IB Chemistry Nucleophilic Substitution SN1/SN2 (HL): FAQ

Most lost marks come from three habits: drawing curly arrows starting from atoms instead of bonds/lone pairs, mixing up which mechanism gives racemisation versus inversion, and stating a rate law without linking it to the rate-determining step. Examiners award marks for precise mechanistic reasoning, not just the right final product.

SN1 is a two-step mechanism via a carbocation intermediate, rate depending only on substrate concentration (first order). SN2 is a single concerted step where the nucleophile attacks as the leaving group departs, giving second-order kinetics (rate depends on both substrate and nucleophile). SN1 gives racemisation; SN2 gives inversion of configuration.

Check three things: the carbon type (tertiary favours SN1, primary favours SN2, secondary can go either way), the nucleophile strength (strong nucleophiles push SN2, weak ones allow SN1), and the solvent (polar protic favours SN1, polar aprotic favours SN2). No single factor decides it alone — you weigh them together.

Steric hindrance slows SN2 as branching increases around the carbon, because the nucleophile can't approach from behind. Carbocation stability speeds up SN1, since tertiary carbocations are stabilised by hyperconjugation and inductive effects from three alkyl groups. Solvent polarity also matters: polar protic solvents stabilise the SN1 transition state and ionic intermediate.

According to the IB Chemistry guide (first exams 2025), Reactivity 3.4 requires HL students to explain SN1 and SN2 mechanisms using curly arrow notation, link mechanism to rate law and reaction kinetics, and explain how substrate structure, nucleophile strength and solvent choice affect which pathway dominates.

Arrows must start from a bond or lone pair, never from an atom's symbol. In SN2, one arrow shows the nucleophile's lone pair attacking the carbon while a second arrow shows the C–leaving group bond breaking, both happening simultaneously in one step. In SN1, draw the ionisation step first, then a separate arrow for nucleophile attack on the carbocation.

Inversion (SN2) flips the spatial arrangement at the carbon, like turning an umbrella inside out — one pure enantiomer becomes the other. Racemisation (SN1) happens because the flat carbocation intermediate can be attacked from either face, giving a roughly 50:50 mixture of both enantiomers, so the product is optically inactive.

Paper 2 mechanism questions typically allocate marks separately for correct curly arrows, correct intermediate/transition state structure, correct product including stereochemistry, and a written explanation linking substrate/nucleophile/solvent to the chosen pathway. Missing the stereochemical outcome (inversion vs racemisation) is one of the most common single-mark losses I see in mock papers.

Don't just memorise the mechanisms — practise predicting SN1 vs SN2 from unfamiliar substrates, then justify your choice in full sentences. Past paper questions on halogenoalkane hydrolysis are the most reliable practice. I tell every student I teach to draw the mechanism from memory five times before a test, not just read it once.

The biggest mistakes: arrows starting from atoms not bonds, forgetting to show the carbocation as planar (sp2) in SN1, stating rate laws without justifying order from the mechanism, and confusing nucleophile strength with nucleophile concentration — they affect different things (pathway choice versus SN2 rate specifically).

Take 2-bromo-2-methylpropane (tertiary) reacting with water versus 1-bromobutane (primary) reacting with hydroxide ion. The tertiary substrate favours SN1: the bulky carbocation is stabilised by three alkyl groups, and water (a weak nucleophile) can't force a back-side attack. The primary substrate undergoes SN2: minimal steric hindrance lets hydroxide attack directly.

It's not conceptually harder than, say, HL equilibrium calculations, but it's mark-dense — small errors in arrow notation or stereochemistry cost points that students often don't notice until moderation. In my experience marking mocks, it's one of the top three topics where predicted grades and actual Paper 2 scores diverge most.

No — the detailed SN1/SN2 mechanistic content, including curly arrows and stereochemical outcomes, is HL-only material under Reactivity 3.4 in the current DP Chemistry guide. SL students cover nucleophilic substitution only in general terms as part of organic reaction types, without the mechanism-level depth or kinetics required at HL.

Most students need targeted practice rather than more classroom time — this topic rewards repeated mechanism drawing and past-paper questions over passive review. Topical worksheets focused specifically on organic mechanisms tend to close gaps faster than general revision, since the mistakes (arrow notation, stereochemistry) are specific and fixable with deliberate practice.