How do I decide between SN1 and SN2 quickly in IB Chemistry?

Start with the substrate, because it usually decides the “default” route. Primary halogenoalkanes tend to go SN2 because the carbon is accessible and the nucleophile can perform a backside attack. Tertiary halogenoalkanes tend to go SN1 because steric hindrance blocks SN2 and the tertiary carbocation is relatively stabilised. Then check the solvent and nucleophile, because they can shift borderline cases (especially secondary). In an IB Chemistry exam, write one sentence of justification using the phrases “steric hindrance” and “carbocation stability” and you usually earn method marks. Finally, confirm with the expected rate law: if only the halogenoalkane concentration appears, you’re describing SN1.

Why does SN2 cause inversion of configuration (and why does IB Chemistry care)?

SN2 happens through backside attack, meaning the nucleophile approaches from the side opposite the leaving group. That geometry forces the carbon’s arrangement to flip as the new bond forms and the old bond breaks in one concerted step. The result is inversion of configuration (often called Walden inversion), which is a clean, testable consequence of the mechanism. IB Chemistry cares because it links mechanism to observable stereochemical outcomes, not just arrows on paper. Even if stereochemistry isn’t heavily assessed in every question, it’s a strong way to show understanding rather than memorisation. Practising a few mechanism drawings with clear 3D wedges helps this become automatic.

What are the most common IB Chemistry mistakes in nucleophilic substitution?

One common mistake is thinking “strong nucleophile equals SN2” no matter what, but substrate structure and solvent can override that. Another is drawing carbocations in every substitution mechanism; carbocations belong to SN1, not SN2. Students also mix up the rate laws, especially writing a two-term rate law for SN1 out of habit. A quieter mistake is ignoring the leaving group entirely; in IB Chemistry, you should explicitly show it leaving with the bonding pair and becoming X⁻. Finally, many students under-explain their choice: one short line about steric hindrance or carbocation stability can be the difference between partial and full marks.

Back to all posts

IB Chemistry: Nucleophilic Substitution Explained

IB Chemistry

RevisionDojo Team

•Last updated: 19 Jul 2026•7 min read

Add as a preferred source on Google

IB Chemistry: Nucleophilic Substitution Explained

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.

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)?

For targeted practice, the fastest way to make this automatic is doing exam-style sets like R3.4.2 Nucleophilic substitution reactions Questionbank.

What nucleophilic substitution actually means

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.

If you want the syllabus-aligned wording and examples in one place, pair this post with Notes for R3.4.2 Nucleophilic substitution reactions.

SN2 in IB Chemistry: one step, tight logic

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.

For HL mechanism detail and stereochemistry, see IB Chemistry R3.4.9 SN1 and SN2 Mechanisms Notes.

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.

You can consolidate the whole “mechanisms of chemical change” unit using IB Chemistry R3.4 Electron-pair Sharing Reactions.

Solvents: the quiet factor that changes everything

Solvent choice is a classic IB Chemistry lever because it changes what gets stabilised.

Polar protic solvents (like water, alcohols) stabilise ions well and can stabilise the carbocation and leaving group, often favouring SN1.
Polar aprotic solvents (like acetone, DMSO) don’t hydrogen-bond strongly to anions, so nucleophiles stay “naked” and reactive, often favouring SN2.

In other words: some solvents comfort charges; others hype the attacker.

To connect substitution to the wider organic toolkit, scan 20.1 Types of organic reactions Notes.

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:

Use the Questionbank for mechanism and rate-law drills: R3.4.2 Questionbank and Reactivity 3 Questionbank.
Lock in definitions and triggers with Flashcards: Reactivity 3 Flashcards.
Rewatch mechanisms until they feel boring (that’s when they stick) with Videos: R3.4.2 Videos and R3.4.9 Videos.
Zoom out with the full topic hub: IB Chemistry Organic Chemistry.

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.