Nucleophilic substitution is the kind of IB Chemistry topic that feels obvious in a textbook and slippery in an exam. You read “a nucleophile replaces a leaving group,” you nod, and then a question asks for the mechanism, the rate law, the stereochemistry, and the solvent effect -- and suddenly your arrows look like they’re trying to escape the page.
The good news is that nucleophilic substitution is predictable. Not because it’s simple, but because it follows a small set of rules that show up again and again in IB Chemistry.
Nucleophile swaps seats comic
Nucleophilic substitution in IB Chemistry (quick checklist)
Use this checklist when you’re deciding what mechanism to draw in IB Chemistry:
Identify the substrate: primary, secondary, or tertiary halogenoalkane?
Check the nucleophile: strong (e.g., OH⁻, CN⁻) or neutral/weaker (e.g., NH₃)?
Look at the solvent: polar protic (often helps SN1) or polar aprotic (often helps SN2)?
Decide SN1 vs SN2: one-step backside attack or two-step carbocation route?
Write the correct rate law: depends on one species (SN1) or two (SN2)?
In IB Chemistry, nucleophilic substitution is a reaction where a nucleophile (electron-rich) replaces a leaving group (takes the bonding pair and departs).
Most exam questions place this on halogenoalkanes because the C--X bond is polar:
Carbon becomes δ+ (electrophilic)
Halogen becomes δ- and can leave as X⁻
That polarity is the invitation: the nucleophile brings a lone pair, attacks the δ+ carbon, and the leaving group exits with electrons.
SN2 stands for bimolecular nucleophilic substitution. In IB Chemistry, the story is “one decisive moment”:
One-step mechanism (single transition state)
Favoured by primary halogenoalkanes (low steric hindrance)
Backside attack leads to inversion of configuration
Rate depends on both reactants
Rate law (SN2):
Rate = k[halogenoalkane][nucleophile]
The key phrase to remember for IB Chemistry is steric hindrance. If the carbon is crowded, the nucleophile can’t reach the reactive centre easily, so SN2 becomes less likely.
SN1 in IB Chemistry: two steps, a risky intermediate
SN1 is unimolecular nucleophilic substitution. It’s slower to explain but sometimes easier to spot:
Two-step mechanism
First step forms a carbocation intermediate
Favoured by tertiary halogenoalkanes (more stable carbocations)
Nucleophile can attack from either side of the planar carbocation
Rate depends on one species
Rate law (SN1):
Rate = k[halogenoalkane]
The exam takeaway in IB Chemistry is that carbocation stability and steric hindrance point in the same direction: tertiary substrates resist SN2 and support SN1.
How to revise nucleophilic substitution with RevisionDojo
The fastest improvement in IB Chemistry usually comes from turning “I understand it” into “I can do it under time pressure.” RevisionDojo is built for that:
If you get stuck mid-mechanism, RevisionDojo’s AI Chat can walk through your arrow pushing step-by-step, and the Grading tools help you spot what examiners actually reward. When exams get close, Predicted Papers, Mock Exams, and support from Tutors can turn weak topics into reliable marks.
Closing: make IB Chemistry substitution feel inevitable
Once you see nucleophilic substitution as a choice between “one crowded step” (SN2) and “two steps with a carbocation pause” (SN1), IB Chemistry questions stop feeling like surprises. They become decisions you can justify.
If you want that confidence on exam day, build repetition with RevisionDojo: start with the nucleophilic substitution Questionbank, reinforce with R3.4.2 notes, and tighten your timing with the wider Reactivity 3 practice. IB Chemistry rewards the student who can stay calm, choose a pathway, and draw it cleanly -- and RevisionDojo is built to train exactly that.
Daniel holds an MSc in Chemistry from Imperial College London and has taught IB Chemistry for over 20 years, including as Head of Chemistry. His focus is IB Chemistry Papers 1-3 and the internal assessment, building the conceptual understanding behind each equation rather than rote recall.
