Question

Question 24 The reaction shown below shows the dehydration of an alcohol 1. What is the name of the alcohol shown 2. How many beta carbons are present in the alcohol 3. How many products are possible from this reaction 4. The product shown is the major product. Explain why this is the best product. OH H2SO4, A

Answer 1

1.                  4-Hydroxy-4methylheptane

2.       All carbons linked to the alpha carbon atom are beta carbons. Alpha carbon is the one to which the funtional group is attached. In this case it is the fourth carbon. ( Note that in this particular case numbering can be done from any side, since it is symmetrical). so there are 3 beta carbons C-3, C-5 and the Methyl carbon at C-4

3. only two products are possible. Elimination of hydrogen from C-3 and C-5 yield the same product in this case due to symmetry in structure.

4. In case of E1 reaction, the more substituted product is the major product. Removal of H can take place from any of the 3 beta carbons, out of which removal from C-3 and C-5 give same product which is the major product. The only other option is removal of H from the methyl carbon. Here the double bond will be formed between Methyl carbon and C-4. C-4 is flanked by two propyl groups whereas methyl carbon is attached to two H atom. Hence less substituted product.

Note that in the major product shown the double bond is between C-4 and C-5 (numbering from left). C-4 is substituted with one methyl and one propyl group and C-5 is substituted with one hydrogen and one ethyl group.