Question

What stereochemistry do you expect for the trisubstituted alkene obtained by E2 elimination of the following alkyl halide on treatment with KOH? (Reddish brown = Br.)

Short answer

The major product obtained after E2 elimination is (Z)-3-methyl-2-pentene.

(Z)-3-methyl-2-pentene

Step-by-step solution

E2 elimination

E2 elimination is dependent on the number of beta hydrogens present in a molecule. Alkyl halides with beta hydrogens undergo elimination and result in the formation of an alkene.

The stereochemistry of trisubstituted alkene obtained by an E2 elimination

Firstly, the alkyl halide structure is drawn and converted into Newman projection. Now, rotate the groups so that –H and –Br should have an anti periplanar relationship and are eliminated.

Structure of alkyl halide and its Newman projection

The major product obtained after E2 elimination is (Z)-3-methyl-2-pentene. In addition, a small amount of 3-methyl-1-pentene is also formed.

(Z)-3-methyl-2-pentene