Question

When (R)-2-chloro-3-methylbutane is treated with potassium tert-butoxide, a monosubstituted alkene is obtained. When this alkene is treated with HBr, a mixture of products is obtained. Draw all of the expected products.

Answer 1

Potassium tert-butoxide is a very strong base (as it is a conjugate of weak acid, butanol). It has a bulky structure and specifically attacks a less hindered carbon. It is sensitive to steric hindrance and on reaction with alkyl halide, it forms the elimination product.

It favors the formation of Hoffman product which is a less substituted alkene.

The mechanism for the reaction of Potassium tert-butoxide with (R)-2-chloro-3-methylbutane is shown below:

H CHз E2 CH3 CHз CHз Hас H2C H Hас -CH3 CH3 CHз CH3 2-methylbut-2-ene 3-methylbut-1-ene (2R)-2-chloro-3-methylbutane More substituted alkene Less substituted alkene forms forms the Minor product because the attack is favored the Major product because the attack is favored from less hindered site. from less hindered site.

Alkene undergoes to the addition reaction with HBr to give the corresponding alkyl halides. The mechanism follows the attack of the pi cloud of the alkene to the hydrogen of the hydrogen bromide molecule. An intermediate involving carbocation is formed which can be rearranged to form the most stable carbocation intermediate. Bromine then attacks this carbocation to form the alkyl halide.

The addition of HBr to the monosubstituted alkene obtained in the mechanism involves the formation of carbocation on 2⁰ carbon.

There are two possibilities:

1. Attack of the carbocation on the 2⁰ carbocation and form product. The formation of the product involves the formation of a new chiral center which suggests that two different products are formed.

2. Rearrangement of the 2⁰ carbocation to 3⁰ carbocation which is more stable due to hyperconjugation. Then follows the attack of the nucleophile, bromide ion to the carbocation forming the third product.

The mechanism is shown below:

Three products are formed from the reaction:

H CHз Нас CH3 CHз Нас Нас CH3 H Br CHз R(c20.c) CH3 3-methylbut-1-ene Br Br Br Br H Br CH3 CH3 Нас Hас 2-bromo-2-methylbutane CH3 Нзс Нзс Br CH3 CHз (2R)-2-bromo-3-methylbutane (2S)-2-bromo-3-methylbutane

Br Br CH3 CНз + H3C Нас CН3З Нас Нас Br CНз CH3З (2R)-2-bromo-3-methylbutane2S-2-bromo-3-methylbutane 2-bromo-2-methylbutane