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.