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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsSteric hindrance slows an SN2 reaction by blocking a nucleophile’s backside approach to the carbon bearing the leaving group. That obstruction raises the transition-state energy and makes the reaction slower. For comparable ordinary alkyl substrates, the qualitative order is methyl > primary > secondary >> tertiary.
Why steric crowding slows SN2
An SN2 reaction happens in one concerted step: a nucleophile forms a bond to an electrophilic carbon as the leaving group departs. The nucleophile must approach from the side opposite the leaving group—the backside trajectory. Groups crowded around that carbon make this approach harder.
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With less accessible backside space, the bond-forming transition state is higher in energy. The larger activation free-energy barrier means a slower reaction. This is an effect on the rate, not a change to the SN2 mechanism: the reaction remains a concerted displacement. OpenStax’s SN2 overview, hosted by Chemistry LibreTexts, describes steric hindrance as raising the transition-state energy and decreasing the reaction rate.
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When comparing similar substrates under comparable conditions, increasing substitution at the carbon bonded to the leaving group generally hinders SN2 attack:
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- Methyl: least crowded and generally most accessible to backside attack.
- Primary: usually accessible, though branching on a neighboring carbon can still create substantial hindrance.
- Secondary: more crowded, so SN2 attack is slower than on a comparable primary substrate.
- Tertiary: so crowded that SN2 displacement at that carbon is generally effectively unavailable.
This is a qualitative trend, not a fixed set of rate ratios. A primary label alone does not guarantee easy backside access: neopentyl substrates have a primary reacting carbon but are strongly hindered by branching on the adjacent carbon. The location of nearby groups matters as well as the number attached directly to the reacting carbon. Roberts and Caserio’s discussion of structural and solvent effects covers these substrate trends.
What else affects an SN2 rate?
Substrate crowding is only one influence on the observed rate. The nucleophile, leaving group, and solvent also matter. To attribute a rate difference to sterics, compare substrates while holding those factors and other relevant conditions as similar as possible. Without matched conditions and measured data, the qualitative substrate order should not be converted into a numerical rate comparison.
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When the usual substrate ranking does not apply
The methyl-to-tertiary trend describes ordinary alkyl substrates; it is not a universal ranking for every carbon–leaving-group bond. Vinylic and aryl halides do not undergo the ordinary SN2 pathway at the sp2 carbon: its geometry does not provide the usual backside approach. Their behavior should not be explained simply by placing them above or below methyl, primary, secondary, or tertiary alkyl substrates.
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Backside displacement at a chiral reacting carbon produces inversion of configuration. The same directional attack that explains the steric barrier also determines this stereochemical outcome.
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