Alkynes on reduction with sodium in liquid ammonia form trans alkenes. Will the butene thus formed on reduction of 2-butyne show the geometrical isomerism?
Alkynes undergo reduction to give alkenes and on further reduction they give alkanes. If the reaction is controlled the reduction step can be stopped at alkenes. Alkynes when treated with Na/liq. NH3 give trans alkenes which can show geometrical isomerism. Here the nature of Na is critical for controlled reduction. Na has very low ionization energy, which allows it to give away its electrons easily.
Step 1: Formation of vinyl radical, and stability
The cis-vinyl radical is less stable due to more electron repulsion on the same side of the bond. Thus it converts into a trans form which is more stable due to staggering of electron and bonded pairs. Then formed trans-isomer of vinyl radical is attacked by the proton from ammonia.
Step 2: double protonation of vinyl radical
The negative charged developed on one carbon attacsks the proton from NH3 and another sodium atom loses its electron to develop a second negative charge on the atom. This negative charge is finally neutralised by attack of second proton to give a trans-but-2-ene.
This but-2-ene produced shows geometrical isomers as cis and trans.