Page 497 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

478 Studies aimed at determining mechanistic details of hydrogen halide addition to
alkenes have focused on the kinetics and stereochemistry of the reaction and on the
CHAPTER 5
effect of added nucleophiles. Kinetic studies often reveal rate expressions that indicate
Polar Addition that more than one process contributes to the overall reaction rate. For addition of
and Elimination
Reactions hydrogen bromide or hydrogen chloride to alkenes, an important contribution to the
overall rate is often made by a third-order term.

Rate = k alkene HX 2

Among the cases in which this type of kinetics has been observed are the addition
4
of HCl to 2-methyl-1-butene, 2-methyl-2-butene, 1-methylcyclopentene, and cyclo-
hexene. 5 The addition of HBr to cyclopentene also follows a third-order rate
2
expression. The TS associated with the third-order rate expression involves proton
transfer to the alkene from one hydrogen halide molecule and capture of the halide
ion from the second, and is an example of general mechanism D (Ad 3 . Reaction
E
occurs through a complex formed by the alkene and hydrogen halide with the second
hydrogen halide molecule.

X
H
fast
C C + H – X
C C
X
H –
H X
C C slow C C
H + X
H–X

The stereochemistry of addition of hydrogen halides to unconjugated alkenes
is usually anti. This is true for addition of HBr to 1,2-dimethylcyclohexene, 6
8
7
2
2
cyclohexene, 1,2-dimethylcyclopentene, cyclopentene, Z- and E-2-butene, and
2
3-hexene, among others. Anti stereochemistry is also dominant for addition of
4
9
hydrogen chloride to 1,2-dimethylcyclohexene and 1-methylcyclopentene. Temper-
ature and solvent can modify the stereochemistry, however. For example, although
the addition of HCl to 1,2-dimethylcyclohexene is anti near room temperature, syn

addition dominates at −78 C. 10
Anti stereochemistry is consistent with a mechanism in which the alkene interacts
simultaneously with a proton-donating hydrogen halide and a source of halide ion,
either a second molecule of hydrogen halide or a free halide ion. The anti stereochem-
istry is consistent with the expectation that the attack of halide ion occurs from the
opposite side of the -bond to which the proton is delivered.
4 Y. Pocker, K. D. Stevens, and J. J. Champoux, J. Am. Chem. Soc., 91, 4199 (1969); Y. Pocker and
K. D. Stevens, J. Am. Chem. Soc., 91, 4205 (1969).
5
R. C. Fahey, M. W. Monahan, and C. A. McPherson, J. Am. Chem. Soc., 92, 2810 (1970).
6 G. S. Hammond and T. D. Nevitt, J. Am. Chem. Soc., 76, 4121 (1954).
7
R. C. Fahey and R. A. Smith, J. Am. Chem. Soc., 86, 5035 (1964); R. C. Fahey, C. A. McPherson, and
R. A. Smith, J. Am. Chem. Soc., 96, 4534 (1974).
8
G. S. Hammond and C. H. Collins, J. Am. Chem. Soc., 82, 4323 (1960).
9 R. C. Fahey and C. A. McPherson, J. Am. Chem. Soc., 93, 1445 (1971).
10
K. B. Becker and C. A. Grob, Synthesis, 789 (1973).