Question

Question: Explain why 2,2,5,5-tetramethylhex-3-yne can’t be made using acetylide anions

Short answer

Answer

• The$3°$ carbon centre is too crowded for the acetylide anion to attack.

• Elimination would be preferred over substitution

Step-by-step solution

Step-by-Step Solution Step 1: Reaction of acetylide with alkyl halide

The reaction between the nucleophilic acetylide ion and the electrophilic alkyl halide is an ${\mathbf{S}}_{\mathbf{N}}\mathbf{2}$ type substitution reaction.The general reaction scheme is shown below:

Reaction scheme

Analyzing the structure of 2,2,5,5-tetramethylhex-3-yne:

The structure of 2,2,5,5-tetramethylhex-3-yne is given below:

Structure of 2,2,5,5-tetramethylhex-3-yne

The acetylide and alkyl halide that would react to form the above compound is:

Formation of two different products

The acetylide anion and alkyl halide that reacts to form the compound

Since the$3°$carbon center would be a sterically hindered center,${\mathrm{S}}_{\mathrm{N}}2$type reactions would not proceed via this mechanism.

Hence, elimination would occur, and we would not get 2,2,5,5-tetramethylhex-3-yne.