Electrophilic Addition - 1,2 versus 1,4

Recall from earlier in the course that alkenes behave like nucleophiles and undergo electrophilic addition.  Remember that the regioselectivity (Markovnikovs rule) is governed by the stability of the carbocation that's formed.  The proton of the HBr adds to the C with the most H's (the rich get richer) and the most stable carbocation forms as indicated below.

Dienes will undergo the same electropilic additions, however two products are now possible, because of the resonance of the allylic carbocation formed.  These two products are referred to as the 1,2-product (Kinetic product) and the 1,4-product (Thermodynamic product).  To predict the products of these reaction you need to beable to show the different resonance structures for allyl carbocations.  Go back now and review Allyl Cations, Anions and Radicals.

Thermodynamic Versus Kinetic Control

In most reactions we have seen thus far, the most stable product forms at a faster rate.  For example the addition of HBr to propene above form the most stable product the fastest.  In these cases the kinetic product and the thermodynamic product are the same.  This does not have to be the case.  In the electrophilic addition of HBr to conjugated dienes the more stable product forms more slowly and the less stable product forms faster.  As we will see as a result of this unique energy situation, the outcomes of these types of reactions can be controlled by adjusting the temperature.  Many other reactions exibit thermodynamic versus kinetic control including Diels Alder reaction and enolate formation and alkylation.

Why is TS_1-4 less stable than TS_1-2?

The transitions state leading to the 1,2-product (TS_1-2 below) is more stable (lower in E) than the transition state leading to 1,4-product (TS_1-4).  This is easily seen since TS1-2 is a more stable secondary allylic carbocation, while TS1-4 is a less stable primary allylic carbocation.  Thus the 1,2-product will form faster, since it has a lower barrier.  See figure below.

Why 1,4-product more stable than 1,2-product?

Recall from our discussion of the Stability of Alkenes, that more substituted alkenes are more stable than less substituted.  The substituents are colored green on the 1,2-and 1,4-products below.  Thus the 1,4-product is more stable.