Page 356 - Mechanism and Theory in Organic Chemistry

Electrophilic Addition to Double and Triple Bonds 343

tetramethylammonium chloride (TMAC). Furthermore, if the stereochemistry
of the reaction is studied by using cyclohexene-1,3,3-d, as substrate, the five pro-
ducts shown in Equation 7.7 are always found but their relative amounts depend

HOAc
D H H
7 8

11 X = OAc

strongly on the chloride ion concentration. Thus the ratio of cyclohexyl chloride
derived from syn addition (71, relative to-<
decreases d y with chloride ion con~entrati~~hm ratio of the sysyn-
the
-----
11
ride adduct (Lt_o-.~L~anti-fnrmed acetate &ct (9) remaiwn-
chxd. (Note that 10 and 11 tell us nothing about the stereochemistry of addi-
6on but. since the amount of 10 formed by svn addition relative to that formed by
, ,
anti addition should be equal to the ratio of 7 : 8, we can focus our discussion on
7,8, and 9 alone.) No acetate formed by syn addition is found.20
- --
Analysis of the rate and product data &o~t5~~eequation
is
com-
posed of three terms (Equation 7.8) ; each includes the concentrations of olefin and
of acid, but one also includes the concentration of chloride ion and another the
concentration of acetic acid.21
rate = k, [HCl] [olefin] + k,[HCl] [olefin] [Cl-] + k3[HC1] [olefin] [HOAc] (7.8)
Fahey has suggested that all the facts are consistent with the products being
formed by three competing reactions. The first, responsible for the second-order
rate term, is an ion-pair mechanism like that found in the hydrochlorination of
t-butylethylene. Such a pathway, which involves collapse of the ion pair (see
above), accountsh._theqtz--t and some of the anti-HC1 adduct. a he-
second
and
sec_~,~n~a~d~-~ns,~far~hr: in the rag
terms
third
ao R. C. Fahey, M. W. Monahan, and C. A. McPherson, J. Amer. Chem. Soc., 92, 2810 (1970);
R. C. Fahey and M. W. Monahan, J. Amer. Chem. SOC., 92, 2816 (1970).
This is actually a simplified form of the rate equation for Reaction 7.6. Fahey found that it is not
the concentration of HCl, but its activity as represented by Satchell's acidity function, A [D. P. N.
Satchell, J. Chem. SOC., 1916 (1958)], that should be included in each term of the rate equation. See
note 20.