Organic Chemistry Chapter 6 Reaction Mechanism - Substitution and Elimination - Nanoplasmonic Research Group

Chemical Reactions in Organic Chemistry So far, you’ve studied addition reactions Now, we will consider substitution and elimination reactions Interestingly, these two are in competition Under a given situation, you can predict which reaction would occur !!!

1. Substrate (steric, orbital overlap), 2. Nucleophile (strong, weak), You need carefully consider 4 reaction variables for the prediction !!!! 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 SN2 Mechanism The reaction involves a transition state in which both reactants are together The rate of the reaction depends on both the nucleophile and the substrate concentrations Every SN2 displacement occurs with inversion of configuration

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 neutral molecules Elements low in the same column In the same row, the less electronegative

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 reaction Polar aprotic solvent (acetone) Generates more nucleopilic anion, accelerating SN2 reaction

Elimination (dehydrohalogenation) The nucleophile acts as a base, removing 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

The E2 Mechanism 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 stereospecifically!!!

The E1 Mechanism The same first step as the SN1 mechanism Compete with SN1 and in some cases E2

E2 E1

Generic Reaction-Energy Diagrams

Predicting the products: Substitution versus Elimination