Organic ReactionsSusbstitution – SN

1

Dr. Sapna Gupta

Kinetics of Nucleophilic Reaction

• Rate law is order of reaction

• 0 order is when rate of reaction is unaffected by change in concentration of the reactants

• 1st order is when rate of reaction doubles when one of the reactants is doubled

• 2nd order is when rate of reaction quadruples when two of the reactants are doubled

• Order of reaction explains the reaction mechanism

Dr. Sapna Gupta/Substitution Reactions 2

• In this reaction a nucleophile is a species with an unshared electron pair which reacts with an electron deficient carbon

• A leaving group is substituted by a nucleophile

• Examples of nucleophilic substitution

Nucleophilic Substitution Reactions

Dr. Sapna Gupta/Substitution Reactions 3

SN1 Reaction

• Unimolecular nucleophilic substitution.

• Two step reaction with carbocation intermediate.

• Rate is first order in the alkyl halide, zero order in the nucleophile.

• Racemization occurs.

Dr. Sapna Gupta/Substitution Reactions 4

The SN1 Reaction Mechanism

Dr. Sapna Gupta/Substitution Reactions 5

SN1 Energy Diagram

• There are three transition states. Step1 crosses the highest energy barrier and is rate determining

Dr. Sapna Gupta/Substitution Reactions 6

Stereochemistry of SN1

Racemization: inversion and retention

Dr. Sapna Gupta/Substitution Reactions 7

Factors Affecting Rates of SN1 Reactions

1. Substrate: 3° > 2° > 1° >> CH3X

2. Leaving group: should be weak base(like SN2)

3. Nucleophile: no effect since its not part of the transition state –but in general weak Nu can also react.

4. Solvent: Polar protic solvent best, it solvates ions strongly with hydrogen bonding.

Dr. Sapna Gupta/Substitution Reactions 8

1) Nature of Substrate

Substrate: 3° > 2° > 1° >> CH3X

a) Order follows stability of carbocations (opposite to SN2)

b) More stable ion requires less energy to form

c) Resonance can also stabilize cation (e.g. allyl cation – C=C-C+)

Dr. Sapna Gupta/Substitution Reactions 9

2) Leaving Group

• Leaving group is key in forming the carbocation intermediate for transition state.

• Larger halides ions are better leaving groups

• In acid, OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halide

• p-Toluenesulfonate (TosO-) is excellent leaving group

3) Nucleophile

• Does not matter since nucleophile is not part of the rate determining step.

Dr. Sapna Gupta/Substitution Reactions 10

4) Solvent

Polar solvents help with ionization and stability of the carbocation.

Dr. Sapna Gupta/Substitution Reactions 11

SN2 or SN1?

SN2 SN1

Substrate Primary or methyl Tertiary

Nucleophile Strong nucleophile Weak nucleophile (may also be solvent)

Solvent Polar aprotic solvent Polar protic solvent, silver salts

Kinetics [substrate][Nu] [substrate]

Stereochemistry Inversion Racemic mixture

Rearrangement No Yes

Dr. Sapna Gupta/Substitution Reactions 12

Solved Problems

• Predict mechanism and stereochemistry of each product

• Predict mechanism of the following reaction

• Predict mechanism and stereochemistry of each product

Cl

CH3OH/ H2O

OH OCH3

HCl+

(R)-2-Chlorobutane

+ +

+ +Na+ CN- Na+ Br -

DMSOBr CN

Br

CH3S-Na+

SCH3

Na+Br-+ +acetone

(R)-2- Bromobutane

Dr. Sapna Gupta/Substitution Reactions 13

Key Words/Concepts

•Substitution Reaction•Nucleophile•Electrophile•Leaving group•1st order reaction (unimolecular)•2nd order reaction (bimolecular)•Transition state•Rate determining step•Carbocation

•Polar protic solvent•Polar aprotic solvent•Non polar solvent•Solvolysis