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Steric crowding slows an SN2 reaction by blocking the nucleophile’s required backside approach to the carbon attached to the leaving group. That obstruction raises the transition-state energy and makes the reaction slower. For otherwise comparable alkyl substrates, the usual qualitative order is methyl > primary > secondary >> tertiary.
Why steric hindrance slows SN2
An SN2 reaction happens in one concerted step: a nucleophile forms a bond to the electrophilic carbon as the leaving group departs. The nucleophile approaches from the side opposite the leaving group, along the backside trajectory needed for displacement.
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Groups surrounding that carbon take up space. As crowding increases, it becomes harder for the nucleophile to reach the required trajectory and form the new bond. The resulting transition state is higher in energy, which increases the activation free energy and decreases the reaction rate. OpenStax explains this relationship in its chapter “11.3: Characteristics of the SN2 Reaction”, last modified September 30, 2024.
How substrate structure affects the trend
For comparable simple alkyl substrates, the standard qualitative order of SN2 reactivity is:
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methyl > primary > secondary >> tertiary
This is a qualitative trend, not a set of universal rate ratios. It assumes other important factors, including nucleophile, leaving group, and solvent, are comparable. The textbook discussion by Roberts and Caserio also treats structural and solvent effects as relevant to nucleophilic substitution rates: “8.8: Structural and Solvent Effects in SN Reactions”.
Methyl and primary substrates
Methyl substrates have the least crowding around the reacting carbon, so the nucleophile has relatively open access to the backside. Primary substrates are also generally favorable, though they have more substitution at the reacting carbon than methyl substrates.
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Secondary substrates
Secondary substrates are more crowded around the reaction site, making backside approach more difficult. Their SN2 reactions are therefore generally slower than comparable methyl or primary reactions.
Tertiary substrates
A tertiary reacting carbon is so crowded that SN2 displacement there is generally effectively unavailable. A poor SN2 outcome does not mean that every possible reaction pathway is impossible; it means this backside-displacement pathway is strongly disfavored at that carbon.
Branching next to the reacting carbon matters too
Do not judge steric hindrance only by counting substituents on the carbon bearing the leaving group. Branching on an adjacent carbon can also obstruct the approach. Neopentyl substrates illustrate the point: the carbon attached to the leaving group is primary, yet nearby branching makes the SN2 reaction strongly hindered.
When the usual alkyl-substrate trend does not apply
Vinylic and aryl halides do not fit the ordinary methyl-to-tertiary alkyl-halide sequence. Their leaving group is attached to an sp2 carbon, where the usual backside approach for an SN2 displacement is not geometrically available. The standard substrate ranking is intended for comparable alkyl substrates, not these different structural classes.
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How to compare two possible SN2 substrates
- Identify the carbon bonded to the leaving group. Determine whether it is methyl, primary, secondary, or tertiary.
- Look for nearby branching. Substitution on adjacent carbons can hinder access even when the reacting carbon is primary.
- Check the geometry. The usual ranking applies to alkyl substrates; vinylic and aryl halides do not undergo the ordinary SN2 pathway.
- Hold reaction conditions in view. Nucleophile, leaving group, and solvent also influence SN2 rate, so a structural comparison is clearest when those factors are comparable.
Stereochemical consequence at a chiral center
Because the nucleophile attacks from the backside as the leaving group departs, displacement at a chiral reacting center produces inversion of configuration. This stereochemical outcome follows from the geometry of the SN2 mechanism.
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