Non-linear Hammett plots

The Hammett plot for hydrolysis of the acid chlorides of benzoic acids in aqueous acetone is very odd indeed. Hammett plots need not be perfectly linear to give useful information, but this one is clearly made up of two intersecting straight lines. This might look like disaster at first but, in fact, it tells us something rather important. The right-hand part of the curve, where the more electron-withdrawing substituents lie, has a slope of +2.5: just what we should expect for rate-determining attack of water on the carbonyl group. As we go to less electron-withdrawing substituents, the rate of the reaction suddenly starts to increase as we pass the para-chloro compound and the left-hand part of the curve has a slope of –4.4. What can this mean? If the reaction becomes faster as we pass the discontinuity in the curve—and it gets faster whether we go from right to left or left to right—there must be a change in mechanism. If there is a choice between two mechanisms, the faster of the two will operate. Mechanism 1 is the rate-determining nucleophilic attack by water on the carbonyl group.

The new mechanism goes faster for more electron-donating substitu ents and has quite a large negative ρ value, suggesting the formation of a cation in the rate-determining step. This mechanism (mechanism 2) must surely be the SN1-like preliminary formation of an acylium ion by loss of chloride ion.

When the Hammett plot bends the other way, so that the rate of the reaction decreases as it passes the discontinuity, we have a single mechanism with a change in rate-determining step. A reaction goes by the fastest possible mechanism but its rate is limited by the slowest of the steps in that mechanism. An example is the intramolecular Friedel–Crafts alkylation of a diphenyl derivative where the alkylating agent is a diarylmethanol attached to one of the benzene rings in the ortho position.

The carbocation intermediate in the Friedel–Crafts reaction is rather stable, being tertiary and benzylic, and the formation of the cation, normally the rate-determining step, with inevitably a negative ρ value, goes faster and faster as the electron-donating power of the substituents increases until it is faster than the cyclization, which becomes the rate-determining step. The cyclization puts electrons back into the carbocation and has a positive ρ value. As the two steps have more or less the reverse electron flow to and from the same carbon atom, it is reasonable for the size of ρ to be about the same but of opposite sign.

● A reaction occurs by the faster of two possible mechanisms but by the slower of two possible rate-determining steps.

We shall see more examples of Hammett ρ values used in conjunction with other evidence as the chapter develops but now it is time to look at what other evidence is available.

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

تسارع الشيخوخة.. متى يبدأ وأي الأعضاء أكثر تأثرا؟

9 مسببات شائعة للالتهاب المزمن قد تهدد صحتك

باحثون يطورون علاجا لهشاشة العظام.. النتائج مذهلة

تحمي من أمراض عديدة.. 6 فوائد لتناول البطاطا الحلوة

المزيد

الآخبار التكنلوجية

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المعلمون في خطر.. خاصية جديدة في "تشات جي بي تي" تشرح الدروس

بشرى لمرضى الشلل.. شريحة في الدماغ قد تتيح التحكم بالأدوات

مدينة غارقة.. هل يخفي قاع البحر الكاريبي سرّ حضارة مفقودة؟

المزيد

مواضيع ذات صلة

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values

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