Conditions for SN1 vs E1 vs SN2 vs E2

What conditions favor SN2?

*Primary > Secondary NO tertiary

*Allylic or benzylic will work

*Want a good nucleophile → can be a SB or WB

*Want a polar aprotic solvent

*Want a good leaving group

**Reaction is concerted

*Summary: A good leaving group, less sterically hindered substrate, and a polar aprotic solvent = faster rate of reaction!

What conditions favor E2?

*Tertiary > secondary > primary

*Allylic or benzylic will work

*Vinyl halides will work (they will form an alkyne)

*Want a good nucleophile → is also a SB

*Want a polar aprotic solvent (BUT reaction can still occur with a polar protic solvent)

*Want a good leaving group

**Reaction is concerted

***The beta-hydrogen and the leaving group MUST BE anti-periplanar

What conditions favor SN1?

*Tertiary NO secondary or primary

*Allylic and benzylic work

*Want a poor nucleophile (and usually a WB)

*Want a polar protic solvent

*Want a good leaving group

**Step-wise reaction

*Summary: A better leaving group, more stable C+, and polar protic solvent = faster rate of reaction

What factors favor E1?

*Tertiary NO secondary or primary

*Allylic and benzylic will work

*Want a poor nucleophile → also a WB

*Want a polar protic solvent

*Want a good leaving group

*NEED HEAT (50 degrees Celsius or more)

**Step-wise reaction

*A better leaving group, more stable C+, and polar protic solvent = faster rate of reaction