Fundamental Concepts of Inorganic Reaction Mechanisms I

Types of Bond cleavage:

Homolytic cleavage:

• Homolytic fission is where each atom of the bond keeps an electron each resulting in species called free radicals.
  R−X→R∙+X∙ 
• Homolytic cleavage occurs generally in the gas phase or in solution in non-polar Solvents and is catalyzed by ultraviolet light or high temperature of or by radical initiators such as peroxides, alkyl hypohalites, and tetraethyl lead (TEL).

Heterolytic cleavage:

• In this case we can see that one of the atoms carry a negative charge after bond cleavage indicating that it has both the electrons of the bond and the other has no electrons at all. Hence it is electron deficient thus positively charged.  As the electrons are not divided equally after bond cleavage this is called Heterolytic Fission.
  R−X→R⁺ + X⁻ 
• Heterolytic cleavage occurs in polar solvents because of the ease of separation of charge and stabilization of the resultant ion pairs through salvation.

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:

• A 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.
  A : B → A + B             Free Radicals  
• 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:
  Cl : Cl → Ċl + Ċl
• 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: