Alkyl Halides

Physical Properties

1. State and Colour:
   C1-C5              Gaseous state, colorless
   C6-C16                 Liquid state, colorless
   C17 to higher       Solid state, colorless

2. Boiling Point:
   RCl < RBr < RI
   B.P. of a akyl halide increases with a increase in their molecular mass. But in the case of isomeric alkyl halide, branched chain of alkyl halide has lesser as compared to the straight chain of alkyl halide .Because in the case of straight chain of alkyl halide, surface area of the molecule becomes more thereby force of attraction between the molecule also increases resulting the higher boiling point.
   Haloalkane have higher B.P. as compared to those of alkene (corresponding alkane). This is due to their polarity  and strong dipole- dipole attractive interaction between the haloalkane molecules.
3. Solubility
   Alkyl halide are the polar compound even they are insoluble in water. This is due the reason that they have not formed intermolecular H- bonding with a water molecule.
   Soluble in benzene, ether, chloroform.
4. Density
   Iodo, bromo and polychloro compounds are more dense than water.

Chemical Properties: SN₂

Nucleophilic substitution reaction( SN reaction)

The reaction in which the substitution of nucleophile by another nucleophile is known as nucleophilic substitution reaction. It is of two types:
 

1. SN₂ Reaction:
   There reaction mechanism which  depends upon the concentration of both the reactant is SN2 reaction and hence bimolecular nucleophilic substitution.
   Kinetics:Consider the reaction between methyl chloride and OH^- ion
   
   Rate ∝ [CH3Cl][OH]
   Rate=K [CH3Cl][OH]
   where,K is rate constant.
   
   This reaction follows the second order kinetics because of that the rate depends upon concentration of both reactants. Because of this reaction acquires a collision between a hydroxide ion and methyl chloride. If the concentration of either of the two factors i.e. CH3Cl and OH^- is increased the rate of reaction also increases and vice versa.
   This is called kinetic study of SN₂ reaction.
   
   SN2 reaction: Mechanism
   The OH^- collides with a methyl chloride at the face most remote from chlorine and having collision with a sufficient energy, a C-OH bond formed and the C-Cl bond breaks with liberating the Cl^- ion.
   SN₁ reaction
   The T.S. is formed during the completion of reaction in which carbon is partially bonded to both -OH and –Cl i.e. C-OH bond is not completely formed and C-Cl bond is not yet completely broken. Finally OH^- ion shared its electron with a carbon and chloride is liberated.Thus the reaction mechanism is bimolecular nucleophilic substitution.
   
   SN1 reaction:Mechanism
   Reaction mechanism which depends upon the concentration of only one reactant is SN1 reaction and hence it is unimolecular  nucleophilic substitution .

Kinetics: Consider there action between tertiary butyl bromide and OH^- ion to yield tertiary butyl alcohol followed first order kinetics i.e. rate depends upon the concentration of only one reactant i.e. substrate.
Rate∝[(CH3)3CBr]
Rate=K[(CH3)3CBr]
Where, K is rate constant.
The kinetic studies have shown that the rate of reaction depends only on the concentration of tert.butyl bromide is increased, the rate of are action increases.Similarly , the rate of reaction decreases when the concentration of tert.butyl bromide decreases.The hydroxide does not affect the rate reaction.

Mechanism
In Step I- tert-butyl bromide slowly dissociate into bromide ion and tert.butyl carbocation.
In StepII-The carbocation then combine rapidly with a hydroxide ion to yield tert.butyl alcohol.The rate of the overall reaction is determined by the slow breaking of the C-Br bond to form the carbocation.Once the Carbocation formed, it react rapidly to form the product.The mechanism is unimolecular nucleophilic substitution.

 

Stereochemistry of SN2 reaction:

SN2 reaction proceeds with complete stereo chemical inversion.For e.g. When (-)-2bromo octane is allowed to react with NaOH under condition where 2^nd order kinetics are followed (+)-2-octanol is obtained.It is seen that the –OH group has not taken position previously occupied by bromide,the alcohol obtained has a configuration opposite to that of the bromide.This is due to the repulsion between attacking nucleophile and existing nucleophile.

Reactivity of SN2 reaction:

The reactivity of SN2 reaction depends on the steric factor but not to electronic factor as the no.of a substituent attached to the carbon bearing the halogen is increased, the reactivity towards SN2 substitution decreases.

In SN2 reaction,the order of reactivity of R-X is:

                   CH3>1°>2°>3°

Stereochemistry of SN₁ reaction

The stereochemistry of SN1 reaction suggest that the product must be the mixture of retention and inversion configuration.The probability of forming both retention and inversion configuration is due to the fact that nucleophile i.e.OH^- can attack the carbocation from either of two faces as shown below:

  If the alkyl halide consists of chiral carbon the product obtained as retention and    inversion configuration would be the racemic mixture.
Reactivity of SN₁ reaction

The rate determing step of an SN1 reaction is the formation of a carbonium ion.The order of reactivity of alkyl halide in SN1 reaction is the same as the order of stability of carbocation or carbonium ion.The order of reactivity of alkyl halide of SN1 reaction is:

3°>2°>1°>CH3X

The rate of SN1reaction is affected largely by electronic factor i.e.by the tendency of substituent to release or with draw electron.

Factors affecting the SN₂ reaction

1. Steric  Effect
   From the mechanism of SN2 reaction there must be formation of T.S. before forming product. Hence the rate of forming the product must depends upon the rate of formation of T.S. the bulkiness or heavier group develops of the atoms attached to the carbon develops over crowding which results in moving a part of the group and increases of each R-C-R angle. Hence the mechanism of SN2 decreases as one passes from methyl to alkyl halide. The reactivity order is:

CH3X > 1° > 2° > 3°

2. Inductive Effect:
   From the mechanism of SN2 reaction there must be formation of T.S. before forming product. Hence the rate of forming the product must depends upon the rate of formation of T.S. the bulkiness or heavier group develops of the atoms attached to the carbon develops over crowding which results in moving apart of the group and increases of each R-C-R angle. Hence the mechanism of SN2 decreases as one passes from methyl to alkyl halide. The reactivity order is:

                                    CH3X >1° > 2° > 3°