Stacked Step 2: Identify the first step of an alkyne alkylation.
Acetylene is H-C=C-H. The difference between the product and acetylene is the substitution of the acetylene protons with alkyl groups. Thus, the alkyne product will be made by two sequential alkylation reactions.
The first step of an alkyne alkylation is:
a) deprotonating of the alkyne proton using NaNH,
b) protonation of the alkyne using H+
c) addition of the alkyl group to the alkyne.
d) deprotonating of the alkyne proton using NaOH.
The Correct Answer and Explanation is :
The correct answer is a) deprotonating of the alkyne proton using NaNH₂.
Explanation:
Alkyne alkylation reactions involve the substitution of the terminal alkyne hydrogens (–C≡CH) with alkyl groups. To achieve this, two sequential reactions are typically required: the first being deprotonation of the alkyne and the second being the nucleophilic attack by the alkyl group. Here’s a breakdown of why a) is the correct choice:
- Deprotonation with NaNH₂:
The first step of the alkyne alkylation process is the removal of one of the acidic protons (H) from the terminal alkyne (–C≡CH). This is achieved by using a strong base like sodium amide (NaNH₂), which is strong enough to deprotonate the alkyne. This creates an alkynyl anion (–C≡C⁻), which is highly nucleophilic. The deprotonation step is crucial because the resulting anion can now attack an electrophile (such as an alkyl halide) in the next step to form a substituted alkyne. - Why not the other options?
- b) Protonation of the alkyne using H+: Protonation of the alkyne does not facilitate alkylation. This would reverse the process, bringing the alkyne back to its neutral form and not making it reactive for alkylation.
- c) Addition of the alkyl group to the alkyne: This step occurs after the deprotonation, not as the first step. The alkyl group can only be added once the alkyne has been deprotonated to form the nucleophilic anion.
- d) Deprotonating of the alkyne proton using NaOH: NaOH is not a sufficiently strong base to deprotonate an alkyne. Sodium amide (NaNH₂) is much stronger and more effective in this role.
Thus, NaNH₂ is the strong base required to deprotonate the alkyne, setting the stage for alkylation to occur in the subsequent steps.