A-Level Chemistry organic reaction mechanisms: curly arrows from free radicals to arenes

Where each mechanism sits

The key ideas

Bonds break in two ways. Homolytic fission gives each atom one electron, making free radicals (shown with a dot); it happens with UV light, as in Cl2 → 2Cl•. Heterolytic fission gives both electrons to one atom, making ions. A nucleophile is an electron pair donor, such as OH-, CN- or NH3, and attacks a positive (δ+) carbon. An electrophile is an electron pair acceptor, such as Br(δ+) in a polarised Br2 molecule, H+ or NO2+, and attacks a region of high electron density such as a C=C bond or a benzene ring.

Each curly arrow means the movement of a pair of electrons. Draw it from a lone pair or the middle of a bond to where the pair ends up. In SN2, the nucleophile's lone pair attacks the δ+ carbon as the C-Br bond breaks, through a single transition state, and the rate depends on both reactants. In SN1, the C-Br bond breaks first to form a carbocation, then the nucleophile attacks, and the rate depends only on the halogenoalkane. Tertiary halogenoalkanes favour SN1 because tertiary carbocations are more stable; primary ones favour SN2.

In electrophilic addition to an unsymmetrical alkene such as propene, the more stable secondary carbocation forms preferentially, so the major product of HBr + propene is 2-bromopropane (Markovnikov's rule). Benzene undergoes substitution rather than addition, because addition would destroy the stable delocalised ring of electrons.

Worked example 1: free-radical substitution of methane with chlorine

1. Conditions: UV light.
2. Initiation: Cl2 → 2Cl•, homolytic fission by UV light.
3. Propagation step 1: CH4 + Cl• → CH3• + HCl.
4. Propagation step 2: CH3• + Cl2 → CH3Cl + Cl•. The chlorine radical is regenerated, so this is a chain reaction.
5. Termination: any two radicals combine, for example Cl• + Cl• → Cl2, CH3• + Cl• → CH3Cl, or CH3• + CH3• → C2H6.
6. Further substitution can give CH2Cl2, CHCl3 and CCl4, which is why this method gives a mixture of products.

Worked example 2: HBr adding to propene (described in words)

1. HBr is polar: H is δ+ and Br is δ-.
2. Arrow 1 goes from the C=C double bond to the δ+ hydrogen of HBr. Arrow 2 goes from the H-Br bond to the bromine, which leaves as Br-.
3. The hydrogen adds to the end carbon (CH2), giving a secondary carbocation CH3-C+H-CH3, which is more stable than the primary alternative because it has two alkyl groups pushing electrons towards the positive carbon.
4. Arrow 3 goes from a lone pair on Br- to the positive carbon.
5. Major product: 2-bromopropane. Minor product: 1-bromopropane, from the less stable primary carbocation.

Worked example 3: SN1 or SN2?

1. Bromoethane, CH3CH2Br, is primary. With aqueous OH-, the lone pair on OH- attacks the δ+ carbon from the side opposite the bromine, the C-Br bond breaks, and ethanol forms in one step: SN2, rate = k[CH3CH2Br][OH-].
2. 2-bromo-2-methylpropane, (CH3)3CBr, is tertiary. The C-Br bond breaks first (slow step) giving the tertiary carbocation (CH3)3C+, then OH- attacks: SN1, rate = k[(CH3)3CBr].
3. Evidence: the rate equation, and with a chiral starting material, SN2 gives inversion of configuration while SN1 gives a racemic mixture because the planar carbocation can be attacked from either side.

Common mistakes that cost marks

• Starting a curly arrow from an atom or a charge instead of a lone pair or bond.
• Missing the lone pair on the nucleophile, or the δ+ and δ- dipoles on a polar bond.
• Drawing a radical without its dot, or using full arrows instead of describing homolytic fission.
• Placing the carbocation on the wrong carbon in electrophilic addition.
• Writing addition for benzene, or forgetting to show how the electrophile is generated.
• Confusing nucleophilic substitution (halogenoalkanes) with nucleophilic addition (carbonyls).

Exam technique and how a tutor helps

Mechanism questions are marked arrow by arrow: each correct arrow, dipole, lone pair and intermediate is usually a mark. Draw large, with full structures around the reacting carbon, and check each arrow starts on electrons. Name the mechanism and the type of reagent (nucleophile, electrophile, radical) when asked.

Learn the mechanisms as a set, comparing what attacks what. A short table of reagent, conditions, mechanism and product for each functional group turns the whole topic into a revision sheet that also covers synthesis questions.

In one-to-one lessons a tutor has the student draw each mechanism on the shared whiteboard from memory and corrects arrows in real time, which is far faster than reading model answers. The tutor then sets unfamiliar examples, such as a different halogenoalkane or alkene, to check the student understands the logic rather than a memorised picture.

Self-check: can you do these?

• Define a nucleophile. (Answer: an electron pair donor)
• What type of fission happens in the initiation step of free-radical substitution? (Answer: homolytic)
• Which product dominates when HBr adds to propene? (Answer: 2-bromopropane)
• Why do tertiary halogenoalkanes react by SN1? (Answer: the tertiary carbocation is stabilised by three alkyl groups, and the crowded carbon hinders backside attack)
• What is the electrophile in the nitration of benzene? (Answer: the nitronium ion, NO2+)