Q9-7P b - Organic Chemistry (Mcmurry)

Question

How would you prepare the following carbonyl compounds starting from an alkyne (reddish-brown Br)?

b.

Step-by-Step Solution

Answer

Answer

Step 1: Hydroboration – oxidation reaction

Borane adds rapidly to an alkyne resulting in vinylic borane can be oxidized by H₂O₂ to yield an enol. Tautomerization then gives either a ketone or an aldehyde. 

• The terminal alkyne leads to the formation of aldehyde 
• The internal alkyne leads to the formation of ketone

Step 2: Formation of compound a

Terminal alkyne undergoes two additions, giving a doubly hydroborated intermediate. Oxidation with H₂O₂ replaces both boron atoms with oxygen and generates the aldehyde.