Page 318 - Advanced Organic Chemistry Part B - Reactions & Synthesis

290 Several other types of addition reactions of alkenes are also of importance and these
are discussed elsewhere. Nucleophilic additions to electrophilic alkenes are covered
CHAPTER 4 in Section 2.6 and cycloadditions involving concerted mechanisms are encountered in
Electrophilic Additions Sections 6.1 to 6.3. Free radical addition reaction are considered in Chapter 11.
to Carbon-Carbon
Multiple Bonds

4.1. Electrophilic Addition to Alkenes

4.1.1. Addition of Hydrogen Halides

Hydrogen chloride and hydrogen bromide react with alkenes to give addition
products. In early work, it was observed that addition usually takes place to give
the product with the halogen atom attached to the more-substituted carbon of the
double bond. This behavior is sufficiently general that the name Markovnikov’s rule
was given to the statement describing this mode of addition. The term regioselective
is used to describe addition reactions that proceed selectively in one direction with
1
unsymmetrical alkenes. A rudimentary picture of the reaction mechanism indicates
the basis of Markovnikov’s rule. The addition involves either protonation or a partial
transfer of a proton to the double bond. The relative stability of the two possible
carbocations from an unsymmetrical alkene favors formation of the more-substituted
intermediate. Addition is completed when the carbocation reacts with a halide anion.

R
R C CH + HX C CH + X – R CCH
2
2
+ 3 2 3
R
X
Markovnikov’s rule describes a specific case of regioselectivity that is based on the
stabilizing effect of alkyl and aryl substituents on carbocations.
+ + + +
+ CH CH R CH CHR CH CHAr CH CR 2 CH C(Ar) 2
3
2
3
3
3
2
increasing stability
A more complete discussion of the mechanism of addition of hydrogen halides
to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or
not discrete carbocations are involved is considered there. Even when a carbocation is
not involved, the regioselectivity of electrophilic addition is the result of attack of the
electrophile at the more electron-rich carbon of the double bond. Alkyl substituents
increase the electron density of the terminal carbon by hyperconjugation (see Part A,
Section 1.1.8).
Terminal and disubstituted internal alkenes react rather slowly with HCl in
nonpolar solvents. The rate is greatly accelerated in the presence of silica or alumina in
noncoordinating solvents such as dichloromethane or chloroform. Preparatively conve-
nient conditions have been developed in which HCl is generated in situ from SOCl or
2
2
ClCO . These heterogeneous reaction systems also give a Markovnikov orientation.
2
1 A. Hassner, J. Org. Chem., 33, 2684 (1968).
2
P. J. Kropp, K. A. Daus, M. W. Tubergen, K. D. Kepler, V. P. Wilson, S. L. Craig, M. M. Baillargeon,
and G. W. Breton, J. Am. Chem. Soc., 115, 3071 (1993).