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SN1 and SN2 Reactions: Difference - Mechanism

SN1 and SN2 Reactions: Difference - Mechanism


Difference Between SN1 & SN2

 

 

Reaction

 

 

SN2

 

 

SN1

 

 

R-x

 

 

Methyle < Primary...

 

 

Tertiary < Secondary…

 

 

Nucleophile

 

 

Strong Nucleophile

 

 

Poor Nucleophile

 

 

Mechanism

 

 

One step

 

 

Two steps

 

 

Rate limiting step

 

 

Bimolecular T.S

 

 

Carbon cation

 

 

Rate Law

 

 

Rate=k[R-X][Nu]

 

 

Rate=k[R-X]

 

 

Sterochemistry

 

 

Inversion of configuration

 

 

Mixed configuration

 

 

Solvent

 

 

Polar a protic

 

 

Polar protic

 

 

 

 

Nucleophilic substitution

Nucleophilic substitution at carbon is of broad synthetic utility and has received exceptionally detailed mechanistic study by organic chemists. The goal developing a coherent mechanistic interpretation was first undertaken by С. К. Ingold and E. D. Hughes in England in the 1930s. Their studies laid the basis for current understanding. Nucleophilic substitution reactions may involve several different

combinations of charged and uncharged species as reactants of.

SN1 and SN2 Reactions: Difference - Mechanism


 

Aliphatic Substitution

In nucleophilic aliphatic substitution the attacking (electron donating) reagent(the nucleophile) brings an electron pair to the substrate, using this pair to form the new bond, and the leaving group (the nucleofuge) comes away with an electron pair:

SN1 and SN2 Reactions: Difference - Mechanism


 

This equation says nothing about charges. Nuclephile Y may be neutral or negatively charged.

 

 

Mechanism of Aliphatic Substitution

Several distinct mechanisms are possible for aliphatic nucleophilic substitution reactions, depending on the substrate, nucleophile, leaving group, and reaction conditions. In all of them, however, the attacking reagent carries the electron pair with it, so that the similarities are greater than the differences.there for these reactions can be classified as bellow :

 

 

The SN2 Mechanism

The designation SN2 stands for substitution nucleophilic bimolecular. In this mechanism, there is backside attack, the nucleophile approaches the substrate from a position 180 away from the leaving group. The reaction is a one-step process with no intermediate The C_Y bond is formed as the C_X bond is broken to generate transition state(I):

SN1 and SN2 Reactions: Difference - Mechanism


 

The energy necessary to break the C_X bond is supplied by simultaneous formation of the C_Y bond. The position of the atoms at the top of the curve of free energy of activation is represented as transition state I. Of course, the reaction does not stop here since this is the transition state. The group X must leave as the group Y comes in, because at no time can the carbon have more than eight electrons in its outer shell. When the transition state is reached, the central carbon atom has gone from its initial sp3 hybridization to an sp2 state with an approximately perpendicular p orbital. One lobe of this p orbital overlaps with the nucleophile and the other with the leaving group.

 

 

The SN1 Mechanism

The most ideal version of the SN1 mechanism (substitutional nucleophilic unimolecular) consists of two steps, (once again, possible charges on the substrate and nucleophile are not shown):

SN1 and SN2 Reactions: Difference - Mechanism


The effect of Concentration [R-X] and [Nu] on SN1&SN2(Knetics for the machanisms):

 

 

1. SN2 mechanism:

The reaction between [-SMe] (as Nucleophile ) and Me-I were indeed SN2 as we would expect:

MeS-Na + Me-I------ <MeS-Me + NaI

First we would keep the concentration of [SMe] constant and very that of [MeI] and see what happened to the rate .Then we would keep the concentration of [MeI] constant and very that of [MeS] and see what happened to the rate.If the reaction is indeed SN2 we should get linear relationship in both cases. The first graph tells us that the rate is proportional to [MeI] , that is rate= k1[MeI] and the second graph that is proportional to [MeSNa] , that is rate =k2[MeSNa] .

 

The Question Why are the slopes different?

 

SN1 and SN2 Reactions: Difference - Mechanism


 

If you look at the rate equation for the reaction you will see that we have incorporated a constant concentration of one of the reagents into what appears to be the rate

Constant for the reaction .the true rate equation is :

Rate =k2[MeSNa][MeI]

 

If [MeSNa] constant the equation becomes:

Rate =k1[MeI] where k1=k2[MeSNa]

 

If [MeI] constant the equation becomes:

Rate =k2[MeSNa] where k2=k1[MeI]

Slop1=k1=k2[MeSNa],but slope2 =k2=k2[MeI].

 

We can easily measure the true rate constant k2 from these slope because we know the constant values for  [MeSNa] in the first experiment and the for[MeI] in the second .

 

 

2. SN1 mechanism:

 

In this mechanism the starting material tertiary alkyl halide [t-BuBr] with base [NaOH] the mechanism following bellow :

He formation of Cation is the rate determining and the the Nucleophile attach the carbon cation .The rate of disappearance of t-BuBr is simply the rate of slow step is called the (the rate determining ) step. It’s a Unimolecular reaction with simple rate of equation :

Rate =k1[t-BuBr]

 

They look like this when we vary [t-BuBr] at constant [NaOH] and then vary[NaOH] at constant [t-BuBr].The slope of the first graph is simply the first order constant because :

Rate =k1[t-BuBr]

 

But the second graph is zero, the rate determining step does not involve NaOH adding more of it does not speed up the reaction. The reaction shows first order kinetics .

SN1 and SN2 Reactions: Difference - Mechanism


 

 

 

References

1.      Advanced Organic Chemistry ,fourth edition ,(Francis A.Carey &Richard J.Sudberg),University of Virginia.

2.     Michael B. Smith and Jerry March March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, Sixth Edition.

3.     Morrison and Boyd Organic Chemistry, Sixth Edition.


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