Page 358 - Mechanism and Theory in Organic Chemistry

Electrophilic Addition to Double and Triple Bonds 345

as shown in 12.23 This conclusion is consistent with the universality of Markowni-
koff addition in hydrochlorinations.
With a variety of different mechanisms available, it is not surprising that the
characteristics of hydrochlorination depend on the reaction conditions. Thus, in
nitromethane, a medium that gives extensive Wagner-Meerwein shifts during
hydrochlorination, olefins react according to the third-order rate law,
k = [olefin] [HC1I2
The fact that rearrangements occur implicates a carbocation intermediate. When
the reaction is carried out with DC1, olefin recovered after several half-lives con-
tains no deuterium. Thus, formationofthe intermediate must be rate-determining.
Apparently, the role of the second HC1 molecule is to assist the first in ionizing,
and the HC1,- anion is produced as shown in Equation 7.9. Predominant anti
addition is observed-presumably because a third HC1 attacks from the back
side in a second fast step.24

The characteristics of the addition of HBr to double bonds are similar to
those of the addition of HCl. However, in acetic acid 1,2-dimethylcyclohexene
gives more anti addition if HBr is the addend.25 Also, as Figure 7.1 shows,
when HX is added to a double bond in acetic acid, the ratio of alkyl halide
to alkyl acetate increases sharply as the concentration of HBr is increased
but is almost independent of the concentration of HC1. Fahey suggests that
the much larger acid dissociation constant of HBr (AK, = lo3-lo4) is respon-
sible for both of these facts. Hydrobromic acid acts as a better halide source,
and Ad3 addition is favored.26
Electrophilic addition of HC1 to triple bonds can apparently also go by bi-
or termolecular mechanisms. Thus in acetic acid 3-hexyne (14) gives predom-
inantly anti addition through an Ad3 pathway, but 1-phenylpropyne (15), which
can form the resonance-stabilized vinyl cation (16), gives predominantly syn
addition through an ion pair Ad,2 mechanism.27

Addition of Halogens
Bromination of double bonds is strongly accelerated by-electron-releasingub-
(see
s a anci -rrcTeX by ..elec~~n~.withd.rawi~ng_pne_s~ Tables 7.4 and 7.7)'
$ndiEIG&iiarly an electrophilic addition. Thcat_e_of~eaca~ays

a3 R. C. Fahey and C. A. McPherson, J. Amer. Chem. Soc., 93, 2445 (1971).
24 Y. Pocker and K. D. Stevens, J. Amer. Chem. Soc., 91, 4205 (1969).
as R. C. Fahey and R. A. Smith, J. Amer. Chem. Soc., 86, 5035 (1964).
R. C. Fahey, C. A. McPherson, and R. A. Smith, J. Amer. Chem. Soc., 96, 4534 (1974); see also
D. J. Pasto, G. R. Meyer, and B. Lepeska, J. Amer. Chem. Soc., 96, 1858 (1974).
R. C. Fahey and D.-J. Lee, J. Amer. Chem. Soc., 88, 5555 (1966); 89,2780 (1967); 90,2124 (1968).