Introduction to Organic Chemistry: Question 3

Worked solution

Step 1: What happens to the shared pair of electrons

A covalent A–B bond is held together by a shared pair of electrons. When the bond breaks, this pair can either be split evenly between A and B, or unevenly, with both electrons going to just one of the two atoms.

Step 2: Homolytic fission

If the pair is split evenly, each atom keeps one electron:

$\text{A–B} \rightarrow \text{A}\bullet + \text{B}\bullet$

Each product, $\text{A}\bullet$ and $\text{B}\bullet$, is a free radical: an electrically neutral species with a single unpaired electron. Homolytic fission is typically favoured when the bond breaks under non-polar conditions, such as by ultraviolet light or heat, and is the first step of free-radical mechanisms.

Step 3: Heterolytic fission

If the pair is split unevenly, one atom keeps both electrons:

$\text{A–B} \rightarrow \text{A}^+ + \text{B}^-\qquad\text{(or }\text{A}^-\text{ and }\text{B}^+\text{, depending on which atom keeps the pair)}$

This produces one cation (the atom that lost both electrons) and one anion (the atom that gained both electrons). Heterolytic fission is typical of polar bonds, and underlies both electrophilic and nucleophilic reaction mechanisms. The cation formed can act as an electrophile (an electron-pair acceptor), while the anion formed can act as a nucleophile (an electron-pair donor) in a subsequent step.

Why the other options are wrong

• B swaps the two definitions: it attributes ion formation to homolytic fission and radical formation to heterolytic fission, which is the reverse of the correct pairing.
• C is wrong because heterolytic fission does not produce free radicals at all. It produces a cation and an anion, not two neutral radicals. The two processes differ fundamentally in how the electrons are distributed, not merely in the energy required.
• D is wrong because fission of this kind applies specifically to a covalent bond breaking within a molecule; both homolytic and heterolytic fission occur in covalent bonds. There is no requirement, or even possibility, for either process to occur “in an ionic bond”.

Final answer

A. Homolytic fission gives two free radicals (one electron to each atom); heterolytic fission gives a cation and an anion (both electrons to one atom).