Reactions with small Hammett ρ values

Small Hammett ρ values arise in three ways. The aromatic ring being used as a probe for the mechanism may simply be too far away for the result to be significant. This trivial case of the alkaline hydrolysis of the 3-aryl propionate ester has a ρ value of + 0.5 and it is surprising that it is even that large.

The second case is the informative one where the reaction is not dependent on electrons flowing into or out of the ring. Pericyclic reactions are important examples and the Diels Alder reaction of arylbutadienes with maleic anhydride shows a small negative ρ value of –0.6. The small value is consistent with a mechanism not involving charge accumulation or dispersal, but the sign is interesting. We explained this type of Diels–Alder reaction in Chapter 34 by using the HOMO of the diene and the LUMO of the dienophile. The negative sign of ρ, small though it is, supports this view because the reaction is somewhat faster with electron donating groups on Ar, which raise the energy of the HOMO of the diene.

The third case is in many ways the most interesting. We have seen that the alkaline hydrolysis of ethyl esters of benzoic acids (ArCO2Et) has a ρ value of +2.6 and that this is a reasonable value for a reaction involving nucleophilic attack on a carbonyl group conju gated with the aromatic ring. The hydrolysis of the same esters in acid solution, which also involves nucleophilic attack on the same carbonyl group, has a ρ value of +0.1. In other words, substituted benzoic esters hydrolyse at more or less the same rate in acid solution, irrespective of their substituents. We need to look at the full mechanism to explain this remarkable result.

Steps 1, 3, and 5 cannot be slow as they are just proton transfers between oxygen atoms, and proton transfer between electronegative atoms is always fast. That leaves only steps 2 and 4 as possible rate-determining steps. The bimolecular addition of the weak nucleophile water to the low concentration of protonated ester (step 2) is the most attractive candidate, as step 4—the unimolecular loss of ethanol and re-formation of the carbonyl group—should be fast. What ρ value would be expected for the reaction if step 2 were the rate-determining step? It would be made up of two parts. There would be an equilibrium ρ value for the protonation step and a reaction ρ value for the addition of water. Step 1 involves electrons flowing out of the molecule and step 2 involves electrons flowing in so the ρ values for these two steps would

have opposite charges. We know that the ρ value for step 2 would be about +2.5 (it’s just like the step in the ester hydrolysis) and a value of about –2.5 for the equilibrium protonation is reasonable. This is indeed the explanation: step 2 is the rate-determining step and the ρ values for steps 1 and 2 almost cancel each other out. All steps before the rate-determining step are present in the rate equation and also affect the Hammett ρ value.

You should not, of course, learn the numbers in this scheme, but you need an idea of roughly what each group of values means. You should see now why it is unimportant whether the Hammett correlation gives a good straight line or not. We just want to know whether ρ is + or – and whether it is, say, 3 or 6. It is meaningless to debate the significance of a ρ value of 3.4 as distinct from one of 3.8.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مركز الثقافة الأسرية يستعد لتقديم خدماته الإرشادية للزائرات في زيارة الأربعين

مجمّع الشيخ الكليني ينهي استعداداته الخاصة بزيارة الأربعين

مجموعة مواكب أمّ البنين (عليها السلام) تباشر بتقديم خدماتها لزائري الأربعين

جامعة الكفيل تقيم موكبًا خدميًّا لزائري الأربعين في النجف

المزيد

الآخبار الصحية

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المزيد

الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with negative Hammett ρ values

Reactions with positive Hammett ρ values

Equilibria with positive Hammett ρ values

The Hammett substituent constant σσ A

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product