Question

Dehydrohalogenation of 1,2-dihalides (with the elimination of two molar equivalents of HX) normally leads to an alkyne rather than to a conjugated diene. However, when 1,2-dibromocyclohexane is dehydrohalogenated, 1,3-cyclohexadiene is produced and not cyclohexyne. What factor accounts for this?

Answer 1

Solution-

E2 elimination anti hydrogen to leaving group should be should be removed and therefore in case of 1,2 dihalide two anti-hydrogen are available and it forms alkyne.

In case of of 1,2-dibromocyclohexane elimination of second halogen.Hydrogen is not available for anti-elimination and it forms , 1,3-cyclohexadiene

Cyclohexane is the strain on the ring that was the deciding factor. Alkynes have minimal energy when the 4 C are atoms in a straight line, difficult when its a ring, unlike the 120 deg of an alkene.

Detail mechanism is shown below