Chemistry · General Principles Of Organic Chemistry · NEET
Bond fission (also called bond cleavage) simply means breaking a covalent bond so a reaction can happen. The shared pair of electrons in the bond has to be split between the two atoms. How that pair is split decides which reactive intermediate you get.
In homolytic fission the two shared electrons split equally: one goes to each atom, giving two neutral free radicals (each with one unpaired electron). In heterolytic fission the two electrons split unequally: both go to one atom, giving one positive ion and one negative ion. Homolytic = radicals; heterolytic = ions.
A free radical is a neutral atom or group that has one unpaired electron. It is formed by homolytic fission. Because the electron is unpaired, radicals are very reactive. Example: when Cl-Cl breaks in sunlight it forms two chlorine radicals.
Both come from heterolytic fission of a bond to carbon. A carbocation is a carbon with only 6 electrons (a sextet) and a positive charge, like CH3+. A carbanion is a carbon with a lone pair and a negative charge, like CH3-. Carbocation = electron-poor; carbanion = electron-rich.
Because the shared electron pair is not split. Both electrons leave with the more electronegative atom. That atom gains an extra electron, so it becomes negative; the atom that lost its share becomes positive. Unequal sharing of charge means you get charged species, i.e. ions.
The full curved arrow (normal arrowhead) shows the movement of a PAIR of electrons and is used for heterolytic fission. The half-headed 'fish hook' arrow shows the movement of a SINGLE electron and is used for homolytic fission. NEET diagrams often test which arrow goes with which cleavage.
Yes. NCERT says reactions that go through heterolytic bond cleavage are called ionic, heteropolar, or just polar reactions. Reactions that go through homolytic cleavage are called free radical, homopolar, or non-polar reactions. So heterolytic = ionic/polar and homolytic = free-radical/non-polar.
The pair of electrons in the given carbanion, CH3-C≡C(-), is present in which of the following orbitals?
A tertiary butyl carbocation is more stable than the secondary butyl carbocation because of which one of the following?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Homolytic fission. The bond breaks so one electron of the shared pair goes to each atom, producing two neutral free radicals, each with a single unpaired electron.
Heterolytic fission. Both shared electrons go to one atom: the atom that keeps them becomes negative (carbanion if it is carbon), and the atom that loses them becomes positive (carbocation if it is carbon).
Non-polar bonds, high temperature, UV light or peroxides favour homolytic (free-radical) fission, for example Cl-Cl breaking in sunlight during chlorination of methane.
Polar bonds (large electronegativity difference), polar solvents that stabilise ions by solvation, and the presence of a good leaving group favour heterolytic (ionic) fission, as in the SN1 breaking of a C-Br bond.
A carbocation is electron-deficient. It has only a sextet (6 electrons) around carbon and a positive charge, so it behaves as an electrophile and is attacked by nucleophiles.
