Fundamental Concepts in Organic Reaction Mechanism

Concept of homolytic and herterolytic fission

Organic reactions usually involve making and breaking of covalent bonds. Fission of bonds can take place in two ways:

Homolytic Fission

When the cleavage of covalent bond between two atoms takes place in a manner, which enables each atom to retain one electron of the shared pair, it is known as homolytic fission.

This fission is symmetrical and leads to the formation of atoms or groups of atoms having unpaired electrons, called free radicals.

 The free radicals are denoted by putting dot over the symbol of atom or group of atoms. For example,

Heterolytic Fission

Heterolytic fission is unsymmetrical wherein one of the fragments takes both the electrons of the shared pair, leaving none on the other. This results into two charged particles as:

Electrophile , nucleophiles and free- radicals

The attacking reagents are classified into three types:

Electrophiles

Positively charged or neutral species, which are deficient of electrons and can accept a pair of electrons are called electrophiles. These are also called electron loving (philic) species. For example,

H⁺, H₃O^+, Cl^+, CH₃^+, NO₂⁺ (Positively charged)

AlCl₃, BF₃, SO₃ (Neutral)

Both Al and B act as electrophiles as they have total of six electrons i.e. two less than the octet, and so they try to complete their octets. These are also called as Lewis acids.

Nucleophiles: A nucleophile is a reagent containing an atom having unshared or lone pair of electrons. As a nucleophile is electron rich it seeks electron deficient sites i.e., nucleus (nucleus loving). According to Lewis concept of acids and bases, nucleophiles behave as Lewis bases. For example,

NH₃, H₂O, ROH, ROR (neutral)

Free Radical: free radical may be defined as an atom or group of atoms having an unpaired electron. Free radicals are produced during the homolytic fission of a covalent bond.

Free radicals are very reactive as they have strong tendency to pair up their unpaired electron with another electron from wherever available. These pairs are very short lived and occur only as reaction intermediates during reactions.

For example, dissociation of chlorine gas in the presence of ultra-violet light produces chlorine free radicals:

The alkyl free radical may be obtained when free radical chlorine attacks methane.

Free radicals may be classified as primary, secondary or tertiary depending upon whether one, two or three carbon atoms are attached to the carbon atom carrying the odd electron:

Stability of free radicals

The order of stability of alkyl free radicals is:

 This order of stability can easily be explained on the basis of hyperconjugation:  Larger the number of alkyl groups, attached to the carbon atom carrying the odd electron, greater is the delocalisation of the odd electron and hence more stable is the free radical.

 Accordingly, the tertiary free radical with three alkyl groups attached to the carbon atom carrying the odd electron is more stable than the secondary free radical containing two alkyl groups and so on.

Structure of alkyl free radical

The carbon atom in alkyl free radicals involves sp² hybridization. Therefore, it has a planar structure. Three hybrid orbitals are used in the formation of three s-bonds with three H atoms or alkyl group. The unpaired electron is present in unhybridized p orbital.