Page 320 - Advanced Organic Chemistry Part B - Reactions & Synthesis

292 1,2-Dimethylcyclohexene is an example of an alkene for which the stereochemistry of

hydrogen chloride addition is dependent on the solvent and temperature. At −78 C
CHAPTER 4 in dichloromethane, 88% of the product is the result of syn addition, whereas at 0 C

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Electrophilic Additions in ether, 95% of the product results from anti addition. Syn addition is particularly
to Carbon-Carbon
Multiple Bonds common with alkenes having an aryl substituent. Table 4.1 lists several alkenes for
which the stereochemistry of addition of hydrogen chloride or hydrogen bromide has
been studied.
The stereochemistry of addition depends on the details of the mechanism. The
addition can proceed through an ion pair intermediate formed by an initial protonation
step. Most alkenes, however, react via a complex that involves the alkene, hydrogen
halide, and a third species that delivers the nucleophilic halide. This termolecular
mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide
complex. This mechanism bypasses a discrete carbocation and exhibits a preference
for anti addition.

Cl
H
C C

Nu:
The major factor in determining which mechanism is followed is the stability of the
carbocation intermediate. Alkenes that can give rise to a particularly stable carbocation

Table 4.1. Stereochemistry of Addition of Hydrogen Halides to Alkenes
Alkene Hydrogen halide Stereochemistry

1,2-Dimethylcyclohexene a HBr anti
1,2-Dimethylcyclohexene a HCl Solvent and temperature dependent
Cyclohexene b HBr anti
Z-2-Butene c DBr anti
E-2-Butene c DBr anti
1-Methylcyclopentene d HCl anti
1,2-Dimethylcyclopentene e HBr anti
Norbornene f HBr syn and rearrangement
Norbornene g HCl syn and rearrangement
E-1-Phenylpropene h HBr syn (9:1)
Z-1-Phenylpropene h HBr syn (8:1)
Bicyclo[3.1.0]hex-2-ene i DCl syn
1-Phenyl-4-(t-butyl)cyclohexene j DCl syn
a. G. S. Hammond and T. D. Nevitt, J. Am. Chem. Soc., 76, 4121 (1954); R. C. Fahey and C. A. McPherson,
J. Am. Chem. Soc., 93, 2445 (1971); K. B. Becker and C. A. Grob, Synthesis, 789 (1973).
b. R. C. Fahey and R. A. Smith, J. Am. Chem. Soc., 86, 5035 (1964).
c. D. J. Pasto, G. R. Meyer, and B. Lepeska, J. Am. Chem. Soc., 96, 1858 (1974).
d. Y. Pocker and K. D. Stevens, J. Am. Chem. Soc., 91, 4205 (1969).
e. G. S. Hammond and C. H. Collins, J. Am. Chem. Soc., 82, 4323 (1960).
f. H. Kwart and J. L. Nyce, J. Am. Chem. Soc., 86, 2601 (1964).
g. J. K. Stille, F. M. Sonnenberg, and T. H. Kinstle, J. Am. Chem. Soc., 88, 4922 (1966).
h. M. J. S. Dewar and R. C. Fahey, J. Am. Chem. Soc., 85, 3645 (1963).
i. P. K. Freeman, F. A. Raymond, and M. F. Grostic, J. Org. Chem., 32, 24 (1967).
j. K. D. Berlin, R. O. Lyerla, D. E. Gibbs, and J. P. Devlin, J. Chem. Soc., Chem. Commun., 1246 (1970).
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K. B. Becker and C. A. Grob, Synthesis, 789 (1973).