6.1 : Alkyl Halides

Structural Properties

Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp³-hybridized and exhibits a tetrahedral shape.

Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp² -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl halides are compounds in which a halogen atom is bonded to an sp²-hybridized carbon atom of an aromatic ring.

Furthermore, depending on the degree of substitution at the carbon atom containing the halogen, alkyl halides are classified into primary, secondary, and tertiary alkyl halides. The carbon bonded to a halogen atom is called the α- carbon. The carbon atom linked to the α-carbon is called β-carbon. The adjacent carbons are subsequently denoted by greek alphabets (γ, δ, and so forth).

Nomenclature

Naming alkyl halides follows the general IUPAC rules of organic chemistry:

1. Identify the longest carbon chain and name it.
2. Find and name the substituent(s).
3. Number the longest carbon chain and designate a number, or locant, for each substituent.
4. Assemble the substituents in alphabetical order.

Reactivity and Applications

Alkyl halides are versatile compounds that serve as solvents, pesticides, and intermediates in the preparation of dyes, drugs, and synthetic polymers. The chemical reactivity of alkyl halides differs depending upon the structural classification. Alkyl halides are classified into primary, secondary, and tertiary alkyl halides depending on the degree of substitution on a carbon-bearing halogen.

Furthermore, the halogen atom’s electronegativity plays a significant role in the alkyl halide’s reactivity. Except for iodine, other halogens have a significantly greater electronegativity than carbon. Due to electronegativity differences between carbon and halogen atoms, the covalent bond between the atoms is polarized; thus, the carbon carries a partial positive charge and the halogen atom a partial negative charge. Consequently, the carbon attached to the halogen is an electrophile.

In a periodic table, going down the halogen family, the electronegativity decreases; inversely, the halogen atom size increases. Thus, the carbon-halogen bond length increases while bond strength and polarity decrease from fluorine to iodine, making it easier to break the bond.

Apart from these factors, the relative stability of the corresponding conjugate bases, or halide anions, also influences the alkyl halide's chemical nature. The stability of halide anions can be measured with respect to the hydrohalic acids' relative acidities. The strongest hydrohalic acid, hydrogen iodide, has a pK_a of -11. Meaning it fully dissociates to the most stable conjugate base, the iodide ion, and a proton. Thus, iodine is an excellent leaving group. Except for hydrofluoric acid with a pK_a of 3.2, other hydrohalic acids have pK_a values of less than 0, making them strong acids with weak and stabilized conjugate bases, which are excellent leaving groups.