Nucleophilic substitution reactions are a major way organic molecules change one functional group into another. In these reactions, a nucleophile donates an electron pair to a carbon while a leaving group departs. The two main pathways, SN1 and SN2, differ in timing, structure, rate law, and stereochemical outcome.
Knowing the difference helps predict products in synthesis and explain why reaction conditions matter.
Key Facts
- SN1 rate law: rate = k[RX], so only the substrate concentration affects the rate.
- SN2 rate law: rate = k[RX][Nu-], so both substrate and nucleophile concentrations affect the rate.
- SN1 mechanism: leaving group leaves first to form a carbocation, then the nucleophile attacks.
- SN2 mechanism: nucleophile attacks from the backside as the leaving group leaves in one concerted step.
- SN1 is favored by tertiary substrates, weak or neutral nucleophiles, polar protic solvents, and stable carbocations.
- SN2 is favored by methyl or primary substrates, strong nucleophiles, polar aprotic solvents, and low steric hindrance.
Vocabulary
- Nucleophile
- A nucleophile is an electron-rich species that donates an electron pair to form a new bond.
- Leaving group
- A leaving group is an atom or group that departs with a pair of electrons during a substitution reaction.
- Carbocation
- A carbocation is a positively charged carbon intermediate with only six electrons in its valence shell.
- Polar protic solvent
- A polar protic solvent has O-H or N-H bonds and can hydrogen bond to ions, often stabilizing carbocations and leaving groups.
- Backside attack
- Backside attack is SN2 nucleophilic attack from the side opposite the leaving group, causing inversion of configuration.
Common Mistakes to Avoid
- Treating SN1 and SN2 as if they have the same rate law is wrong because SN1 depends only on substrate concentration, while SN2 depends on both substrate and nucleophile concentrations.
- Choosing SN2 for a tertiary alkyl halide is usually wrong because bulky groups block backside attack and make the one-step collision very difficult.
- Forgetting stereochemistry in SN2 is wrong because backside attack causes inversion at the reacting chiral carbon.
- Assuming all strong nucleophiles favor SN1 is wrong because strong nucleophiles usually speed up SN2, while SN1 often works with weak nucleophiles if the carbocation is stable.
Practice Questions
- 1 For the SN2 reaction CH3Br + OH- -> CH3OH + Br-, if rate = k[CH3Br][OH-], k = 0.25 M^-1 s^-1, [CH3Br] = 0.10 M, and [OH-] = 0.20 M, calculate the reaction rate.
- 2 An SN1 reaction has rate = k[(CH3)3CCl]. If k = 0.015 s^-1 and [(CH3)3CCl] = 0.40 M, calculate the reaction rate. What happens to the rate if the nucleophile concentration is doubled?
- 3 A student reacts 2-bromobutane with a strong nucleophile in a polar aprotic solvent. Predict whether SN1 or SN2 is more likely and explain the expected stereochemical result at the reacting carbon.