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Nucleophilic substitution reaction of saturated carbon

As we see The Cl group has been replaced by OH. This a nucleophilic substitution reaction the nucleophile OH attacked the E+ which is the carbon bonded to halogen.

The reaction mechanism may be one of two which are called SN1 and SN2.

fig.2

The Differences between SN1 and SN2:

Kinetic overview:

SN1 SN2
It is Nucleophilic Substitution of Unimolecular reaction It is Nucleophilic Substitution of Bimolecular reaction
It is first order reaction: Its rate depends on the concentration of electrophile only not nucleophile so It is called unimolecular It is second order reaction: Its rate depends on the concentration of electrophile and nucleophile so It is called bimolecular
R = K [E+] R = K [E+] [Nu]

Mechanism:

SN1

It reacts in two steps:

  • At first the leaving group (halide) moves leading to formation of carbocation.
  • Second the nucleophile attacks carbocation.

SN2

This reaction occurs in one step:

The nucleophile attacks electrophile before leaving group leaves the compound making a transition state in which the carbon becomes planner Nu dash lines represents the bond formation while X dash lines represents bond breaking.

Factors determine the mechanism of reaction:

SN1:

Stabilization of carbocation is the very important factor which makes a reaction undergoes SN1 mech.

Different structures that stabilizes carbocation:

1- Alkyl substituents stabilize a carbocation:

3 Factors That Stabilize Carbocations

Tertiary carbocation very stable then It will undergo SN1 mechanism.

(CH3)3–C–Br + OH —->

2- An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations:

Allylic carbocation:

The carbocation is also stabilized by π or lone pair electrons.

Examples:

  • Symmetrical allylic:

 Cyclohexenol + HBr

fig.8

Symmetrical allyic has only one product fig.8

  • Asymmetrical allylic:

When this cation reacts with Br−, about 80% goes to one end and 20% to the other, giving a mixture of butenyl bromides. This regioselectivity (where the nucleophile attacks) is determined by steric hindrance: attack is faster at the less hindered end of the allylic system.

Benzylic carbocation:

3- Carbocations are stabilized by an adjacent lone pair:

Methyl chloromethyl ether, MeOCH 2 Cl, reacts very well with alcohols to form ethers.

Note: Always oxonium ion is formed In lone pair stabilizing carbocation by oxygen.

SN2:

1- The low substitunet carbon:

Low substitunet carbon doesn’t stabilize carbocation which then induces SN2 mechanism.

  • No substituted carbon:
  • Mono substituted carbon:

2- Adjacent C=C or C=O π systems increase the rate of SN2 reactions:

We mentioned above the C=C stabilizes carbocation inducing SN1 mechanism, but also It induces SN2 mechanism and this due to:

Allyl compounds react rapidly by the SN2 mechanism because the π system of the adjacent double bond can stabilize the transition state by conjugation. The p orbital at the reaction centre (shown in
brown) has to make two partial bonds with only two electrons—it is electron deficient, and so any additional electron density it can gather from an adjacent π system will stabilize the transition state and increase the rate of the reaction.

Notes:

Effect of Medium

In case of using di-substituted carbon and allylic electrophile mechanism depends on medium 

SN1 SN2

Using of Protoic or polar medium 

Charged medium stabilizes carbocation formation


Ex:
CH3OH, CH3CH2-OH, H2O

Using of aprotic medium

Non charged medium doesn’t stabilize the carbocation

Ex: Benzne , Ether  , Acetone

Electronic effect:

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