We sneaked a new reaction into that sequence. The removal of a hydrogen atom (note: not a proton) from HBr by the peroxide radical is an abstraction reaction. The bromine radical will also abstract hydrogen atoms and will do so from the same alkene we have just used but with yet another different outcome, as you see in the margin. When light shines on bromine, the weak Br–Br bond breaks to give two bromine radicals. Heat will do the job too but light is cleaner and, as bromine is brown, it absorbs most wave lengths of visible light.

Radicals are very unstable and reactive, and these bromine radicals may simply recombine or they may react with other compounds. You already know that bromide anions are good nucleophiles in SN2 reactions, but bromine radicals do two quite different reactions: abstraction and addition. The Br radical may abstract a hydrogen atom from the alkene or it may add to the π bond. Notice that each reaction produces a new carbon-centred radical and, in the first case, a molecule of HBr. Whereas the Br–Br bond is weak, the H–Br bond is much stronger (366 kJ mol⁻¹) and, unlike ionic reactions, radical reactions are dominated by bond strength.

The first reaction introduces another important aspect of regioselectivity: why does the radical abstract that H atom, and not one from the alkene?

Removal of an alkene H gives a carbon-centred radical localized on the sp2 atom but the removal of an H from a methyl group gives a much more stable delocalized allylic radical. In addition, there are six such H atoms but only two alkene H atoms. The reaction obviously cannot end there with the formation of another radical, however stable, and this allylic radical collects a bromine atom from a bromine molecule. Note that the allylic radical doesn’t react with a bromine radical in this step: radicals are very un-stable and the concentration of radicals at any one time is so low that it is rare for two of them to meet.

This step also produces a new bromine radical that can start a new series of reactions. Like the addition of HBr above, the reaction is a radical chain reaction, and only a small amount of Br₂ needs to break down to Br^• to get the reaction going. This is important as you already know what happens when bromine molecules react with alkenes: addition occurs by an ionic mechanism. Add too much Br₂ and the bromine molecules attack the alkene directly and do not abstract H atoms.

If we want to make the dibromide, we use plenty of bromine, but if we want to use a radical process to make the allylic bromide we must take advantage of the greater reactivity of the radical and keep the bromine concentration low. A good way to do this is to use the compound NBS (N-bromosuccinimide), NBS acts as a sort of turnstile which only lets a molecule of Br₂ out when a molecule of HBr is formed (and of course HBr is the by-product in the radical bromination).

Br₂ is slowly released into the reaction as it proceeds, and the concentration never builds up enough to generate the dibromide. In this example, dibenzoyl peroxide is the initiator and allylic bromination gives the useful cyclohexenyl bromide.

These radical reactions will be described in much greater detail. For the moment you need only notice that they can have quite different regioselectivity from ionic reactions with the same reagents.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الدينية يُطلق دورة فقهية دينية لملاكات مستشفى الكفيل

شعبة فاطمة بنت أسد للدراسات القرآنية تعلن نتائج مسابقة (ميثاق الرسول)

شعبة الخطابة الحسينية النسوية تنظّم مسابقة فرقية لطالبات برنامج ثمار العطاء

قسم العلاقات ينظم ورشة عن فن الإقناع والتواصل الفعّال للطلبة المنسقين

المزيد

الآخبار الصحية

تغيير بسيط في نمط التفكير قد يحدث فرقا كبيرا لصحة الدماغ ؟

لماذا ينعشنا المشروب الساخن في الطقس الحار؟

علماء يكتشفون فئة دم فريدة من نوعها في العالم !

لماذا ينصح الأطباء بزيت الزيتون يوميا؟

المزيد

الآخبار التكنلوجية

حساس الإطارات.. ما هو وعلامات تلفه وكيفية صيانته؟

ما معنى أودي Quattro؟ سر الأداء والتميز الهندسي ؟!

فصل الشاحن أم الهاتف أولا؟.. عادات بسيطة تطيل عمر البطارية

تعود للقرن الرابع قبل الميلاد.. اكتشاف بقايا "إيمت" المصرية

المزيد

مواضيع ذات صلة

Alkylation of nitroalkanes

Alkylation of nitriles

Some important considerations that affect all alkylations

Conjugate addition

Electrophilic attack on conjugated dienes

Regiospecific preparation of allylic chlorides

Nucleophilic attack on allylic compounds

Radical addition

Electrophilic attack on alkenes

Regioselectivity of intramolecular reactions

Regiocontrol by choice of route

Regioselective reactions of naphthalene