Question

A student wanted to know whether the greater proximity of the nucleophile to the C-2 carbon in the transition state is what causes the 1,2-addition product to be formed faster when 1,3- butadiene reacts with HCl. Therefore, she decided to investigate the reaction of 2-methyl- 1,3-cyclohexadiene with HCl. Her friend told her that she should use 1-methyl-1,3-cyclohexadiene instead. Should she follow her friend’s advice?

Answer 1

Reaction of 1,3-butadiene with HCl are as follows:

The relative amounts of the product is highly temperature dependent.

At, formation of product I is 71% and product II is 29%.

At, formation of product I is 15% and product II is 85%.

At low temperature reaction is kinetically stable as 1,2-addition product is favored than 1,4-addition product. This is due to the ion pair mechanism, the chloride ion is in close proximity with C-2 carbon than C-4 carbon. Therefore , 1,2-addition product is fomed faster than 1,4 addition product.

In case of 1-methyl-1,3-cyclohexadiene, when it reacts with HCl, hydrogen get attached to carbon with more number of hydrogen atoms as follows:

But in this case positive charge is not in conjugation with double bond. Hence there is no transition state and it is bound to only give 1,2 addition product. Therefore, 1-methyl-1,3-cyclohexadiene is not a good choice to investigate the attack of nucleophile in transition state.

In case of 2-methyl-1,3-cyclohexadiene, reaction with HCl is as follows:

Transition state for the above reaction will be:

Now, 1,2-addition is more stable because in the transition state tertiary allylic carbon is more stable than secondary allylic carbon. Also, The greater proximity of nucleophile to the C-2 carbon in the transition state causes 1,2 addition faster when 2-methyl-1,3-cyclohexadiene reacts with HCl.

Therefore, she should not follow her friend’s advice and should not choose 1-methyl-1,3-cyclohexadiene.