November 26 - Substitution Reactions Flashcards
Sn2
A leaving group is replaced by a nucleophile in a single step
What bases are better leaving groups?
Weak bases, such as halogens
What happens to stereochemistry during Sn2?
Stereochemistry is inverted
Factors affecting nucleophilicity
For atoms of similar size, stronger base = stronger nucleophile
Larger atom = better nucleophile
Steric hindrance = worse nucleophile
Factors affecting electrophilicity
Less steric hindrance = better electrophile
Better leaving group (weaker base) = more reactive
Rate law for Sn2
What affects it?
k[nucleophile][electrophile]
How good is the nucleophile?
How hindered is the electrophile?
How good is the leaving group?
Rate law for Sn1
What affects it?
k[electrophile]
How stable is the carbocation?
How good is the leaving group?
Basicity affecting leaving
The weaker the base, the better the leaving group
SN2
- how many steps
- rate of rxn controlled by what?
- which molecules do SN2?
- product configuration
- does strength of nucleophile affect rxn rate?
- Two-step rxn
- Rate of rxn controlled by steric hindrance
- Methyl halides + 1°/2° alkyl halides do SN2
- Product has inverted configuration
- The better the nucleophile, the faster the rxn rate
Tertiary halides don’t do SN2
SN1
- how many steps
- rate-determining step
- which molecules do SN1?
- product configuration
- does strength of nucleophile affect rxn rate?
- A 2-step rxn with carbocation intermediate
- Rxn rate depends on [alkyl halide] only
- Only tertiary alkyl halides do SN1 rxns
- They form 2 stereoisomers
- Strength of nucleophile doesn’t affect rxn rate
8.3, p. 274
Which alkyl halides undergo SN1 and SN2 reactions?
Only tertiary alkyl halides undergo SN1
Primary and secondary alkyl halides undergo SN2
Which elecrophiles react the fastest in Sn2 reactions?
methyl > 1° > 2° > 3°
