Hydrocarbon of Class 11
Nomenclature of alkynes:
In IUPAC system the compounds are named as alkynes in which the final – ane of the present alkane is replaced by the suffix – yne. The position of the triple bond is indicated by a number.
When both double and triple bonds are present, hydrocarbon is named as alkenyne.
Within presence of undergoes electrophilic substitution at o- and p- position and in the absence of and in the presence of light or high temperature undergoes substitution in the side chain (free radical substitution).
For side chain halogenation, case of abstraction of hydrogen atom is as follows:
Allylic or benzylic > 3° > 2° > 1°> CH4 > Vinylic the stability order of free radicals is Brominew which is more selective than chlorine.
Benzene ring is usually very resistant to oxidation, hence the side chain is always attacked. Whatever the length of side chain the ultimate oxidation product is benzoic acid.
When two side chains are present, it is possible to oxidize them at same time.
But, if the C attached to benzene ring does not have any hydrogen then it will not give benzoic acid.
When an electron withdrawing group (-I and / or –R) is present, the ring is stable and the result of oxidation is a substituted benzoic acid.
If –OH or is present, the ring is very sensitive to oxidation and is largely broken down, whatever be the nature of oxidizing agent. Ring rupture can be prevented by protection of the group.