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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.
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