Question

Alkyne anions react with the carbonyl groups of aldehydes and ketones to form alkynyl alcohols, as illustrated by the following sequence. Propose a mechanism for the formation of the bracketed compound, using curved arrows to show the flow of electron pairs in the course of the reaction.

Short answer

Question: Describe the mechanism for the formation of alkynyl alcohols from alkyne anions and carbonyl groups of aldehydes and ketones. Answer: The mechanism for the formation of alkynyl alcohols from alkyne anions and carbonyl groups of aldehydes and ketones involves a nucleophilic attack of the alkyne anion on the carbonyl group, formation of an alkoxide intermediate, and protonation of the alkoxide intermediate to form the alkynyl alcohol. Curved arrows are used to show the flow of electron pairs during the reaction.

Step-by-step solution

Identify the nucleophilic and electrophilic centers

The alkyne anion has a negative charge on the carbon atom, making it a nucleophile. On the other hand, the carbonyl group of aldehydes and ketones has a partially positive carbon atom due to the polarization of the C=O bond, making it an electrophile.

Attack of the nucleophile on the electrophile

The nucleophilic carbon atom of the alkyne anion attacks the electrophilic carbon atom of the carbonyl group, displacing the electrons in the C=O bond towards the oxygen atom. This generates an alkoxide intermediate with a new C-C bond between alkyne and carbonyl carbon atoms.

Protonation of the alkoxide intermediate

The negatively charged oxygen atom in the alkoxide intermediate abstracts a proton (H+) from a solvent molecule or a protonated base, and this results in the formation of the final alkynyl alcohol product. The proposed mechanism can be summarized as follows: 1. Nucleophilic attack of the alkyne anion on the carbonyl group. 2. Formation of an alkoxide intermediate. 3. Protonation of the alkoxide intermediate to form the alkynyl alcohol. Use the curved arrow notation to illustrate the mechanism: 1. The curved arrow starts from the negatively charged carbon atom of the alkyne anion and goes towards the electrophilic carbon atom of the carbonyl group. 2. The curved arrow starts from the π bond of the C=O bond and goes towards the oxygen atom, forming a new O- species. 3. A proton is transferred from a solvent molecule or protonated base (indicated as H+) to the negatively charged oxygen atom.