S_N1 and S_N2 Mechanism

• The S_N2 Reaction

Mechanism and Kinetics

The reaction between methyl bromide and hydroxide ion to yield methanol follows second order kinetics; that is, the rate depends upon the concentrations of both reactants :

 CH₃Br +^-OH → CH₃OH + Br^-

rate = K [CH₃Br] [OH^–]

The simplest way to account for the kinetics is to assume that reaction requires a collision between a hydroxide ion and a methyl bromide molecule. In its attack, the hydroxide ion stays far away as possible from the bromine; i.e. it attacks the molecule from the rear and begin to overlap with the tail of the sp³ hybrid orbital holding Br. The reaction is believed to take place as shown:

In the T.S. the carbon is partially bonded to both -OH and -Cl; the C-OH bond is not completely formed, the C-Cl bond is not yet completely broken. Hydroxide has a diminished -ve charge, since it has begun to share its electrons with carbon. Bromine has developed a partial negative charge, since it has partly removed a pair of electrons from carbon. At the same time, of course, ion dipole bonds between hydroxide ion and solvent are being broken and ion-dipole bonds between bromide ion and solvent are being formed.

As the -OH becomes attached to C, 3 bonds are forced apart (120^o) until they reach the spoke arrangement of the T.S ; then as bromide is expelled, they move on to the tetrahedral arrangement opposite to the original one. The process has often been likened to the turning  inside out of an umbrella in a gale.

Stereochemistry

Both 2-bromo-octane and 2-octanol are chiral.

The (-) bromide and the (-) alcohol have similar configurations, i.e. -OH occupies the same relative position in the (-) alcohol as -Br does in the bromide.

When (-)-2-bromooctane is allowed to react with sodium hydroxide under S_N2 conditions, there is obtained (+)-2-octanol.

In Fisher projection the above reaction can be represented as follows 

We see that -OH group has not taken the position previously occupied by -Br; the alcohol obtained has a configuration opposite to the bromide. A reaction that yields a product whose configuration is opposite to that of the reactant is said to proceed  with inversion of configuration.

Refer to the following video for S_N2 reactions

Reactivity

In S_N2 reactions the order of reactivity of RX is CH₃X>1^o>2^o>3^o.

Differences in rate between two S_N2 reactions seem to be chiefly due to steric factors (bulk of the substituents) and not due to electronic factors i.e. ability to withdraw or release electrons.

• The S_N1 Reaction

Mechanism and Kinetics

The reaction between tert-butyl bromide and hydroxide ion to yield tert-butyl alcohol follows first order kinetics; i.e., the rate depends upon the concentration of only one reactant,
tert-butyl bromide.

Rate = K[Br]

S_N1 reaction Þ follows first order kinetics.

Stereochemistry

When (-)-2-bromo octane is converted into the alcohol under conditions where first-order kinetics are followed, partial racemization is observed.

The optically active bromide ionizes to form bromide ion and the flat carbocation. The nucleophilic reagent then attaches itself to carbonium ion from either face of the flat ion.

It the attack were purely random, we would expect equal amounts of two isomers; i.e. we would expect only the racemic modification. But the product is not completely racemized, for the inverted product exceeds its enantiomer.

We can say in contrast to S_N2 reaction, which proceeds with complete inversion; an S_N1 reaction proceeds with racemization though may not be complete.

r.d.s --> formation of carbonium ion.

Reactivity of an alkyl halide depends chiefly upon how stable a carbonium ion it can form.

In S_N1 reactions the order of reactivity of alkyl halides  is Allyl, benzyl >3^o>2^o>1^o>CH₃ X.

3⁰ alkyl halides undergo S_N1 reaction very fast because of the high stability of 3⁰ carbocations.

Refer to the follwoing video for S_N1 reactions

Order of reactivity of alkyl halides towards S_N1 and S_N2 reactions as follows:

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