Indian Forest Service (Mains), Chemistry optional · 2023 · Paper II

Q1(e). Dehydrobromination through an ion pair

Question

Complete the reaction of cis-1,2-dibromoethylene with triethylamine and name the elimination mechanism.

The initial elimination product is bromoacetylene, HC≡CBr; the classic mechanistic assignment is (E1cB)ip.

Triethylamine removes a vinylic proton, forming a closely associated vinyl-carbanion/triethylammonium ion pair.
Within that pair, bromide elimination forms the carbon–carbon triple bond. The net products are bromoacetylene and triethylammonium bromide.
The characteristic evidence in DMF includes second-order kinetics, a near-unity H/D kinetic isotope effect and lack of exchange with added deuterated triethylammonium.
These observations motivated a tight ion-pair model rather than a freely equilibrating carbanion or a routine E2 assignment.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

CoversE1cB mechanismion-pair E1cBdehydrobrominationbromoacetylenekinetic isotope effect