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SN1 and SN2 Mechanism

 

SN1

SN2

 

Steps

Two :  
(1) R:Xl → R+ + X-

 (2) R+ + Nu- l →RNu

One :
R:X + Nu- l → RNu + X-

 

Rate

            =K [RX] (1st order)

=K[RX] [:Nu-] (2nd order)

TS of slow step

Stereochemistry

Inversion and racemization

Inversion (backside attack)

Molecularity

Unimolecular

Bimolecular

Reactivity
structure of R
Determining Factor

Nature of X
Solvent effect on rate

3o> 2o> 1o> CH3

Stability of R+

RI> RBr> RCl> RF

Rate increases in polar solvent

CH3> 1o> 2o> 3o

Steric hindrance in R group

RI> RBr> RCl> RF

with Nu- there is a large rate increase in polar aprotic solvents.

Effect of nucleophile

No effect as it does not appear in the rate expression.

Rate depends on nucleophilicity

I-  > Br-  > Cl; RS-  > RO-

Catalysis

Lewis acid, eg. Ag+, AlCl3, ZnCl2

None

Competitive reaction

Elimination, rearrangement

Elimination

 

  • The SN2 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 (120o) 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 SN2 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 SN2 reactions

Reactivity

In SN2 reactions the order of reactivity of RX is CH3X>1o>2o>3o.

Differences in rate between two SN2 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 SN1 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]

SN1 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 SN2 reaction, which proceeds with complete inversion; an SN1 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 SN1 reactions the order of reactivity of alkyl halides  is Allyl, benzyl >3o>2o>1o>CH3 X.

30 alkyl halides undergo SN1 reaction very fast because of the high stability of 30 carbocations.

Refer to the follwoing video for SN1 reactions

Order of reactivity of alkyl halides towards SN1 and SN2 reactions as follows:

Click here to access the Organic Chemistry Revision Notes and IIT JEE Chemistry Syllabus

To read more, Buy study materials of Halo Alkanes & Halo Arene comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.

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