Page 504 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

+ 32
correlated with
. 2-Methyl-1-butene and 2-methyl-2-butene appear to react via 485
the 2-methylbutyl cation, and 3-methyl-1-butene gives the products expected for a
SECTION 5.3
carbocation mechanism, including rearrangement. These results are consistent with
rate-determining protonation. 33 Addition of Halogens

CF 3 CO 2 H
(CH 3 ) 2 CHCH CH 2 (CH 3 ) 2 CHCHCH 3 + (CH 3 ) 2 CCH 2 CH 3
O 2 CCF 3
O 2 CCF 3
The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of
water is greatly increased by ERG substituents. The reaction of vinyl ethers with water
in acidic solution is an example that has been carefully studied. With these reactants,
the initial addition products are unstable hemiacetals that decompose to a ketone and
alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results
pertain to this step. The mechanistic features are similar to those for hydration of
simple alkenes. Proton transfer is rate determining, as demonstrated by general acid
catalysis and solvent isotope effect data. 34

+ OH
OR' H + OR' OR' H 2 O O
RCH C RCH 2 C + RCH 2 C RCH 2 C OR' RCR"
slow fast
R" RDS R" R" R"

5.3. Addition of Halogens

Alkene chlorinations and brominations are very general reactions, and mecha-
nistic study of these reactions provides additional insight into the electrophilic addition
reactions of alkenes. 35 Most of the studies have involved brominations, but chlori-
nations have also been examined. Much less detail is known about fluorination and
iodination. The order of reactivity is F > Cl > Br > I . The differences between
2 2 2 2
chlorination and bromination indicate the trends for all the halogens, but these differ-
ences are much more pronounced for fluorination and iodination. Fluorination is
strongly exothermic and difficult to control, whereas for iodine the reaction is easily
reversible.
The initial step in bromination is the formation of a complex between the alkene
and Br . The existence of these relatively weak complexes has long been recognized.
2
Their role as intermediates in the addition reaction has been established more recently.

32 A. D. Allen, M. Rosenbaum, N. O. L. Seto, and T. T. Tidwell, J. Org. Chem., 47, 4234 (1982).
33
D. Farcasiu, G. Marino, and C. S. Hsu, J. Org. Chem., 59, 163 (1994).
34 A. J. Kresge and H. J. Chen, J. Am. Chem. Soc., 94, 2818 (1972); A. J. Kresge, D. S. Sagatys, and
H. L. Chen, J. Am. Chem. Soc., 99, 7228 (1977).
35
Reviews: D. P. de la Mare and R. Bolton, in Electrophilic Additions to Unsaturated Systems, 2nd
Edition, Elsevier, New York, 1982, pp. 136–197; G. H. Schmidt and D. G. Garratt, in The Chemistry
of Double Bonded Functional Groups, Supplement A, Part 2, S. Patai, ed., Wiley-Interscience, New
York, 1977, Chap. 9; M.-F. Ruasse, Adv. Phys. Org. Chem., 28, 207 (1993); M.-F. Ruasse, Industrial
Chem. Library, 7, 100 (1995); R. S. Brown, Industrial Chem. Library, 7, 113 (1995); G. Bellucci and
R. Bianchini, Industrial Chem. Library, 7, 128 (1995); R. S. Brown, Acc. Chem. Res., 30, 131 (1997).