NucleophilicNucleophilic SubstitutionSubstitution

Nucleophilic Substitutions(SN1, SN2) of Halides

The Unimolecular (SN1) Reaction: 2 steps

R X R + X(1)

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.

R + Nu R-Nu(2)

Carbocation and nucleophile combine.

Fast step.

Overall reaction: R-X + Nu- R-Nu + X-.

Rate of the reaction = k[R-X]

SN1 Reaction

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.

C X

R

RR

C+

R

RR

rate determining step Nu

C Nu

R

RR

+ CNu

R

R

R

x

The Bimolecular (SN2) Reaction: 1 step

(1)

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-.

(2)

Transition state dissociates to form product, Nu-R, and halide ion, X-.

Rate of the reaction = k[R-X][Nu-]

Nu + R-X R XNuδ-δ+

R XNuδ-δ+

Nu-R + X

SN2 reaction

C X

H

HH

N u : C XNu

H H

H

C

H

NuH

H

Steric factors are important: 1º > 2º > 3º.

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.

x

OEt+ AgCl

Experiment: Structural Effects (Part A only)

OEt Cl

Br

Possible Products 0.2 mL (4 drops) Possible Products

OEt

95% EtOHAgNO3

Clacetone

NaI

I+ NaCl

Cl

OEt I

ISN1 SN2

Br

2 mL 2 mL

OEt

I

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 Students mustmust include statements in their include statements in their lab reportslab reports about the following:about the following:1. Alkyl halide structure (1º, 2º, 3º) and SN22. Alkyl halide structure (1º, 2º, 3º) and SN13. The leaving group (Br vs. Cl)4. Aryl halide reactivity5. Temperature and rate

All statements must be based on observations.

Grading: 50% on the written report and50% on observations, results, discussion and conclusions.