Question

Which stereoisomer ― cis− or trans−1−bromo−3−tert−butylcyclohexane ― will react faster in an E2 elimination reaction? Select the single best answer.

Answer 1

• Cis-1-bromo-3-tert-butylcyclohexane will give faster reaction in E2 elimination.

Step-by-step explanation:

• E2 is known as second order reaction . Such reaction take place in a single step. E2 involve breaking of C-H and C-X bond in order to form double bond.

Role of cis geometry:

• The base mostly remove hydrogen which is anti-palanar to halogen. So when both group i.e bromide and tert-butyl group will be on one side, (cis), then base can abstract hydrogen easily as a result double bond formation take place so fastly.

Answer 2

The cis molecule will react faster.

Step-by-step explanation:

In the E2 mechanism, we have to have an anti-peri-planar configuration. The hydrogen that is removed and the leaving group should have a configuration of 90º in an anti position (see the figure red hydrogens).

In the trans configuration the ter-butyl group would exercise a steric hindrance upon the hydrogens that can be removed. In the cis configuration we don't have this problem. Therefore the cis molecule would react faster.