Question

Question: Draw a stepwise mechanism for each reaction.

a.

b.

Short answer

Answer

a.

The reaction of the acetylide with the ${\mathrm{CH}}_2=\mathrm{O}$

Addition of a proton

b.

Formation of a resonance stabilized carbocation

Nucleophilic attack of the water molecule

Removal of a proton

Attack of ${\mathrm{H}}_2{\mathrm{SO}}_4$ to form the resonance stabilized carbocation and the desired product

Step-by-step solution

Step-by-Step Solution Step 1: Addition of water to alkynes

In the presence of a strong acid, the water molecule adds up to the triple bond of the alkyne to produce an enol in the initial step. The enol produced is highly unstable, which rearranges itself into a carbonyl group.

The addition reactions of the alkynes are similar to alkenes.

Reactions of the terminal alkynes

The terminal alkynes are highly reactive and can be deprotonated by the presence of a strong base.The acetylide ions which are produced by the loss of a proton can react with a variety of electrophiles.

The mechanism of the given alkene reactions

a. In the given reaction, the terminal alkyne loses a proton to form the carbanion, as shown in the first step of the given reaction. In the next step, the formaldehyde ${\mathrm{CH}}_2=\mathrm{O}$ acts as an electrophile and reacts with the terminal carbanion, as shown in the following reaction:

The reaction of the acetylide with the ${\mathrm{CH}}_2=\mathrm{O}$

In the final step, there is an addition of proton, which results in the following product:

Addition of a proton

b. In the first step of the given reaction, there is protonation of the -OH group. Thereafter, there is a loss of a water molecule to form the carbocation, which is resonance stabilized, as shown in the following structure:

Formation of a resonance stabilized carbocation

In the next step, a nucleophilic attack of a water molecule on the carbocation takes place, followed by the abstraction of the proton by the ${{\mathrm{HSO}}_4}^{-}$ ion:

Nucleophilic attack of the water molecule

In the next step, ${\mathrm{H}}_2{\mathrm{SO}}_4$ reacts to produce the desired product, as shown in the following steps:

Removal of a proton

Attack of ${\mathrm{H}}_2{\mathrm{SO}}_4$to form the resonance stabilized carbocation and the desired product