Reactions with positive Hammett ρ values
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
الجزء والصفحة:
ص1044-1045
2025-08-04
475
Reactions with positive Hammett ρ values
The size and sign of the value of ρ tell us about what is happening in the rate-determining step of a reaction. Any reaction that involves nucleophilic attack on a carbonyl group as the rate-determining step is going to have a ρ value of about 2–3, the same as for the hydrolysis of esters, as we have already seen. Large positive ρ values usually indicate extra electrons in the transition state delocalized into the ring itself. A classic example is nucleophilic aromatic substitution by the addition–elimination mechanism. The ρ value is +4.9, but even this large value does not mean a complete anion on the benzene ring as the nitro group, present in all cases, takes most of the negative charge. The substituent X merely helps.
We get the full value when there are no nitro groups to take the brunt of the negative charge. This vinylic substitution has a ρ value of +9.0. It cannot be an SN2 reaction or it would have a small ρ value and it cannot be an SN1 reaction or it would have a negative ρ value (fewer electrons in the transition state). It must be an addition–elimination mechanism through a benzylic anion delocalized round both benzene rings.
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