Be sure of the structure of the product

This seems a rather obvious point. However, there is a lot to be learned from the detailed structure of the product: its connectivity (which atom goes where) as well as its stereochemistry. You will see that it may be necessary to alter the structure of the starting material in subtle ways to make sure that we know exactly what happens to all its atoms by the time it reaches the product. Suppose you are studying the addition of HCl to this alkene. You find that you get a good yield of a single adduct and you might be a bit surprised that you do not get a mixture of the two obvious adducts. You may wonder if there is some participation of the ether oxygen or whether perhaps the ketone enolizes during the reaction and controls the outcome.

If you are cautious you might check on the structure of the product before you start a mechanistic investigation. The NMR spectrum (above) tells you at once that the product is neither of these suggestions. It contains a (CH₂)₃Cl unit and can no longer have an eight membered ring. A ring contraction has given a five-membered ring and a mechanistic investigation is hardly needed. Simply knowing what the product is allows us to propose a mechanism : number the atoms in the starting material and find them in the product. This is quite easy as only one numbering system makes any sense.

This numbering suggests that the carbon skeleton is unaffected by the reaction, that protonation has occurred at C5, that the ether oxygen has acted as an internal nucleophile across the ring at C4, and that the chloride ion has attacked C7. The mechanism is straightforward.

It may be disappointing to find that every step in this mechanism is well known and that the reaction is exactly what we ought to have expected with an eight-membered ring as these rings are famous for their transannular (across-ring) reactions to form 5/5 fused systems. However, it is good that a prolonged invesigation is not necessary.

● Find out for sure what the structure of the product is before you start a mechanistic investigation.

A more subtle distinction occurred in a study of the bromination of alkynes. Bromination of benzyl alkynes in acetic acid gave the products of addition of one molecule of bromine— the 1,2-dibromoalkenes. The reaction was successful with a variety of para substituents and there seems at first to be no special interest in the structure of the products.

Closer investigation revealed an extraordinary difference between them, not at all obvious from their NMR spectra: the compound from X=OMe was the Z-dibromoalkene from cis addition of bromine while the product from X=CF₃ was the E alkene from trans addition. What mechanism could explain this difference?

The anti-addition is more easily explained: it is the result of formation of a bromonium ion, similar, in fact, to the normal mechanism for the bromination of alkenes. Bromine adds from one side of the alkene and the bromide ion must necessarily form the E-dibromo product regardless of which atom it attacks.

So why does the p-methoxy-substituted compound behave differently? It cannot react by the same mechanism and a reasonable explanation is that the much more electron-donating ring participates in the reaction to give a carbocyclic three-membered ring intermediate that is attacked in an anti-fashion to give the Z alkene. Both intermediates are three-membered ring cations and both are attacked with inversion but the p-MeO compound undergoes double inversion by participation of the ring.

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

Variation in the structure of the aldehyde

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