6.15 : תגובות אלימינציה

When an alkyl halide reacts with a nucleophile, the nucleophile can displace the halogen to give the substitution product or it can abstract a neighboring hydrogen to form an alkene through an elimination reaction.

In elimination reactions, nucleophiles function as Lewis bases by donating a pair of electrons to a proton. Some of the common bases used to promote elimination reactions include hydroxides such as sodium hydroxide, alkoxides like potassium tert-butoxide, or alcohols like ethanol.

Elimination reactions typically involve the loss of small molecular fragments from a substrate to form at least one π bond. In alkyl halides, the elimination reaction proceeds with the loss of one hydrogen atom and one halogen atom, hence the name dehydrohalogenation.

Since the carbon bonded to the leaving group is an α carbon and the hydrogen on the adjacent carbon is a β hydrogen, these reactions are often called β-elimination or 1,2-elimination reactions.

Most elimination reactions occur via an E2 or E1 mechanism.

For E2 reactions, strong bases like sodium ethoxide are used. The concerted mechanism is initiated by the deprotonation of the β carbon followed by the departure of the halide leaving group leading to the formation of a π bond between the α and β positions.

In contrast, the E1 reaction proceeds in two steps. The first involves the departure of the leaving group to form a carbocation intermediate, followed by deprotonation of the carbocation by the base to form a π bond.

With alkyl halides containing two different β carbons, elimination reactions can produce more than one alkene. Here, the more substituted alkene is the most stable and is called the Zaitsev product, while the less substituted alkene is called the Hofmann product. Thus, elimination reactions are said to be regioselective.

Additionally, elimination reactions favor the formation of trans-alkenes over the cis-isomers, making them stereoselective.