Reactions with negative Hammett ρ values
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص1045-1046
2025-08-04
470
Reactions with negative Hammett ρ values
Negative ρ values mean electrons flowing away from the ring. A representative example is the SN2 displacement of iodide from EtI by phenoxide anions. This has a ρ value of exactly –1.0. Although the transition state has a negative charge, that charge is decreasing on the aromatic ring as the starting material approaches the transition state.
An SN1 reaction on the carbon atom next to the ring has a large negative ρ value. In this example, a tertiary benzylic cation is the intermediate and the rate-determining step is, of course, the formation of the cation. The cation is next to the ring but delocalized round it and the ρ value is –4.5, about the same value, though negative, as that for the nucleophilic substitution on nitrobenzenes by the addition–elimination mechanism that we saw in the last section.
The largest negative ρ values come from electrophilic aromatic substitution, where the electrons of the ring are used in the reaction, leaving a positive charge on the ring itself in the intermediate. Some of this charge is already there in the transition state. This simple nitration has ρ = –6.4 and ρ values for electrophilic aromatic substitution are usually in the range –5 to –9. Negative ρ values mean electrons flowing out of the ring.
الاكثر قراءة في مواضيع عامة في الكيمياء العضوية
اخر الاخبار
اخبار العتبة العباسية المقدسة
