Reaction Mechanism- Substitution and Elimina-

tion -

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 6

Chemical Reactions in Organic Chemistry

• So far, you’ve studied addition reac-tions

• Now, we will consider substitution and elimination reactions

• Interestingly, these two are in competi-tion

• Under a given situation, you can predict which reaction would occur !!!

You need carefully consider 4 reaction variables for the pre-

diction !!!!

1. Substrate (steric, orbital overlap), 2. Nucleophile

(strong, weak), 3. Leaving group (good, poor),

4. Solvent (protic, aprotic)

Nucleophilic Substitution

• In a typical reaction, a nucleophile displaces the leaving group binding to a substrate

• Nucleophile & Leaving group• Substitution mechanisms– SN1 vs SN2, depending on how many

molecules involve in transition state

• The reaction involves a transition state in which both reactants are to-gether

• The rate of the reaction depends on both the nucleophile and the sub-strate concentrations

• Every SN2 displacement occurs with inversion of configuration

The SN2 Mechanism

Order of Reactivity in SN2- Steric Effect -

The SN1 Mechanism (I)

• Distinct two steps while SN2 occurs with both events at the same time

• The rate of the reaction does not depend on the concentration of the nucleophile

The SN1 Mechanism (II)

• If the carbon bearing the leaving is stereogenic, the reaction occurs mainly with loss of optical activity (racemization)

Order of Reactivity in SN1- Carbocation stability -

SN2 SN1

The Nucleophile• Strong nucleophiles favor SN2

• Negaive ions > corresponding neu-tral molecules

• Elements low in the same column• In the same row, the less elec-

tronegative

The Leaving Group• A good leaving group reduces the

barrier to a reaction• Stable anions that are weak bases

are usually excellent leaving groups and can delocalize charge

The Solvent• Polar vs Nonpolar• Polar protic solvent– Stabilize carbocation, accelerating SN1 reac-

tion

• Polar aprotic solvent (acetone)– Generates more nucleopilic anion, accelerating

SN2 reaction

• The nucleophile acts as a base, re-moving a proton from the carbon next to the one that contains the halogen (leaving group)

• Please refer to page 195• As with the substitution, there are two

possible mechanisms for elimination reaction

Elimination (dehydrohalogena-tion)

• A proton is transferred to the base as leaving group begins to depart

• Transition state combines leaving of X and transfer of H

• Product alkene forms stereospecifi-cally!!!

The E2 Mechanism

• The same first step as the SN1 mech-anism

• Compete with SN1 and in some cases E2

The E1 Mechanism

E2 E1

Generic Reaction-Energy Dia-grams

Predicting the products: Substitution versus Elimination