Question 7.FP.6: In a family of reactions A + B → C in which cycloaddition oc......
In a family of reactions
A + B → C
in which cycloaddition occurs, some values of \Delta\,V^{\neq *} and \Delta\,V^{\theta} are as follows:
\mathrm{Bu~is ~CH_{3}C H_{2}C H_{2}C H_{2}}.Comment on these values, and give the values of \Delta\,V^{\neq{*}} for the reverse reactions. Predict the trend in {\Delta{S}^{\neq*}}.
| A | B | Reaction medium |
\frac{\Delta\,V^{\neq*}}{\mathrm{cm}^{3}\,\mathrm{mol}^{-1}} | \frac{\Delta\,V^{\theta}}{\mathrm{cm^{3}\,m o l^{-1}}} |
| {\mathrm{CH_{2}}}={\mathrm{CH}}-{\mathrm{C}}({\mathrm{CH_{3}}})={\mathrm{CH_{2}}} | {\mathrm{CH_{2}}}={\mathrm{CHCOOCH}_{3}} | BuBr (21 °C) | -30.8 | -36.9 |
| {\mathrm{CH_{2}}}={\mathrm{C}}({\mathrm{CH_{3}}})-{\mathrm{C}}({\mathrm{CH_{3}}})={\mathrm{CH_{3}}} | {\mathrm{CH_{2}}}={\mathrm{CHCOOCH}_{3}} | BuBr (40 °C) | -30.2 | -37.0 |
| {\mathrm{CH}}_{2}{=}C({\mathrm{CH}}_{3}){-}{\mathrm{C}}({\mathrm{CH}}_{3}){=}{\mathrm{CH}}_{3} | CH_{3}OOC- C(H)=C(H)COOCH_{3} (trans) |
BuBr (40 °C) | -32.9 | -37.2 |
| {\mathrm{CH}}_{2}{=}C({\mathrm{CH}}_{3}){-}{\mathrm{C}}({\mathrm{CH}}_{3}){=}{\mathrm{CH}}_{3} |  |
BuBr (30 °C) | -41.3 | -36.3 |
| {\mathrm{CH}}_{2}{=}{\mathrm{CH}}-{\mathrm{C}}({\mathrm{CH}}_{3})={\mathrm{CH}}_{2} |  |
ClCH_{2}CH_{2}Cl (35 °C) |
-37 | -35.5 |
For reactions 1–3, \Delta\,V^{\neq{*}} and \Delta\,V^{\theta} are very similar with \Delta\,V^{\neq{*}} only slightly less negative than \Delta\,V^{\theta}. This suggests that the activated complex is very similar to the products, but not quite as compact.
For reactions 4 and 5, \Delta\,V^{\neq{*}} is more negative than \Delta\,V^{\theta}, which suggests an activated complex which resembles the product, but is even more compact than the product. Such possibilities complicate possible interpretations of values of \Delta\,V^{\neq{*}} . For the reverse reactions
\Delta\,V_{b}^{\neq\,*}\,\mathrm{values~are}+6.1,\,+6.8,\,+4.3,\,-5.0\,~\mathrm{and}\,-1.5\,\mathrm{cm^{3}\,m o l}^{-1},\,\mathrm{and}\,.
\Delta\,V_{b}^{\theta}\,\mathrm{values}\,\,\mathrm{are}+36.9,\,+37.0,\,+37.2+35.5\,\mathrm{cm}^{3}\,\mathrm{mol}^{-1}.
None of the \Delta\:V_{b}^{\neq{*}} values is very large and all are very different from the corresponding \Delta{{V}}_{b}^{\theta} values. The reverse reactions are 1 → 2, and so the \Delta\:V_{b}^{\neq{*}} values would be expected to be positive and fairly large. This is not the case, which suggests that the activated complexes are fairly similar to the reactant C.
A similar trend in \Delta\,S^{\neq{*}} values for both forward and back reactions would be expected, though the \Delta\,S^{\neq{*}} values are complicated by the fact that the complexity of the reactants must be considered.