06 Substitution and Elimination Reactions Free Online FlashCards

How do substitution and elimination differ?

Substitution replaces a leaving group on carbon with a nucleophile. Elimination removes a leaving group and a neighboring hydrogen to form a carbon–carbon double bond.

How does an SN2 reaction proceed?

In one concerted step, the nucleophile attacks from the side opposite the leaving group as the carbon–leaving-group bond breaks.

What stereochemical outcome is characteristic of SN2?

Backside attack causes inversion of configuration at the stereogenic reaction center. The R/S label may not change if substituent priorities change.

What conditions commonly favor SN2?

SN2 is favored by strong, unhindered nucleophiles and accessible carbons, especially methyl and primary substrates. Polar aprotic solvents commonly favor it.

What stereochemical outcome is common in SN1?

A planar carbocation can be attacked from either face, so a chiral substrate commonly gives partial to substantial racemization, not necessarily a 50:50 mixture.

Can an SN1 carbocation rearrange?

Yes. Hydride or alkyl shifts can occur if they produce a more stable carbocation.

How does E2 form an alkene?

A base removes a β-hydrogen as the leaving group departs; electrons from the C–H bond form the double bond in one concerted step.

What geometry does E2 generally require?

The C–H and C–leaving-group bonds generally must be anti-periplanar: in the same plane and pointing in opposite directions.

How can base size affect E2 alkene regioselectivity?

A small base often favors the more substituted Zaitsev alkene. A bulky base may favor the less substituted Hofmann alkene by removing a less hindered β-hydrogen more readily.

Why can E1 compete with SN1, and what effect can heating have?

E1 and SN1 share carbocation formation as their first step, so they often compete under conditions that support carbocations. Heating often increases elimination relative to substitution.

How does leaving-group ability affect SN1, SN2, E1, and E2?

A better leaving group generally facilitates all four pathways. Iodide, bromide, and tosylate are common good leaving groups; hydroxide is generally poor unless converted to a better leaving group.

Which concentrations determine the rate of an SN2 reaction?

Its rate law is $\text{rate}=k[\text{substrate}][\text{nucleophile}]$, so the rate depends on both concentrations.