Alkyl Halide Reactions
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SN2 stands for
substitution, nucleophile, bimolecular
What effect does an SN2 reaction have on stereochemistry?
Configuration is inverted (e.g. R to S or vice versa)
- Most reactive with methyl, primary, and sometimes secondary.
- Will not react with tertiary, vinylic halides and aryl halides.
- Nucleophilicity roughly parallels basicity
- Usually increases going down periodic table
- Usually negatively charged nucleophiles are more reactive than neutral ones
SN2 Leaving Group
- Good leaving groups stabilize the negative charge:
- TosO, I, Br, Cl
- Poor leaving groups:
- OH, NH2, OR, F
- Polar aprotic solvents are the best for SN2
- Good solvents include:
- HMPA, CH3CN, DMF, DMSO
The SN1 reaction occurs through....
A carbocation intermediate
What effect does an SN1 reaction have on stereochemistry?
Theoretically no effect, except that racemic products are not exactly 50/50 due to ion pairs getting in the way.
- Stability of the carbocation increases reaction rate:
- Tertiary > Secondary = Benzylic = Allylic
SN1 Leaving Group
- Often carried out in acidic conditions.
- Good: TosO, H2O, I > Br > Cl > OH
Does not effect reaction since it does not take part in the rate-limiting step
- Stabilizes transition state, not reactant.
- Strong polar solvents are good: Water
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