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Addition of Hydrogen Halides to Alkenes

Substituent Effects on the Electrophilic Addition to Alkenes

Ethylene is protonated very slowly resulting in an unstable primary carbenium ion.

Substituent effects
In addition reactions, substituents at the double bond can effect the reaction rate as well as the regioselectivity of a reaction.

The secondary carbenium ion formed by the protonation of propylene, however, is stabilized by hyperconjugation and the +I effect of the methyl group. By partially donating electron density to the empty p orbital of the sp2 carbon atom, the methyl group shows a positive inductive effect and distributes the positive charge over a larger space. Therefore, the reactivity of alkenes in electrophilic addition increases with the degree of substitution.

Positive inductive effect
Relative reaction rates of electrophilic addtion at 25 °C

Hydrogen halide addition to propylene proceeds 2 x 106 times faster than the addition to ethylene. Addition to methyl vinyl ether is even 5 x 1014 times faster.

One could expect the methoxy group (OCH3-) at the double bond of methyl vinyl ether to reduce the reaction rate by destabilizing the intermediate carbenium ion due to its negative inductive effect. But the positive mesomeric effect caused by the free electron pair at the oxygen atom supersedes the negative inductive effect resulting in an overall stabilization of the carbenium ion by the methoxy group.

Additionally, the transition state of the carbenium ion is stabilized according to the Hammond postulate. The activation energy to reach the transition state is reduced by the methoxy group and the reaction rate is increased because the formation of the carbenium ion is the rate-determining step of the electrophilic addition. Because of the additional stabilizing mesomeric effect of the methoxy group, HX addition to methyl vinyl ether is the fastest of the three reactions shown in Fig. 2.

Resonance stabilization (mesomeric effect)
Substituents with +M effect
The reaction rate of electrophilic addition to alkenes is increased by substituents at the double bond which show a positive mesomeric effect (+M effect).

This is in contrast to the behavior of halomethyl-substituted alkenes, which show a noticeably reduced reaction rate in electrophilic additions because the strong negative inductive effect of halogens results in a higher partial positive charge of the vicinal CH2 group.

Effect of negative inductive effects on reaction rates
R = H Br Cl
Relative reaction rate 1 10 -4 10 -4

Protonation of the double bond results in an energetically unfavored carbenium ion in direct vicinity to a δ+ charged CH2 group. In this case, the -I effect of the halomethyl group is not offset by a positive mesomeric effect as is the case for methyl vinyl ether, for example. Consequently, the intermediate carbenium ion is richer in energy resulting in higher activation energy of the rate-determining step.

Substituent effects and the reaction rate of additions
To summarize:
The reaction rate of electrophilic additions to alkenes is increased by substituents which show a positive mesomeric effect (+M effect) or a positive inductive effect (+I effect).
The reaction rate of electrophilic additions to alkenes is reduced by substituents which show a negative mesomeric effect (-M effect) or a negative inductive effect (-I effect).
Mesomeric and inductive effects of different as well as same substituents at a double bond can work against each other. The mesomeric effect always dominates an inductive effect.

Exercise 1

Exercise 2

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