Question

a. Why does deuterium add to C-1 rather than to C-4 in the preceding reaction?

b. Why was DCl rather than HCl used in the reaction?

Short answer

a) Addition at C-1 forms the most stable carbocation because the positive charge is shared by two secondary allylic carbons. If the deuterium had added to C-4 the positive charge would have been shared by a secondary and a primary allylic carbon.

b) DCl was used to cause the 1,2- and 1,4- products to be different. If HCl had been used, the 1,2- and 1,4- products would have been the same.

Step-by-step solution

How reaction proceed

At C-1 addition

Final answer

a) Addition at C-1 forms the most stable carbocation because the positive charge is shared by two secondary allylic carbons. If the deuterium had added to C-4 the positive charge would have been shared by a secondary and a primary allylic carbon.

b) DCl was used to cause the 1,2- and 1,4- products to be different. If HCl had been used, the 1,2- and 1,4- products would have been the same.