Question

Question: Write a stepwise mechanism for each of the following reactions. Explain why a more stable alkyne (but-2-yne) is isomerized to a less stable alkyne (but-1-yne), but under similar conditions, 2,5-dimethylhex-3-yne forms 2,5-dimethylhexa-2,3-diene.

Short answer

Answer

a. The mechanism of the given reaction is as follows.

Formation of the diene

b.

Formation of the terminal butyne

c.

Formation of 2,5-dimethylhexa-2,3-diene

Step-by-step solution

Step-by-Step SolutionStep 1: Reaction of the alkynes with strong bases

The alkynes reacting with a strong base, such as , or , leads to the formation of acetylide anions. The hydrogen atoms, which are present on the sp-hybridized carbon atoms, are very acidic in nature and result in the formation of the acetylide anions.

The acetylide anions are very strong nucleophiles and can readily react with the electrophiles, such as alkyl halides and epoxides.

Stepwise mechanism of the first reaction

a. The ${{\mathrm{NH}}_2}^{-}$ abstracts a proton from 2-butyne to produce the resonance-stabilized carbanion, as shown in the mechanism. The carbanion further reacts with the carbanion to form the diene, as shown below.

Formation of the diene

The diene then further reacts with ${{\mathrm{NH}}_2}^{-}$ to form the terminal butyne, as shown below.

Formation of the terminal butyne

In the above reaction, a more stable terminal alkyne can isomerize into a less stable terminal alkyne. This is because the butyne contains sp-hybridized carbon and hydrogen bonds.

The terminal hydrogen atom in butyne is more acidic, as compared to any other hydrogen atom in 2-butyne.

Under the given reaction conditions, the proton from the butyne can be easily removed by a strong base, and the formation of the acetylide anion will take place. The acetylide anion causes the equilibrium to favor its formation.

Therefore, the protonation of the acetylide ion will result in the formation of the less stable terminal alkyne.

Stepwise mechanism of the second reaction

b. The reaction of the 2,5 dimethylhex-3yne under similar conditions leads to the formation of 2,5-dimethylhexa-2,3-diene, as shown in the following mechanism:

Formation of 2,5-dimethylhexa-2,3-diene

The formation of 2,5-dimethylhexa-2,3-diene takes place because there are no more hydrogen atoms present on the second carbon. The further reaction does not take place, and the more stable internal alkyne is formed as a product.