Page 390 - Mechanism and Theory in Organic Chemistry

Nucleophilic Addition to Multiple Bonds 377

synthetically useful because, unlike El and E, eliminations, there are no accom-
panying side reactions such as substitution or rearrangement. Both types of
pyrolyses give predominantly Hofmanri elimination. Xanthates decompose at
considerably lower temperatures than the corresponding esters, and therefore
often give a higher yield of olefin and a lower yield of tar. They are conveniently
prepared in situ by the reactions shown in Equation 7.48.
H R H R S
I I I I 1 I CH31
61
R-C-C-OH + CS, + NaOH d R-C-C-0-C-S-Na+ - (7.48)
I I I I
R R R R
Isotope effects and the large negative entropies of activation for the pyrolyses
make it appear probable that the transition states for Reactions 7.46 and 7.47 are
62 and 63, respectively. Substituent effects, however, indicate that the transition
states do have some polar character.

7.3 NUCLEOPHILIC ADDITION TO MULTIPLE BONDS1
When the electron density of a carbon-carbon bond is reduced by strongly
electron-withdrawing substituents, nucleophilic attack at one of the vinylic or
acetylenic carbons may occur. Electron withdrawal may be either by induction
or by resonance. Examples of nucleophilic addition are shown in Equations
CFz=CFz + RSH -
7.49-7.53.
+
-
&H,N(CH3130H
CF
CF2
1 ,-I (7.49)'19
SR H
(1) KCN
ArCH--C(CN), ArCH-CH(CN), (7.50)120
(2) H I
CN
0

OEt

llB (a) S. Patai and Z. Rappoport, in The Chemistty of Alkenes, S. Patai, Ed., Wiley-Interscience, New
York, 1964, p. 464; (b) E. Winterfeldt, Angew. Chem. Znt. Ed., 6, 423 (1967).
119 W. K. R. Musgrave, Quart. Rev. (London), 8, 331 (1954).
120 See note 118(a).
121 J. A. Markisz and J. D. Gettler, Can. J. Chem., 47, 1965 (1969).