Optimal Reaction Conditions

E1, E2, SN1, and SN2 mechanisms

SN2 Substrate

Methyl > 1° > 2° (Faster with less steric hinderance)

SN1 Substrate

3° > 2° (Faster with a more stable carbocation)

E2 Substrate

3°>2°>1° (Faster with more substituted alkene product formed)

E1 Substrate

3°>2° (Faster with more stable carbocations and more substituted alkene product)

SN2 Nucleophile or Base

Favored by stronger nucleophiles, no bulky bases

SN1 and E1 Nucleophile or Base

Favored by weaker nucleophiles

E2 Nucleophile or Base

Favored by stronger bases; bulky bases favor E2 over SN2

SN2 Leaving group

Better leaving group means faster reaction

SN1 Leaving group

Better leaving group means faster reaction

E2 Leaving group

Better leaving group means faster reaction

E1 Leaving group

Better leaving group means faster reaction

SN2 Solvent

Polar aprotic

SN1 Solvent

Polar protic

E2 Solvent

Polar aprotic

E1 Solvent

Polar protic

SN2

Mechanism: Concerted where the nucleophile attacks from backside and leaving group leaves at the same time

SN2

Stereochemistry: Backside attack of nucleophile causes inversion of configuration

SN2

k[substrate][nucleophile]

SN1

Mechanism: Stepwise, where the leaving group leaves first, forming a carbocation intermediate. The nucleophile attacks the carbocation intermediate in the next step.

SN1

Stereochemistry: Not stereospecific. Nucleophile attacks from front and back, resulting in identical molecules, enantiomers, or diastereomers.

SN1

k[substrate]

E2

Mechanism: Concerted where base attacks βC-H, the electrons from that C-H bond form the new pi bond, and the leaving group leaves

E2

Stereo/Regiochemistry: βC-H and LG must be anti periplanar, resulting in stereospecific products. If multiple E2 products, the more stable alkene is the major E2 product.

E2

k[substrate][base]

E1

Mechanism: Stepwise, where the leaving group leaves first, forming a carbocation intermediate. As the new base attacks, the electrons from the βC-H bond moves to form the new pi bond.

E1

Stereo/Regiochemistry: Not stereospecific. The more stable alkene is the major E1 product.

E1

k[substrate]

Identical molecules

Superimposable

Enantiomers

Non-superimposable, but mirror images

Diastereomers

Non-superimposable and not mirror images

Weak nucleophiles and bases

NH3, R2NH, H2O, ROH

SN1 and E1 competition

Strong nucleophiles and strong bases

R-, OH-, H2N-, RO-, R2N-

SN2 and E2

Strong base, poor nucleophile

H-

E2

Strong nucleophile, weak base

Halides: Cl-, Br-, I-

Sulfur nucleophiles: HS-, RS-

SN2

Leaving groups

OH- and ethers

Can be protonated with strong acids to form good leaving groups

Polar protic solvents

Stabilizes leaving groups and nucleophiles

NH3, OH hydrogen bonding

Polar aprotic solvents

Cannot stabilize leaving groups and nucleophiles