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Table 4.9 EFFECT OF ELECTRON-DONATING -WITHDRAWING SUBSTITUENTS
AND
ON THE RATE OF DISPLACEMENT REACTIONS CARBON AND SULFUR
ON
a Data from M. A. Sabol and K. K. Andersen, J. Amer. Chem. Soc., 91, 3603 (1969).
Data from R. Fuchs and D. M. Carletan, J. Amer. Chem. Soc., 85, 104 (1963).
Reproduced by permission of the American Chemical Society.
Table 4.10 COMPARISON THE RATES OF SN2 REACTIONS SULFUR AND AT CARBON
AT
OF
Relative Rates
R R-S-SO3 - + SOZ2 - R-S-SR + - SR' RSCN+ C4HgNH2 RCH2X + Y -
CH3 100 100
C2H, 50 40
n-C4H9 40
i-C3H7 0.7 1 .O 3.0
t-C,Hg 0.0006 0.00015 0.000125 0.001 1
SOURCE: Data from E. Ciuffarin and A. Fava, Prog. Phys. Or~g. Chem., 6, 81 (1968). Reproduced by
permission of Wiley-Tnterscience.
Even in a one-step S,2 displacement, sulfur might use one of its empty d
orbitals to accept the incoming pair of electrons at the same time as the leaving
group begins to break away. If this did occur, backside displacement, which
occurs in the S,2 displacement on carbon because of the stereoelectronic require-
ments of the transition state, would not be required, and the nucleophile could
enter forming a very small angle with the leaving group. Evidence against even
this kind of participation of the d orbitals comes from. the decrease in rates for
Reactions 4.30-4.32 as the substrates are increasingly substituted with alkyl
groups. As Table 4.10 shows, the decreases in rate due to increasing steric
* *
R-S-SO3- + S032- + R-S-SO3- + S032- (4.30)
R-S-S-R + R'S- ----t R-S-S-R' + RS- (4.31)
R-S-CN + C4HgNH2 ----t R-S-NHC4Hg + HCN (4.32)
requirements of the central sulfur run parallel to those of analogous substi-
tutions on carbon.60 The similarity in relative rates probably arises from a
similarity in transition states-that is, bimolecular displacement at divalent
sulfur, as at tetrahedral carbon, occurs from the back side (8).
For similar data for other displacements on dicoordinated sulfur, see: (a) C. Brown and D. R.
Hogg, Chem. Commun., 38 (1967); (b) E. Ciuffarin and A. Fava, Prog. Phys. Org. Chem., 6, 81 (1968),
p. 86.