Question

For the deprotonation of a terminal alkyne, why is it that NaNH_2 in liquid ammonia is used and not NaNH_2 in water?

a) NaNH_2 in water causes excessive side reactions.
b) Liquid ammonia acts as a better solvent for the reaction.
c) NaNH_2 in water does not sufficiently deprotonate terminal alkynes.
d) Liquid ammonia stabilizes the deprotonated product.

Answer 1

NaNH2 in liquid ammonia is used for the deprotonation of a terminal alkyne instead of NaNH2 in water due to excessive side reactions, liquid ammonia being a better solvent, insufficient deprotonation with NaNH2 in water, and the ability of liquid ammonia to stabilize the deprotonated product.

Step-by-step explanation:

The reason why NaNH2 in liquid ammonia is used for the deprotonation of a terminal alkyne instead of NaNH2 in water is because:

1. NaNH2 in water causes excessive side reactions. When NaNH2 is dissolved in water, it reacts with water to produce NH3 and OH- ions, which can lead to unwanted reactions and lower yields in the deprotonation reaction.
2. Liquid ammonia acts as a better solvent for the reaction. Liquid ammonia has a lower dielectric constant compared to water, which promotes the formation of the deprotonated product and inhibits the formation of undesired side products.
3. NaNH2 in water does not sufficiently deprotonate terminal alkynes. The strong basicity of NaNH2 in liquid ammonia is necessary to fully deprotonate a terminal alkyne, yielding the corresponding alkynide ion.
4. Liquid ammonia stabilizes the deprotonated product. The alkynide ion formed during the deprotonation reaction is stabilized by solvation in liquid ammonia, which prevents its re-protonation and allows for further reactions.