Indian Forest Service (Mains), Chemistry optional · 2022 · Paper II

Q1(c). Tracing the nitrile carbon

Question

Use isotope labelling to establish the origin of the nitrile carbon in RCOO⁻ + BrCN → RCN + Br⁻ + CO₂.

The nitrile carbon is derived predominantly from the original carboxylate carbon; the cyanogen-bromide carbon is lost as CO₂.

Perform the reaction with carboxyl-carbon-labelled R¹⁴COO⁻ and unlabelled BrCN; isolate the nitrile and determine where its radioactivity resides.
Retention of the label as R¹⁴CN establishes the carboxylate origin; hydrolysing the nitrile back to the acid can confirm positional labelling.
As a reciprocal test, use unlabelled carboxylate and Br¹⁴CN; trap the evolved CO₂ separately to show the fate of the reagent carbon.
Published tracer work on sodium benzoate found the CO₂ carbon came almost entirely from cyanogen bromide.

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.

Coversisotopic labelling14C tracercyanogen bromidemechanism determinationcarboxylate to nitrile