SlideShare a Scribd company logo

Nucleophilic Substitution

•
•
U
University of Houston

This document discusses nucleophilic substitution reactions, specifically SN1 and SN2 reactions. It describes the two-step SN1 reaction, where the first step is rate determining and involves formation of a carbocation intermediate. It also describes the one-step SN2 reaction, where the rate determining step involves formation of a pentacoordinate transition state. Finally, it provides an experiment to observe the effects of structure on SN1 and SN2 reactions.

1 of 9
Downloaded 450 times
Nucleophilic Substitution
Nucleophilic Substitutions (SN1, SN2) of Halides
Nucleophilic Substitution
The Unimolecular (SN1) Reaction: 2 steps (1) R X R + X R-X dissociates, forming a carbocation intermediate, R+, and the leaving group, X-. Slow step = rate determining step (r.d.s.). Entails only one species, R-X. (2) R + Nu R-Nu Carbocation and nucleophile combine. Fast step. Overall reaction: R-X + Nu- R-Nu + X-. Rate of the reaction = k[R-X]
SN1 Reaction R R rate determining s tep Nu R C X C+ R R R R R R C Nu + Nu C R R R x Carbocation stability is important factor: 3º > 2º> 1º. Polar, protic solvents such as ethanol support carbocation formation (stabilize intermediate). Aprotic solvents such as acetone do NOT support carbocation formation. Weak nucleophile is OK.
The Bimolecular (SN2) Reaction: 1 step δ+ δ- (1) Nu + R-X Nu R X Nucleophile combines with alkyl halide to form pentacoordinate transition state. Slow step = rate determining step (r.d.s.). Entails two species, R-X and Nu-. δ+ δ- (2) Nu R X Nu-R + X Transition state dissociates to form product, Nu-R, and halide ion, X-. Rate of the reaction = k[R-X][Nu-]
SN2 reaction H H H Nu : C X Nu C X Nu C H H H H H H Steric factors are important: 1º > 2º > 3º.x Polar, aprotic solvents such as acetone increase rate of SN2. Aprotic solvent destabilizes nucleophile (increases its reactivity). Strong nucleophile is necessary. The nature of the leaving group is important in both SN1 and SN2 reactions: I > Br > Cl.
Experiment: Structural Effects (Part A only) Possible Products 0.2 mL (4 drops) Possible Products OEt Cl I OEt Br I 2 mL 2 mL 95% EtOH NaI + AgCl AgNO3 acetone + NaCl OEt Cl I SN1 SN2 OEt Cl I OEt Br I
Experiment - You will have 2 series of 5 test tubes with 0.2 mL (4 drops) of each reactant. - You will monitor time for cloudiness. - If NaI/acetones do not react after 30 min, heat to 50 °C in water bath (loose corks!!!) - If 95% EtOH/AgNO3 tubes do not react after 30 min, also heat to 50 °C. Students must include statements in their lab reports about the following: 1. Alkyl halide structure (1º, 2º, 3º) and SN2 2. Alkyl halide structure (1º, 2º, 3º) and SN1 3. The leaving group (Br vs. Cl) 4. Aryl halide reactivity 5. Temperature and rate All statements must be based on observations. Grading: 50% on the written report and 50% on observations, results, discussion and conclusions.

More Related Content

Nucleophilic Substitution

  • 2. Nucleophilic Substitutions (SN1, SN2) of Halides
  • 4. The Unimolecular (SN1) Reaction: 2 steps (1) R X R + X R-X dissociates, forming a carbocation intermediate, R+, and the leaving group, X-. Slow step = rate determining step (r.d.s.). Entails only one species, R-X. (2) R + Nu R-Nu Carbocation and nucleophile combine. Fast step. Overall reaction: R-X + Nu- R-Nu + X-. Rate of the reaction = k[R-X]
  • 5. SN1 Reaction R R rate determining s tep Nu R C X C+ R R R R R R C Nu + Nu C R R R x Carbocation stability is important factor: 3º > 2º> 1º. Polar, protic solvents such as ethanol support carbocation formation (stabilize intermediate). Aprotic solvents such as acetone do NOT support carbocation formation. Weak nucleophile is OK.
  • 6. The Bimolecular (SN2) Reaction: 1 step δ+ δ- (1) Nu + R-X Nu R X Nucleophile combines with alkyl halide to form pentacoordinate transition state. Slow step = rate determining step (r.d.s.). Entails two species, R-X and Nu-. δ+ δ- (2) Nu R X Nu-R + X Transition state dissociates to form product, Nu-R, and halide ion, X-. Rate of the reaction = k[R-X][Nu-]
  • 7. SN2 reaction H H H Nu : C X Nu C X Nu C H H H H H H Steric factors are important: 1º > 2º > 3º.x Polar, aprotic solvents such as acetone increase rate of SN2. Aprotic solvent destabilizes nucleophile (increases its reactivity). Strong nucleophile is necessary. The nature of the leaving group is important in both SN1 and SN2 reactions: I > Br > Cl.
  • 8. Experiment: Structural Effects (Part A only) Possible Products 0.2 mL (4 drops) Possible Products OEt Cl I OEt Br I 2 mL 2 mL 95% EtOH NaI + AgCl AgNO3 acetone + NaCl OEt Cl I SN1 SN2 OEt Cl I OEt Br I
  • 9. Experiment - You will have 2 series of 5 test tubes with 0.2 mL (4 drops) of each reactant. - You will monitor time for cloudiness. - If NaI/acetones do not react after 30 min, heat to 50 °C in water bath (loose corks!!!) - If 95% EtOH/AgNO3 tubes do not react after 30 min, also heat to 50 °C. Students must include statements in their lab reports about the following: 1. Alkyl halide structure (1º, 2º, 3º) and SN2 2. Alkyl halide structure (1º, 2º, 3º) and SN1 3. The leaving group (Br vs. Cl) 4. Aryl halide reactivity 5. Temperature and rate All statements must be based on observations. Grading: 50% on the written report and 50% on observations, results, discussion and conclusions.