Explain the following:
(a) Alkynes are less reactive than alkenes towards electrophilic addition reaction.
(b) Although benzene is highly unsaturated, it does not undergo addition reactions.
(c) Birch reduction of but-2-yne gives trans-2-butene.
Answers
a) Due to its higher density in electron, alkynes with triple bonds are easily attacked by "electrophiles", but are "less reactive" than "alkenes" due to the "compact" "C-C electron" cloud. As the electrophilic addition of non-symmetric alkenes follows the Markovnikov rule, adding the electrophilic to the "less substituted" carbon.
b) "Benzene" is highly unsaturated but does not normally undergo any additional reactions. Reason is it is an "aromatic compound" and is "stabilized" because of the resonance or the delocalization of electrons. It loses resonance stability if it undergoes additional reactions.
c) "Birch reduction" is an "organic reaction" in which aromatic rings are reduced by 1 and 4 to provide "unconjugated cyclohexadienes". The reduction in liquid ammonia and in presence of alcohol is carried out by sodium or lithium metal.