Question

How would you get trans alkenes from alkyne? Give its mechanism.

Answer 1

Answer on Question #61064 - Chemistry - Organic Chemistry

Task:

How would you get trans alkenes from alkyne? Give its mechanism.

Solution:

Trans-isomers alkenes are obtained when recovering of non-terminal alkynes with solution of sodium or lithium in liquid ammonia at $-33^{\circ}\mathrm{C}$. Solutions of alkali metals in liquid ammonia possess the properties of a strong reducing agent. Restoring alkynes sodium in liquid ammonia to flow completely stereospecifically with the formation only the trans-isomer.

As an example restoring alkynes sodium in liquid ammonia can propose restoring decyne-5 to trans-decene-5 in $78\%$ yield and stereochemical purity of $100\%$.

For this process, mechanism is proposed a two-electron recovery with the formation triple bond as intermediatov sequentially anion radical, vinyl radical and vinyl anion. The first electron is directed to the antibonding $\pi$-orbital with the formation the anion radical. Then anion radical is protonated by ammonia, and gives the vinyl radical which appends another electron with the formation vinyl anion. After all vinyl anion splits off proton from the ammonia performing the role of N-H acid, which results in a trans-alkene and imide anion:

one-electron restoring

1) $\mathrm{RC} \equiv \mathrm{CR} + \mathrm{Na} \longrightarrow \mathrm{Na}^{\oplus} + \left[\mathrm{RČ} = \mathrm{CR}\right]^{-}$

2) $\left[\mathrm{RČ} = \mathrm{CR}\right]^{-} + \mathrm{NH}_{3} \longrightarrow \mathrm{NH}_{2}^{\ominus} + \begin{array}{c} \mathrm{R} \\ \mathrm{C} = \mathrm{R} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \\ \mathrm{CH} \end{array} \text{vinyl radical}$

one-electron restoring

3) $\mathrm{R} \underset{\mathrm{H}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}} + \mathrm{Na} \longrightarrow \mathrm{Na}^{\oplus} + \underset{\mathrm{H}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}}$

vinyl anion

4) $\mathrm{R} \underset{\mathrm{H}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}} + \mathrm{NH}_{3} \longrightarrow \mathrm{NH}_{2}^{\ominus} + \underset{\mathrm{H}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}} = \underset{\mathrm{R}}{\overset{\mathrm{R}}{\mathrm{C}}}$

trans-alkene