Question

Why is a single pure product obtained from the hydroboration–oxidation of 2-butyne, whereas two products are obtained from hydroboration–oxidation of 2-pentyne?

Answer 1

The hydroboration–oxidation of but-2-yne forms a single pure product because but-2-yne is a symmetrical alkyne. Pent-2-yne forms two products because it is an unsymmetrical alkyne.

The hydroboration-oxidation of an internal alkyne forms a ketone:

R-C≡C-R' → RCH₂C(=O)R'

But-2-yne is a symmetrical alkyne. It has the same R groups (CH₃) on each end.

The ketone group could form either on C-2

or on C-3.

But these are these are the same compound — butan-2-one.

Pent-2-yne is an unsymmetrical alkyne. It has different R groups on each end. One end has a CH₃ group; the other end has a CH₂CH₃ group.

The ketone group could form either on C-2

or on C-3.

These are two different compounds. The first is pentan-2-one.The second is pentan-3-one.