Reaction Mechanism. Part 2 of 8

Mechanism of Organic Reaction:Organic compounds are covalent, organic reactions involve the breakingof the old and making of the new covalent bonds. Breaking of a covalentbond between two atoms can take place mainly in two alternative ways,

(i) Homolytic Fission

(ii) Heterolytic Fission

(i) Homolytic Fission:It involves the fission of covalent bond in such a way that each of thebonded atom takes away one electron out of shared electron resulting intwo electrically neutral fragments or atoms generally know asfree-radicals, e.g.,

Radical are formed by homolytic bond cleavage in which one electron of the bond remains with each of the atom

Notethat an arrow with only half of an arrow-heads used to show themovement of a single electron. Species are called free radicals.

A free radical defined as the atom or group of atoms having a single or unpaired electron. These species are paramagnetic.

-They are electrically neutral

-They are extremely reactive

-Their stability is in the order.

(ii) Heterolytic Fission:In this type of cleavage the shared pair is taken away by one of themore electronegative atom. Heterolysis always produces ionic species.Thus the reactions involving Heterolytic fission are known as ionicreactions.

The Heterolytic fission of the covalent bond can occur in either of the following two ways.

(a)Heterolytic fission results in the formation of cation. Such organicspecies which has a positive charge at its carbon centre is known ascarbonium ion or carbocation.

(b) When organic species which has a negative charge at its carbon centre is known as carbanion.

Although most of the organic reactions are either homolytic (free radical reactions) or heterolytic (ionic reactions),a few reaction occur by electron transfer in cyclic order within amolecule or between molecules. The transition state is usually asix-member ring expect in some cases were a four-member ring is found.Such mechanism is known as cyclic or molecular mechanism.

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Example 1- For each of the following bond cleavages use curved arrows to describe the electron flow

Solution:

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[SolvedExample]

Example 2- Two products are formed when methylene-cyclohexane react with N-Bromosuccinimide. Explain how each is formed?

Solution: With NBS, there is bromination by free radical substitution

Forms free radical by homolytic fission of C-H bond

As resonance structure

Br. (from NBS) attack at a to give I and b to give II

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