Alkynes are reduced to trans alkenes by a process called dissolving metal reduction. The reaction uses sodium or lithium metal as the reducing agent and liquid ammonia as the solvent. The method is specific in the formation of trans alkenes from alkynes. The method involves two successive transfers of single electrons from the alkali metal to the triple bond, with abstraction of protons from the ammonia solvent. Draw curved arrows to show the movement of electrons in this step of the mechanism. Arrow-pushing Instructions

Explanation:

The movement of the electrons is illustrated in the picture attached to this answer. It is a four-step reaction mechanism.

First STEP: The first step involves the transfer of an electron from sodium to form a radical anion.

Second STEP: This radical anion then removes a proton/hydrogen from ammonia in a bid to neutralize itself (hence the hydrogen becomes bonded to the anion).

Third STEP: The sodium (from NaNH₂ formed) transfers an electron again to produce a vinyl carbanion.

Fourth STEP: The carbanion then removes a proton/hydrogen from ammonia (like in the second step) to form a neutral trans-alkene.

NOTE: The circled numbers denote each step while the mechanism on the left represents the use of any alkyl group (R and R') while the mechanism on the right assumes both alkyl groups are methyl. Hence, 2-butyne started the reaction and the final product was trans-2-butene.

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