Variation in the structure of the aldehyde

Before leaving the Cannizzaro reaction, look at these rates of reactions for aromatic aldehydes with different substituents in the para position. These aldehydes may be divided into two classes: those that react faster than unsubstituted benzaldehyde and those that react more slowly. Those that go slower all have something in common—they all have substituents on the ring that donate electrons.

We have already seen how substituents on a benzene ring affect the rate of electrophilic substitution . Electron-donating groups such as MeO and Me2N dramatically speed up the rate at which an aromatic ring is attacked by an electrophile, whereas electron withdrawing groups, particularly nitro groups, slow the reaction down. The Cannizzaro reaction is not taking place on the benzene ring itself, but substituents on the ring still make their presence known. The fact that the Cannizzaro reaction goes much slower with electron donating groups and faster with electron-withdrawing groups tells us that, for this reaction, rather than a positive charge developing, as in the case of electrophilic substitution on an aromatic ring, there must be negative charge accumulating somewhere near the ring. The accumulation of more negative charge is disfavoured by the presence of a group that is already offloading electron density into the ring. In agreement with this, our mechanism has mono- and dianion intermediates, which are stabilized by electron-withdrawing groups and destabilized by electron-donating groups.

The rest of the chapter is devoted to discussions of methods similar to those we have briefl y surveyed for the Cannizzaro reaction, with examples of the use of each method. You can assume that the mechanisms we have discussed in this book have been verifi ed (not, of course, proved) by these sorts of methods.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أهالي كربلاء يشيعون النعش الرمزي للإمام الحسن (عليه السلام) عند العتبتين المقدستين

العتبة العباسية المقدسة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن (عليه السلام)

قاعة العرض المتحفي في العتبة العباسية المقدسة تشهد إقبالًا واسعًا من الزائرين

خلال لقائه ملاكات مدارس العميد.. سماحة السيّد الصافي يؤكّد أهمّية التعلّم المستمرّ الذي يفتح آفاق الفهم والمعرف

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

Be sure of the structure of the product

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis

Ene-yne metathesis

Metathesis catalysts

Alkene metathesis

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes