In the previous example, the alkene substrate is a symmetric alkene. In contrast, an unsymmetrical alkene has the two groups bonded to one of the doubly-bonded carbon differ from those on the other carbon. For example, in propene, one of the sp² carbon has two H bonded to it, while the other sp² carbon has an H and a methyl group bonded to it.

When propene undergo addition reaction with HBr, two constitutional isomeric products are possible depending on which carbon in the double bond bonds to the H and which to the Br.

The two products are formed from two separate reaction pathways starting from the same reactants—these two pathways are in competition (competing for the same starting material). Experimentally, 2-bromopropane is observed to be the major product, while 1-bromopropane is the minor product (in fact, negligible amount of it is formed under typically reaction conditions).  In other words, this reaction is regioselective, where one of the possible constitutional isomer products are formed in excess over the other(s).

Under typical conditions, the HX addition to alkene reaction is kinetically controlled, where the product selectivity is dictated by the relative rates of the competing reaction pathways. This is because the addition of Br^– to the carbocation is not reversible under typical conditions, preventing the reaction from reaching equilibrium.

Therefore, the rate of 2-bromopropane formation must be faster than the rate of 1-bromopropane formation in order for it to be the major product. For both reaction pathways, the rate of the overall reaction is the rate of the first step, the RDS:

Rate = k[propene][HBr]

Given that the starting material concentrations are the same in both cases, the difference here lies in the rate constant, k,

k = Ae^−E_a/RT

where A is the pre-exponential factor, E_a is the activation energy, R is the gas constant (8.314 ), and T is the temperature. The competing reactions are occurring in the same reaction vessel, therefore they are under the same temperature. Similarly, starting from the same reactants, the pre-exponential factor would be the same. Hence, the dictating factor here is the difference in the activation energy, and the pathway with the lower E_a will have a faster rate, leading to the major product.

Hammond’s Postulate

Consider the two competing pathways outlined in Figure 1. They have different rates because they differ in their RDS (step 1). And because they start from the same reactants, if we know which transition state 1 structure is lower in energy, we will know which E_a is lower and therefore which pathway would be faster.

However, transition state complexes are highly unstable, existing on maxima on the reaction coordinate diagrams. They have fleeting existences and are extremely difficult to study directly. To gain insight into a particular transition state, we invoke Hammond’s postulate, which states that “If two states, as, for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures.”

In other words, for the HX addition to alkene reaction, the structure and energy of the first transition state can be approximated by the structure and energy of the carbocation intermediate. Therefore, any structural features that can lower the energy of a carbocation intermediate will also lower the energy of the corresponding transition state, thereby speeding up the reaction. If we understand the relative energies of carbocations, we can predict the relative energies of the transition states and, by extension, which pathway is the faster one.

Relative Energy of Carbocations

The sp² carbon atom in a carbocation is electron deficient—it is surrounded by only six valence electrons, which are involved in the three σ bonds to the three groups arranged in a trigonal planar geometry around that carbon. It has an unoccupied p orbital that is perpendicular to the plane created by the σ bonds.

Carbocations can be classified based on the number of alkyl groups attached to the sp² carbon: a tertiary (3°) carbocation has three alkyl groups, a secondary (2°) carbocation has two alkyl groups, and a primary (1°) carbocation has one alkyl group. If no alkyl groups are attached, that is a methyl carbocation.

For an example of relative carbocation stability, consider the following Δ_fH° values of three C₄H₉⁺ carbocation isomers.

n-butyl cation		803 kJ/mol	1° carbocation
sec-butyl cation		769 kJ/mol	2° carbocation
tert-butyl cation		711 kJ/mol	3° carbocation

These values show that more alkyl substituents on the sp² carbon leads to lower energy carbocation, with a tertiary carbocation being lowest in energy. Similar to the alkene isomers, this trend is due to a combination of bond strengths and hyperconjugation.

The bond strength consideration is essentially the same as alkene isomers: a carbocation with more alkyl substituents has more σ_{C(sp³)-C(sp²)} bonds and less σ_{C(sp³)-C(sp³)} bonds, making it lower in energy compared to an isomer with less substituents.

However, unlike the difference in energy between alkene isomers, which are typically <10 kJ/mol per substituent, the relative energies of carbocation isomers differ by ~50 kJ/mol per substituent.  This larger difference is accounted for by hyperconjugation, where the overlap of the filled C-H (or C-C in larger alkyl groups) σ bonding orbitals with the empty p orbital leads to delocalization of the electron densities. As a result, the two electrons that are nominally involved in the C-H σ bond have some of their densities delocalized to the electron-deficient sp² carbon, thereby stabilizing the high energy complex.

Each off-plane C-H bond in an alkyl substituent can overlap with the empty p orbital, and as the number of alkyl substituents increases, the number of σ bonds available for hyperconjugation increases. Hence, the tert-butyl cation in the table above is the lowest energy isomer. If we look at the actual charges in the tert-butyl cation, the central carbon only has a charge of +0.469, while each methyl group has a charge of +0.178. In other words, electron delocalization via hyperconjugation spread out the +1 charge in the carbocation to the alkyl substituents.

[should the sigma bond resonance structure drawing be included here?]

Just as electron delocalization via hyperconjugation can lower the energy of a carbocation, electron delocalization via a π conjugation can also lower the carbocation’s energy. However, we must first consider whether the conjugated π system involves the nominally empty 2p orbital.

Regioselectivity

We can now understand why 2-bromopropane is observed to be the major product in the reaction between propene and HBr. In the two competing pathways depicted in Figure 1, the one leading to the formation of 2-bromopropane involves forming a lower energy 2° carbocation as opposed to a 1° carbocation. Therefore, 2-bromopropane forms much faster than 1-bromopropane, and it is the major product.

This regioselectivity of electrophilic addition of HX to alkenes follows Markovnikov’s rule, proposed by Russian chemist Vladimir Markovnikov in 1869: the H (from HX) attaches to the double-bonded carbon with the fewer alkyl substituents, and the X attaches to the double-bonded carbon with the more alkyl substituents.

Markovnikov’s rule allows us to predict the major product in electrophilic addition of HX to alkene reactions. But to understand why Markovnikov’s rule applies for this class of reactions, we must invoke Hammond’s postulate and understand the relative energies of carbocations.