Level 2 – Case 17
118
The values of the enthalpy of activation ('H
‡ ) complement the trends marked
by the entropy of activation and reflect the energy changes between reactants and
activated complex ascribable to the breaking and reforming of bonds. Breaking
old bonds requires energy and forming new bonds releases energy. Low values of
'H
‡ (lower than 100 kJ mol
–1 ) are estimated for reactions in which two species are
involved (bimolecular processes). All 'H
‡ values in Table 17.1 are lower than 100
kJ mol
–1 , as expected for an overall second-order reaction, and are particularly low
in the case of the less basic amines 3 and 4 (46 and 43 kJ mol
–1
, respectively)
which supports the arguments in favor of an (E1cB) irr mechanism in these cases.
r
,QQ6XPPDU\\
The aminolysis of 4-nitrophenyl-N-benzylsulfamate (NPBS) follows a base-catalyzed E1cB mechanism. However, the rate-determining step of the reaction
changes depending on the pK a of the amine employed. At lower amine pK a values,
an (E1cB) irr reaction takes place and the removal of the proton by the base is the
rate-determining step of the reaction. At higher pK a values, the removal of the proton is fast and the departure of 4-nitrophenol is the slow step of the reaction
(E1cB R mechanism). The change in the rate-determining step occurs at approximately the point where the substrate pK a is equal to the pK a of the catalytic amine.
$GGLWLRQDOO&RPPHQWVV
This problem is based on the work by Spillane WJ, McGrath P, Brack C, O´Byrne
AB (2001) J. Org. Chem. 66:6313-6316.
6XEMHFWVVRII5HYLVLRQQ
Brønsted catalysis law. Brønsted coefficients. Activation parameters: physical significance.
118
The values of the enthalpy of activation ('H
‡ ) complement the trends marked
by the entropy of activation and reflect the energy changes between reactants and
activated complex ascribable to the breaking and reforming of bonds. Breaking
old bonds requires energy and forming new bonds releases energy. Low values of
'H
‡ (lower than 100 kJ mol
–1 ) are estimated for reactions in which two species are
involved (bimolecular processes). All 'H
‡ values in Table 17.1 are lower than 100
kJ mol
–1 , as expected for an overall second-order reaction, and are particularly low
in the case of the less basic amines 3 and 4 (46 and 43 kJ mol
–1
, respectively)
which supports the arguments in favor of an (E1cB) irr mechanism in these cases.
r
,QQ6XPPDU\\
The aminolysis of 4-nitrophenyl-N-benzylsulfamate (NPBS) follows a base-catalyzed E1cB mechanism. However, the rate-determining step of the reaction
changes depending on the pK a of the amine employed. At lower amine pK a values,
an (E1cB) irr reaction takes place and the removal of the proton by the base is the
rate-determining step of the reaction. At higher pK a values, the removal of the proton is fast and the departure of 4-nitrophenol is the slow step of the reaction
(E1cB R mechanism). The change in the rate-determining step occurs at approximately the point where the substrate pK a is equal to the pK a of the catalytic amine.
$GGLWLRQDOO&RPPHQWVV
This problem is based on the work by Spillane WJ, McGrath P, Brack C, O´Byrne
AB (2001) J. Org. Chem. 66:6313-6316.
6XEMHFWVVRII5HYLVLRQQ
Brønsted catalysis law. Brønsted coefficients. Activation parameters: physical significance.