A covalent bond is a shared pair of electrons. When the bond breaks, the two electrons can be shared out in two ways.Homolytic fission: each atom takes one electron from the bond. This forms two radicals.
Cl-Cl → Cl• + Cl•
A radical is a species with an unpaired electron, shown by a dot. Radicals are very reactive because the unpaired electron readily pairs up with an electron from another species. Homolytic fission needs energy, often from ultraviolet light or high temperature, and is common for non-polar bonds such as Cl-Cl and C-C.Heterolytic fission: one atom takes both electrons. This forms two ions: a positive ion and a negative ion.
H-Br → H+ + Br-
The electrons usually go to the more electronegative atom. Heterolytic fission is common for polar bonds, such as C-Br in halogenoalkanes. When a carbon atom is left with a positive charge, the ion is called a carbocation.Curly arrows show how electrons move in a mechanism:
a full-headed arrow shows the movement of a pair of electrons (used for heterolytic fission);
a half-headed (fish-hook) arrow shows the movement of a single electron (used for homolytic fission).
Arrows start at a bond or lone pair and point to where the electrons end up.
Worked example: classify CH3-CH3 → 2CH3•. Each carbon keeps one electron and two methyl radicals form, so it is homolytic fission.
A radical is a species with an unpaired electron, shown by a dot. Radicals are very reactive because the unpaired electron readily pairs up with an electron from another species. Homolytic fission needs energy, often from ultraviolet light or high temperature, and is common for non-polar bonds such as Cl-Cl and C-C.Heterolytic fission: one atom takes both electrons. This forms two ions: a positive ion and a negative ion.
The electrons usually go to the more electronegative atom. Heterolytic fission is common for polar bonds, such as C-Br in halogenoalkanes. When a carbon atom is left with a positive charge, the ion is called a carbocation.Curly arrows show how electrons move in a mechanism:
a full-headed arrow shows the movement of a pair of electrons (used for heterolytic fission);
a half-headed (fish-hook) arrow shows the movement of a single electron (used for homolytic fission).
Arrows start at a bond or lone pair and point to where the electrons end up.
Worked example: classify CH3-CH3 → 2CH3•. Each carbon keeps one electron and two methyl radicals form, so it is homolytic fission.