Common and special organic reactions

What the exam asks

Expect a substrate, a reagent and a set of conditions, and a question asking for the product, the mechanism, or what changing one variable does to the rate. The commonest items are: choose SN1 or SN2 from the three variables and state the stereochemical consequence, predict the major alkene from an elimination, apply Markovnikov to an addition, and place an incoming group on a substituted benzene ring. The trap that catches most candidates is treating SN1 versus SN2 as a substrate question alone. Substrate is the loudest vote but it is not the only one: a secondary carbon is genuinely undecided, and then the nucleophile and solvent decide it. When a stem bothers to name the solvent, the solvent is the answer. The second trap is expecting an SN2 product to be racemic. SN2 inverts, cleanly and every time, which is the opposite of racemising, and a stem offering a single inverted product is describing a backside attack.

Four families cover almost everything asked here: substitution at a saturated carbon, elimination to make an alkene, addition across a double bond, and the chemistry of the carbonyl. Aromatic substitution is a fifth, and it behaves differently from the rest for one reason worth holding on to, which is that benzene will not give up its aromaticity.

Almost none of this is reagent memorisation. The exam supplies the reagents and asks what they do, or gives you a product and asks which conditions made it. The reasoning underneath is nearly always the same two questions: where does electron density sit, and which intermediate is the most stable one available. A nucleophile is somewhere with electrons going looking for somewhere without them. A carbocation forms where the molecule can best afford the positive charge. Markovnikov, SN1 versus SN2, and Zaitsev are all consequences of those two questions rather than separate facts.

The substitution decision is the highest-value skill in the topic, because it is a genuine three-variable judgement and the exam knows it. Substrate, nucleophile and solvent each vote. A tertiary carbon cannot be attacked from behind and makes a stable cation, so it goes SN1. A methyl or primary carbon is wide open and makes a terrible cation, so it goes SN2. And the two mechanisms leave different fingerprints at the stereocentre, which is how a question can ask about mechanism while appearing to ask about a product.

What to hold

• SN2 is one concerted step: the nucleophile attacks the carbon from the side opposite the leaving group, so the rate depends on both substrate and nucleophile, and the configuration at that carbon inverts.
• SN1 is two steps: the leaving group departs first to give a carbocation, which the nucleophile then attacks from either face, so the rate depends only on the substrate and the product is close to racemic.
• Substrate decides most of it: SN2 needs an unhindered carbon and runs best on methyl and primary, while SN1 needs a stable carbocation and runs best on tertiary.
• A strong, anionic, concentrated nucleophile pushes towards SN2; a weak neutral nucleophile, often the solvent itself, pushes towards SN1.
• Polar aprotic solvents favour SN2 by leaving the nucleophile bare and reactive; polar protic solvents favour SN1 by hydrogen bonding to the nucleophile and stabilising the carbocation.
• Carbocation stability runs tertiary above secondary above primary, because neighbouring alkyl groups donate electron density into the empty orbital.
• Any mechanism with a carbocation can rearrange to a more stable one via a hydride or alkyl shift, so an SN1 product may appear at a carbon that never carried the leaving group.
• E2 is concerted and needs a strong base, with the departing hydrogen and leaving group anti-periplanar; E1 shares the carbocation intermediate with SN1 and competes with it.
• Zaitsev's rule says elimination favours the more substituted, more stable alkene, but a bulky base such as tert-butoxide cannot reach that hydrogen and gives the less substituted alkene instead.
• Heat favours elimination over substitution, because elimination increases the number of particles and so gains more from the entropy term.
• Markovnikov addition places the electrophile so that the more stable carbocation forms, which puts the hydrogen on the carbon that already had more hydrogens.
• Hydroboration and oxidation adds water across an alkene the other way round, anti-Markovnikov, and it does not go through a free carbocation so it cannot rearrange.
• Aldehydes and ketones undergo nucleophilic addition at the carbonyl carbon, which is electron poor because oxygen holds the pi electrons close.
• Carboxylic acid derivatives undergo substitution rather than addition, because they carry a leaving group, and their reactivity falls in the order acid chloride, anhydride, ester, amide.
• Benzene substitutes rather than adds, because substitution restores the aromatic ring and addition would destroy it permanently.