height: 80px;" data-altupdatedat="1747908711000" data-alt="Figure1">
Hydroxy-functionalized carboxylic acids undergo intramolecular Fischer esterification to form lactones. The cyclic five- and six-membered lactones are formed spontaneously.
<img alt="Fischer esterification, chemical reaction diagram, intramolecular process, organic chemistry." src="/files/ftp_upload/12348/12348_PT_Figure_2.svg" style="text-align: center; height: 100px;" data-altupdatedat="1747908711000" data-alt="Figure2">
The reaction rate of Fischer esterification is greatly dependent on steric factors. Primary alcohols react fastest with carboxylic acids to form esters, while tertiary alcohols undergo Fischer esterification at a slower rate, forming alkene by-products.
Fischer esterification is an inherently slow reaction with a low equilibrium constant value and never attains completion. During the reaction, an equilibrium is always established between the reactant and the product. Consequently, traces of unreacted acid are always present along with the product ester. Using excess alcohol as a solvent directs the equilibrium towards the product, according to Le Chatelier's principle. Alternatively, removing the water from the reaction mixture using a Dean–Stark trap can also drive the reaction to completion.

Carboxylic Acids to Esters: Acid-Catalyzed Fischer Esterification Overview

The Fischer esterification reaction was developed by the German chemist Emil Fischer&#160;in 1895. It is a condensation reaction between carboxylic acids ...

Carboxylic Acids

KEY TERMS AND CONCEPTS

Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Mechanism

Carboxylic acids react with alcohols to yield esters via an acid-catalyzed condensation reaction called Fischer esterification. This is a nucleophilic acyl substitution reaction that proceeds via a tetrahedral intermediate, where a water molecule is eliminated as the leaving group.
<img alt="Esterification process equation, acid-catalyzed reaction diagram, shows ester and water formation." src="/files/ftp_upload/13088/13088_PT_Figure_1.svg" style="text-align: center; height: 68px;" data-altupdatedat="1747908711000" data-alt="Figure1">
The mechanism of this reaction was confirmed by Robert and Urey (1938) through a radioisotope labeling experiment, where esterification of a carboxylic acid was carried out using 18O-labeled alcohol. The resulting ester was found to be labeled with an 18O atom, establishing the fact that the –OH group of the carboxylic acid was replaced by the –OR group from the alcohol.
<img alt="Esterification reaction formula, chemical equation diagram, alcohol and carboxylic acid reaction process." src="/files/ftp_upload/13088/13088_PT_Figure_2.svg" style="text-align: center; height: 55px;" data-altupdatedat="1747908711000" data-alt="Figure2">
Modifications of Fischer esterification use either a boron trifluoride ether complex or an organotin complex as the catalyst in an acid-free condition.
<img alt="Chemical reaction scheme for esterification; cyclopropane carboxylic acid with methanol, 81% yield." src="/files/ftp_upload/13088/13088_PT_Figure_3.svg" style="text-align: center; height: 48px;" data-altupdatedat="1747908711000" data-alt="Figure3">

Carboxylic Acids to Esters: Acid-Catalyzed Fischer Esterification Mechanism

Carboxylic acids react with alcohols to yield esters via an&#160;acid-catalyzed condensation reaction called Fischer esterification. This is a ...

Esters to Carboxylic Acids: Acid-Catalyzed Hydrolysis

Hydrolysis of esters under acidic conditions proceeds through a nucleophilic acyl substitution. In the presence of excess water, the reaction proceeds in a reversible manner, forming carboxylic acids and alcohols.
During hydrolysis, the ester is first activated towards nucleophilic attack through the protonation of the carboxyl oxygen atom by the acid catalyst. The protonation makes the ester carbonyl carbon more electrophilic. In the next step, water acts as a nucleophile and adds to the carbonyl carbon, followed by the loss of a proton to give the tetrahedral addition intermediate. Further, the protonation of the alkoxy group converts it into a better leaving group. Subsequently, the carbonyl group is reconstructed with the expulsion of alcohol as a leaving group. Finally, deprotonation yields a carboxylic acid as the final product and regenerates the acid catalyst.
<img alt="Organic reaction mechanism diagram, showing aldol reaction steps with hydroxide ions and methanol molecules." src="/files/ftp_upload/12369/12369_PT_Figure_1_.svg" style="text-align: center; height: 225px;" data-altupdatedat="1747908711000" data-alt="Figure1">
An acid catalyst protonates the carbonyl oxygen, thereby increasing the formation rate of the tetrahedral intermediate. Moreover, the catalyst decreases the basicity of the leaving group. Notably, the reaction is exactly the reverse of a Fischer esterification reaction.

Hydrolysis of esters under acidic conditions proceeds through a nucleophilic acyl substitution. In the presence of excess water, the reaction proceeds in ...

Esters to Alcohols: Grignard Reaction

The reaction of an ester with a Grignard reagent, followed by hydrolysis of the magnesium alkoxide salt in aqueous acid, yields a tertiary alcohol. In the case of formate esters, secondary alcohols are formed.
The reaction requires two equivalents of the Grignard reagent and introduces two identical alkyl groups, derived from the Grignard reagent, bonded to the hydroxyl-bearing carbon of the alcohol.
The reaction follows the typical nucleophilic acyl substitution mechanism. The Grignard reagent, a source of carbanions, functions as a nucleophile and attacks the carbonyl carbon of the ester to form a tetrahedral intermediate. Next, the carbonyl group is reconstructed with the departure of the alkoxide ion as a leaving group. The product of the reaction is a ketone intermediate. The reaction does not stop at this stage because ketones are more reactive than esters toward nucleophilic attack. Therefore, an attack from a second equivalent of the carbanion generates a tertiary alkoxide ion, which gives tertiary alcohol as the final product upon protonation.
<img alt="Grignard reaction mechanism diagram showing chemical equations and bond formation steps." src="/files/ftp_upload/12828/12828_PT_Figure_1.svg" style="text-align: center; height: 225px;" data-altupdatedat="1747908711000" data-alt="Figure1">

The reaction of an ester with a Grignard reagent, followed by hydrolysis of the magnesium alkoxide salt in aqueous acid, yields a tertiary alcohol. In the ...

&alpha;-Hydroxy Ketones via Reductive Coupling of Esters: Acyloin Condensation Overview

The pinacol and McMurry reactions involve the reductive coupling of ketones or aldehydes. Similarly, the bimolecular reductive coupling of two ester molecules in the presence of sodium metal in an aprotic solvent yields an &#945;-hydroxy ketone product. The &#945;-hydroxy ketone is also called acyloin, so the reaction is referred to as &#8216;acyloin condensation.&#8217;
<img alt="Figure1" src="/files/ftp_upload/12908/12908_PT_Figure_1.svg" style="text-align: center; height: 65px;" />
In the acyloin condensation reaction, esters are converted to ketyls, which undergo radical dimerization to form an unstable tetrahedral intermediate. Further, this intermediate collapses to give a 1,2-diketone intermediate. The 1,2-diketones are comparatively more reactive than ketones towards electrophiles and reducing agents because of lower &#960;* energy. Consequently, two electrons sequentially transfer to the 1,2-diketone to form an enediolate. Lastly, the enediolate intermediate is quenched with acid to yield an &#945;-hydroxy ketone with a good yield.
Often some by-products are obtained alongside the major product because of the reactivity of nucleophilic enediolate. To overcome the side reactions and improve the yield of an acyloin, trimethylsilyl chloride is used to silylate the enediolate. This results in a bis-silyl ether, further hydrolyzed with aqueous acid to give &#945;-hydroxy ketones.

&amp;#945;-Hydroxy Ketones via Reductive Coupling of Esters: Acyloin Condensation Overview

The pinacol and McMurry reactions involve the reductive coupling of ketones or aldehydes. Similarly, the bimolecular reductive coupling of two ester ...

Radical Chemistry

Esters to &beta;-Ketoesters: Claisen Condensation Mechanism

Regular Claisen condensation involves the synthesis of &#946;-ketoesters by combining identical ester molecules bearing two &#945; hydrogens in the presence of an alkoxide base. The reaction commences with the deprotonation of the acidic &#945; hydrogen by the base to form a resonance stabilized ester enolate. This nucleophilic ion then attacks the carbonyl center of another ester molecule to generate a tetrahedral alkoxide intermediate. Next, the expulsion of the alkoxide group from the intermediate restores the carbonyl center and produces an acyl-substituted ester. The alkoxide by-product subsequently abstracts the second &#945; proton from the &#946;-dicarbonyl compound to form a doubly-stabilized enolate ion. This step is the driving force of the reaction to completion and suggests the essential requirement of two &#945; protons in starting ester. Finally, acidification of the enolate produces the desired &#946;-ketoester. The utility of the Claisen condensation process is also observed in biological systems. For instance, the synthesis of acetoacetyl-CoA from the condensation of acetyl-CoA in the presence of thiolase enzyme.

Esters to &#946;-Ketoesters: Claisen Condensation Mechanism

Regular Claisen condensation involves the synthesis of &#946;-ketoesters by combining identical ester molecules bearing two &#945; hydrogens in the ...

&alpha;-Carbon Chemistry: Enols, Enolates, and Enamines

Alkylation of &beta;-Ketoester Enolates: Acetoacetic Ester Synthesis

Acetoacetic ester synthesis is a method to obtain ketones from alkyl halides and &beta;-keto esters. The reaction occurs in the presence of an alkoxide base that abstracts the acidic proton of the &beta;-keto esters. The step results in an enolate ion which is doubly stabilized. The enolate then reacts with an alkyl halide via the SN2 process to produce an alkylated ester intermediate with a new C&#8211;C bond. The hydrolysis of the intermediate, followed by acidification, results in an alkylated &beta;-keto acid. Under high-temperature conditions, the &beta;-keto acid undergoes decarboxylation to form a ketone. However, if the alkylation is repeated before hydrolysis and decarboxylation steps, a disubstituted ketone is obtained.

Alkylation of &#946;-Ketoester Enolates: Acetoacetic Ester Synthesis

Acetoacetic ester synthesis is a method to obtain ketones from alkyl halides and &beta;-keto esters. The reaction occurs in the presence of an alkoxide ...

Synthesis of Esters Via a Greener Steglich Esterification in Acetonitrile

Summary

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