Radicals form by homolysis of weak bonds

This is the most important way of making radicals: unpairing a pair of electrons by homolysis, making two new radicals. Temperatures of over 200 °C will homolyse most bonds; on the other hand, some weak bonds will undergo homolysis at temperatures little above room temperature. Light is a possible energy source for the homolysis of bonds too. Red light has associated with it 167 kJ mol⁻¹; blue light has about 293 kJ mol⁻¹. Ultraviolet (200 nm), with an associated energy of 586 kJ mol−1, will decompose many organic compounds (including the DNA in skin cells: sunbathers beware!). There are a number of compounds whose homolysis is particularly important to chemists, and the most important ones are discussed in turn below. They all have weak σ bonds, and generate radicals that can be put to some chemical use. The halogens are quite readily homo lysed by light, as you can see from the bond strengths in the table above, a fact that drives the radical halogenation reactions that we shall discuss later, dibenzoyl peroxide is an important compound because it can act as another initiator of radical reactions. It undergoes homolysis simply on heating.

Another compound that is often used in synthetic reactions for the same reason (although it reacts with a different set of compounds) is AIBN (azobisisobutyronitrile).

This decomposition mechanism accounts for the separate movements of all the electrons, but we can also draw the mechanism in a slightly different way: we show two radical (‘fish hook’) arrows forming the molecule of nitrogen but only one arrow to break each of the C–N bonds. It can be assumed that the electrons ‘left behind’ form radicals as well.

Another way of cutting back on the number of arrows without losing precision in the mechanism is to draw one arrow for each step all in the same (which can be either) direction. The fi rst mechanism has the advantage of complete clarity; the other two make for neater diagrams. Choose which suits you best.

The important thing is to use the right type of arrow and to make it clear whether you are moving one, and not two, electrons. A simpler example is the abstraction of a hydrogen atom by an oxygen-centred radical: any of the mechanisms below is fine.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بيان لمكتب المرجعية العليا حول نشر صور السيد السيستاني في الأماكن العامة

متحف الكفيل: إنجاز المرحلة الرابعة من مشروع متحف العتبة العسكرية المقدسة

تضمن عرضًا مسرحيًّا.. شعبةُ الخطابة النسويّة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن (عليه السّلام)

الأمين العام للعتبة العبّاسية المقدّسة يتفقد مجموعة مواكب أمّ البنين (عليها السلام) لخدمة الزائرين

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

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct