Q1(b). Secondary kinetic isotope effect

Answer in brief

The isotope is not broken off: an α-secondary isotope effect is expected, usually inverse, k_H/k_D < 1, for rate-controlling addition to the carbonyl.

Key points

• A primary isotope effect involves breaking/forming a bond to the isotopic atom in the rate-determining event; here the aldehydic C–H/C–D bond remains intact.
• Cyanide attacks the planar carbonyl carbon, changing it from approximately sp² to sp³. Protonation of the resulting alkoxide gives the cyanohydrin.
• The change in bending vibrations and zero-point energies favours the D substrate for this sp²-to-sp³ change, so the D-labelled aldehyde generally adds slightly faster.
• The size depends on how far rehybridization has progressed in the transition state; no universal numerical ratio should be assigned.