Bond cleavage and reagents

In a reaction, a covalent bond can break in two ways:

Heterolysis (heterolytic cleavage): one atom takes both bonding electrons, so ions are formed. The pair usually goes to the more electronegative atom. CH₃–Br → CH₃⁺ + Br⁻ (bromine takes the pair). But in CH₃–Cu, carbon is the more electronegative, so CH₃–Cu → CH₃⁻ + Cu⁺.

• A carbocation (CH₃⁺) has a carbon with only six valence electrons (a sextet). It is sp² hybridised and planar, and electron-poor.
• A carbanion (CH₃⁻) has a carbon with a lone pair and a negative charge.

Carbocation stability follows 3° > 2° > 1° > methyl (by the number of alkyl groups on the positive carbon). Alkyl groups push electrons towards the electron-poor carbon. Think of someone short of money: with three friends each lending ₹100, they are much better off than with one friend.

Homolysis (homolytic cleavage): each atom keeps one electron of the pair, giving free radicals, species with an unpaired electron. It is shown with half-headed (fish-hook) arrows. CH₃O–OCH₃ → 2CH₃O•. Free radical stability also follows 3° > 2° > 1°.

Reagents that attack organic molecules are of two kinds:

• A nucleophile ('nucleus-loving') is electron-rich and donates an electron pair: OH⁻, CN⁻, HS⁻, C₂H₅O⁻, and neutral molecules with lone pairs such as NH₃ and R₃N.
• An electrophile ('electron-loving') is electron-poor and accepts an electron pair: H⁺, Cl⁺, NO₂⁺, CH₃C⁺O, neutral BF₃, and the slightly positive carbon of a C=O group.

Chemists show how electrons move with curved arrows. An arrow always starts from an electron pair (a bond or a lone pair) and points to where it goes.