8.7.1. Fission of a Covalent Bond

A covalent bond can get cleaved either by: (i) heterolytic cleavage, or by (ii) homolytic cleavage.

Covalent bonds can break in two primary ways—heterolytic cleavage and homolytic cleavage. In heterolytic cleavage, the bond breaks, and one atom takes both of the electrons from the bond, leading to the formation of charged species—one with a full positive charge (carbocation) and the other with a full negative charge (carbanion). In contrast, homolytic cleavage results in each atom taking one of the shared electrons, forming two radicals, which have unpaired electrons and are usually neutral.

Imagine a tug-of-war game. In heterolytic cleavage, one team yanks the rope all the way to their side, taking full control (like an atom taking both electrons), while in homolytic cleavage, each team pulls on the rope equally and ends up with half of it (each atom taking one electron).

In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments. After heterolysis, one atom has a sextet electronic structure and a positive charge and the other, a valence octet with at least one lone pair and a negative charge.

Heterolytic cleavage results in two charged fragments. For example, in the cleavage of bromomethane, the bond between carbon and bromine breaks, where bromine takes both electrons, forming a bromide ion (Br–) and a carbocation (C+H3). The positively charged carbon atom now has six electrons instead of the usual eight, making it highly reactive, while the bromine, with extra electrons now, has a complete octet but carries a negative charge.

Think of a teacher giving a prize to a student. If the teacher hands over the entire prize to one student (like Br taking both electrons), that student becomes the clear winner (carbocation) and now has all the attention, while the other students watch without participation (like the bromide ion).

Carbocations are classified as primary, secondary, or tertiary depending on whether one, two, or three carbons are directly attached to the positively charged carbon. Carbocations are highly unstable and reactive species.

The stability of a carbocation increases with the number of alkyl groups attached to the positively charged carbon atom. Primary carbocations have one alkyl group, secondary have two, and tertiary have three. Tertiary carbocations are the most stable because the surrounding alkyl groups can help disperse the positive charge through inductive and hyperconjugation effects.

Imagine a person balancing on a seesaw. The more friends (alkyl groups) that person has sitting on their side to keep them balanced, the more stable they become. A tertiary carbocation is like a person with three supportive friends, making it much less likely to tip over precariously!

In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms, resulting in the formation of neutral species (free radicals) containing an unpaired electron.

Homolytic cleavage occurs when the bond breaks such that each atom retains one of the electrons. This leads to the production of free radicals, which are highly reactive species because they have unpaired electrons. The movement of one electron is represented by a 'half-headed' arrow.

Consider a pair of scissors that cuts a piece of string. Each blade of the scissors takes half of the string (one electron each). The resulting loose ends (free radicals) from the string are reactive, similar to how free radicals want to quickly bind with other atoms to find a stable state.

Alkyl radicals are classified as primary, secondary, or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary.

Just like carbocations, the stability of alkyl radicals also increases with the number of alkyl groups attached to the carbon atom that carries the unpaired electron. Tertiary radicals are the most stable due to the stabilization provided by the surrounding alkyl groups. This effect is vital in determining the products of reactions involving radicals.

Imagine a skateboarder (the free radical) trying to land a trick. The more friends they have around to help steady them (like the surrounding alkyl groups), the more likely they are to land it smoothly—making tertiary radicals the expert skaters who have mastered their tricks with friends' support.

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