Page 354 - Mechanism and Theory in Organic Chemistry

Electrophilic Addition to Double and Triple Bonds 341

Alkynes ge-rgo acid-catalyzed hvdration to form vinyl alc~&ols,
-.snge tp_ ketones, These hydrations exhibit general acid
catalysis,12 and unreacted acetylenes recovered after partial reaction have not
exchanged deuterium with the solvent.13 Noyce and Schiavelli have found that
the rate of hydration of ring-substituted phenylacetylenes is very dependent on
the nature of the substituent, giving a linear correlation with o+ (p = - 3.84).14
Thus all the evidence points to the transient slow formation of the unstable vinyl
cation in a mechanism entirely analogous to that for hydration of alkenes as
shown in Equation 7.4. As Table 7.2 shows, the rate of hydration of alkynes is also
slow
~@CR + H30+ + RC=C-R + Hz0 +
I
+
H

comparable to that of alkenes.15 his is most surprising in view of the much greater
stability of a tric~ordinated as opposed to a vinyl carbocation.

Addition of Hydrohalides
The addition of HX to double bonds in the dark and in the absence of free-radical
initiators is closely related to hydration: The orientation of the elements of HX i~
$he _adductws to MarluumkdT additinn;16 no deuterium
exchange wjbh solvent is found in unreacted olefins recovered after partial reac-
tion, nor is recovered starting material isomerized after partial reaction.17 How-
ever, the addition of HX apparently can proceed b2.a number of different
--
mechsnisms depending on the niure oi the substrate and on M-
&Thus when HC1 is added to t-butylethylene in acetic acid, the rate is first-
order in each reactant and the products are those shown in Equation 7.5.18
Since 4 and 6 were demonstrated to be stable to the reaction conditions, the
rearranged product (5) can be formed only if a carbocationic intermediate is
formed during reaction. However, the carbocation exists almost solely in an inti-
mate ion pair, and the rate of collapse of the ion pair to products must be faster
than, or comparable to, the rate of diffusion of C1- away from the carbocation.
This must be so because the ratio of chloride to acetate products is unaffected by

l2 (a) W. Drenth and H. Hogeveen, Rec. Trav. Chim., 79, 1002 (1960); (b) E. ,J. Stamhuis and
W. Drenth, Rec. Trav. Chim., 80, 797 (1961); (c) E. J. Stamhuis and W. Drenth, Rec. Trav. Chim., 82,
394 (1963); (d) H. Hogeveen and W. Drenth, Rec. Trav. Chim., 82, 410 (1963); (e) D. S. Noyce and
M. D. Schiavelli, J. Amer. Chem. Soc., 90, 1020 (1968).
l3 See note 12(d).
l4 See note 12(e).
l6 (a) Note 12(a); (b) D. S. Noyce, M. A. Matesich, M. D. Schiavelli, and P. E. Peterson, J. Amer.
Chem. Soc., 87,2295 (1965); (c) K. Yates, G. H. Schmid, T. W. Regulski, D. G. Garratt, H.-W. Leung,
and R. McDonald, J. Amer. Chem. Soc., 95, 160 (1973).
l6 See note 1 (a), p. 337.
IT Y. Pocker, K. D. Stevens, and J. J. Champoux, J. Amer. Chem. Soc., 91, 4199 (1969).
18 (a) R. C. Fahey and C. A. McPherson, J. Amer. Chem. Soc., 91, 3865 (1969); (b) Rearranged
acetate corresponding to 5 is not stable to the reaction conditions but reacts with C1- to form 5.