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Substituent Effects on Strengths of Brransted Acids and Bases  161

      Table 3.16  ENTHALPY AND  ENTROPY CONTRIBUT~ONS RELATIVE FREE
                                                TO
                ENERGIES SOLUTION AMINES
                       OF
                                  OF
      Amine                     SAHs(B)             - S TAS,(B)




      SOURCE: Reprinted with  permission from  E.  M.  Arnett, F.  M. Jones,  111,  M. Taagepera,  W.  G.
      Henderson, J. L. Beauchamp, D. Holtz, and R. W. Taft, J. Amer. Chem. Soc.,  94,4724 (1972). Copy-
      right by the American Chemical Society. Values in kcal mole-= at 25°C. See text for explanation of
      symbols.
           Table  3.15  dissects  the  important  GAG,(BHf)  term  into  enthalpy  and
      entropy  contributions.  Increasing  substitution  on  BHf  makes  both  of  these
      quantities less favorable in the gas -+ solution direction. For the bases themselves,
      on the other hand, the solution enthalpies and entropies are in opposition (Table
      3.16), enthalpies  being  more favorable to  the solution  process the more substi-
      tuents,  but  entropies  becoming  less  favorable  with  more  substituents.  The
      cancelation of these opposing effects leaves the  small  GAG,(B)  values shown in
      Table 3.14.  As  we have noted  in  Section 2.4,  these results are not easily inter-
      preted at the molecular level, although the most important effect, that on solution
      free energy of BHf , is undoubtedly caused by decreasing opportunity for hydro-
      gen bonding as hydrogens are replaced by alkyl groups.97

      Oxygen and Sulfur Bases
      The oxygen and sulfur bases are weaker  than the nitrogen bases, and accurate
      solution basicities are not available. Arnett's  heat of protonation studies indicate
      that the order of decreasing basicity is ROR > ROH > H,0,98 a result that is
      in agreement with gas-phase  measurement^.^^ Hydrogen sulfide in the gas phase
      has basicity comparable to that of water  (Table 3.18), and substitution of H  by
      alkyl produces stronger gas-phase bases just as does similar substitution on oxygen.
           Comparisons among the alcohols are difficult to make in solution. Titration
      in acetic acid indicates an order of basicity isopropyl alcohol  > ethyl  > methyl,
      but water was found to be a stronger base than any of these alcohols,100 a result
      in  disagreement  with  the  gas-phase  data.  In  the  gas  phase  (Table 3.18),  the
      basicity order  (strotgesths~ tn &-a>
      CH,CH,OH  > CH,OH  > -H20. Againeore and larger aljddgmqssxm to
      uize charge.
           Substitution  by  an  aromatic group  has  a  marked  effect  on solution  base
      strength. One might be tempted  to attribute the low basicity of aniline, di- and
      triphenylamines, and phenol compared with reference compounds  (Table 3.17)
      to partial delocalization of the nonbonded electron pair on the nitrogen or oxygen
      into the  err  orbital system of the ring. But gas-phase results indicate the basicity


      97 (a) See note 94  (a), p.  159; (b) R. W. Taft,  M. Taagepera, K. D. Summerhays, and J. Mitsky,
      J. Amer. Chem. Soc.,  95, 381 1  (1973).
        E. M. Arnett, R. P. Quirk, and J. J. Burke, J. Amer. Chem. Soc.,  92, 1260 (1970).
      99 M. S. B. Munson, J. Amer. Chem. Soc.,  87, 2332  (1965).
      loo I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc.,  78, 1  (1956).
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