The regioselectivity in the reaction below is opposite: a primary alkyl bromide is formed, by a different mechanism involving radicals.

In radical reactions, bonds break homolytically with one electron going one way and one the other. The radicals that are formed have an odd number of electrons, one of which must be unpaired. This makes them very reactive and they are not usually isolated. Even strong bonds can break into ions provided they are polarized, but to make radicals we need weak symmetrical bonds such as O–O, Br–Br or I–I. Dibenzoyl peroxide, the Ph(CO₂)₂ catalyst in this reaction, readily undergoes homolysis like this—the one-electron movements are represented by ‘fish-hook’ arrows having one barb and odd electrons on atoms are represented by dots.

Now we can use the new radicals we have just made to cleave the strong HBr bond homo lytically because a new and very strong OH bond will be formed. As we start with one radical intermediate that must have an unpaired electron, we must finish with another radical with an unpaired electron. In this case, it is a bromine radical.

If we do this reaction in the presence of the alkene we have just reacted with HBr, the bro mine radical adds to the alkene in one of the two possible ways. Although radicals are neutral, they are electron-deficient (the C atom is one electron short) and, rather like cations, are more stable the more substituents they have. So the tertiary radical is formed rather than the primary radical, and the bromine ends up at the primary position.

We have still not reached the end of the reaction as our product is still a radical. How can it become a molecule with only paired electrons? The answer is simple. It reacts with another HBr molecule to produce more bromine radicals. Now you see something important to all radical reactions: only a small amount of the radical is needed as more radicals are produced every time the reaction gives product. The overall process is a radical chain reaction.

Because of this we also need only very small amounts of dibenzoyl peroxide, the radical initiator, which is just as well as it is potentially explosive, like many radical generators. Here is the reaction being used to make a bromoacid:

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 abstraction

Electrophilic attack on alkenes

Regioselectivity of intramolecular reactions

Regiocontrol by choice of route

Regioselective reactions of naphthalene