Nucleophilic SubstitutionNucleophilic Substitution

Nucleophilic SubstitutionsNucleophilic Substitutions(S(SNN1, S1, SNN2) of Halides2) of Halides

S – Substitution

N – Nucleophilic

1 – Unimolecular

2 – Bimolecular

r.d.s. – Rate Determine Step

SSNN1 Reaction1 Reaction

R X R + Xr.d.s.

(1)

R + Nu R-Nu(2)

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

SSNN1 Reaction1 Reaction

Carbocation stability is important factor: 3º > 2º> 1º.

Rate of the reaction = k[R-X]

Polar, protic solvents such as ethanol support carbocation formation (stabilize

transition state).

Aprotic solvents such as acetone do NOT support carbocation formation.

Weak nucleophile is OK.

C X

R

R

RC+

R

RR

rate determining step Nu

C Nu

R

R

R

+ CNu

R

R

R

X

The Bimolecular (SThe Bimolecular (SNN2) Reaction2) Reaction

(1)

(2)

Nu + R-X R XNu--r.d.s. +

R XNu-+

Nu-R + X-

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

SSNN2 reaction2 reaction

C X

H

HH

Nu : C XNu

H H

H

C

H

NuH

H

r.d.s

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

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

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

Experiment: Structural EffectsExperiment: Structural Effects

OEt Cl I + NaCl

Possible SN1 Products Reactant Possible SN2 Products

BrOEt I + NaBr

OEt

+ AgClAgNO3

EtOH

Clacetone

NaI

I

+ NaCl

OEt + AgCl Cl

OEt Br I

ISN1 SN2

+ AgCl

+ AgBr

+ AgBr

+ NaCl

+ NaBr

ExperimentExperiment

- Only perform PART A. Use bromobenzene instead of crotyl chloride.

- You will have 2 sets of 5 test tubes with 0.2 mL (4 drops) of each reactant.

- You will monitor time for cloudiness. - If NaI/acetone does not react in 30 min, heat to 50 °C in water bath

(loose cover cork!!!)- If EtOH/AgNO3 tubes do not react in 30 min, also heat to 50 °C.

The followingThe following discussions discussions must be included in the must be included in the lab report:lab report:

1. RX structure affect on SN22. RX structure affect on SN13. The leaving group effect.4. ArX reactivity5. Temperature v.s. rate

All statements must be based on observations.

Grading: 50% on the written report and 50% on observations / results / discussion /

conclusions.