When do E2 and SN2 compete?

Elimination via E two competes with substitution via S N two when a strong base is used on secondary or tertiary carbon centers, and factors like base strength, substrate structure, and temperature push the reaction toward one pathway or the other.

Think of the carbon with the leaving group as a fork in the road where two things can happen: the base can pull off a hydrogen to make a double bond, which is elimination, or the base can attack the carbon and replace the leaving group, which is substitution. If the carbon is primary and the base is not bulky, substitution by S N two usually wins because it can reach the carbon easily; if the base is bulky or the carbon is secondary or tertiary, it has a harder time attacking the carbon directly, so E two becomes more likely because removing an adjacent hydrogen is easier. Temperature matters too because higher temperatures favor elimination; stronger, nonnucleophilic bases and steric hindrance favor E two, while small, strong nucleophiles at lower temperature favor S N two when the substrate allows it.

Students often think that strong bases always give elimination, but on primary carbons a small strong nucleophile can still do S N two, so check the substrate; use less hindered bases for S N two on primary centers. Another common mistake is ignoring temperature, since higher heat pushes toward elimination; lowering the temperature can help get more substitution. People also mix up bulky bases with weak bases, so remember that bulky strong bases hinder backside attack and favor elimination while weak bases usually do not promote E two strongly.

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E2 vs SN2: When They Compete

1. Consider substrate sterics
2. Use small nucleophiles for SN2
3. Bulky bases favor E2
4. Higher temperature favors elimination
5. Strong bases favor elimination

Common E2 versus SN2 Mistakes

1. Assuming strong bases always cause elimination
2. Ignoring temperature effects on pathway
3. Confusing bulky bases with weak bases

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