Reactions And Mechanisms Of Alkyl Halides Biology Essay


Alkyl halides belong to the class of hydrocarbon containg carbon and hydrogen atom along with a substitued halogen atom.the carbon atom is bonded to a halogen atom and exhibits a tetrahedral shape.the carbon attached to halogen is sp3 hybridized. Halogen atoms being more electronegative than carbon atoms make the σ covalent bonds between the atoms more polar. Due to bond polarity the carbon atom acquires a slightly positive charge and halogen atom acquires partially negative charge.Thus,the carbon atom acts as an electrophile and halogen atom acts as a nucleophile.As we go down a group in the periodic table,the size of halogen atom increases and electronegativity decreases.due to this reason the bond length between the carbon and oxygen atom becomes longer and less polar as the halogen atom changes from fluorine and iodine.


These are usually in liquid state except ethyl chloride which are in gaseous state.some of them are also in gaseous state and alkyl halides having more than 18 carbons are solids.

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Alkyl halides are sweet smelling compounds.

They are generally insoluble in water but soluble in organic solvents like benzene,carbon tetrachloride etc.

The boiling point of different alkyl halides containing same halogen increases with the increasing length of chain.

For a given alkyl halide molecule,the boiling point increases as the halogen is changed from fluorine to iodine.

In case of isomeric haloalkanes,the boiling point decreases with increase in branching.


Methyl halides-have only one carbon atom,CH3X

Primary alkyl halides-molecules in which carbon attached to halogen has only one c-c bond,CH3CH2Br

Secondary alkyl halides-molecules in which carbon attached to halogen has two c-c bonds.CH3CH(Cl)CH3

Tertiary alkyl halides-molecules in which carbon attached to halogen has three c-c bonds.(CH3)3Br


Tertiary alkyl halides have longer life stability as compared to those of secondary and primary alkyl halides and thus is less reactive in comparison to other classes of alkyl halides. Thus greater the stability of an organic molecule lesser will be its reactivity and hence the product formation.



In sn1 nuclophilic substitution reaction,the reaction involves ionization of the alkyl halide to form the carbocation.this step is slow and is called as rate determining step of the reaction.The carbocation thus formed is then attacked by a nucleophile in the second step of the reaction to form the product.This is the fast step involved in the reaction.The more stable the carbocation is to form,the faster will be the thereaction.In case of sn1 mechanism tertiary alkyl halides are more stable than secondary and primary alkyl halides and this facilitates the formation of stable carbocation which then undergoes the rapid attack by the nucleophile and hence results in the formation of product.

Stability of carbocation-The bulky substituents on the central carbon atom increases the rate of carbocation formation because of the relief of steric strain that occurs.The resultant carbocation is further stabilized by inductive stabilization and hyperconjugation from the attached alkyl groups. The SN1 mechanism therefore dominates in reactions at tertiary alkyl centers and is further observed at secondary alkyl centers in the presence of weak nucleophiles.

Order of reactivity-  (CH3)3C-  >  (CH3)2CH-   >   CH3CH2-  >  CH3-

SN1 is a substitution,nucleophilic,unimolecular reaction whose rate depends upon the concentration of the alkyl halide.

Rate=k[alkyl halide],which implies that it is a first order reaction in which retention of configuration of takes place.


The reaction involves two steps-

1.Formation of carbocation

This step is slow and reversible and hence is the rate-determining step of the reaction.

2.Attack by nucleophile

In the second step,the carbocation being a reactive chemical species,is immediately attacked by a nucleophile to give a substitution product.This step is fast and hence doesnot affect the rate of reaction.

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3.Deprotonation-This steps involves the removal of proton on the protonated nucleophilewhere water acts as a base forming alcohol and a hydronium ion.

Stereochemistry-The nucleophile can attack from either side of the plane on the carbocation formed,and can result in the mixture of two stereoisomers.If the central carbon atom is the only stereocentre in the reaction,then racemization may occur.

Factors involved in sn1 reaction-

1.Substituent effect-tertiary alkyl halides>secondary alkyl halides>primary alkyl halides

2.Concentration of nucleophile-No effect since reaction is zero order in respect to the concentration of nucleophile.

3.Strength of nucleophile-weak nucleophiles like solvent molecules favor sn1 reaction,solvolysis.

4.Solvent effect-polar protic solvents with high dielectric constants favor sn1(strong hydrogen bonding solvates and stabilizes carbocations. Ex-carboxylic acids, alcohol, water etc.

5.Nature of leaving group-A leaving group is a substituent that can leave as a relatively stable entity.The best leaving group are weak bases which are relatively stable.

Intermediates formed-1.Formation of carbocation

2.attack by nucleophile on the carbocation.


Sn2 is a substitution ,nucleophilic, bimolecular second order reaction whose rate depends upon the concentration of alkyl halide and nucleophile.

Rate=k[alkyl halide][nucleophile]

It is a single step reaction in nucleophile displaces the leaving group.

A positive charge develops on the carbon atom attached to the leaving group due to electronegativity difference between carbon atom and leaving group.This polar σ bond facilitates the attack of nuclophile on the carbon atom resulting in the formation of a transititon state in which central carbon atom is partially bonded to five groups.

When a nucleophile attacks in the sn2 it is on the opposite side to the leaving group.As a result the reaction proceeds with the inversion of configuration.

This reaction is very sensitive to steric hinderance and is fastest for primary halides.In case of tertiary halides the attack of nucleophile is blocked due to steric hindrance.

Reactivity order :  CH3-  >  CH3CH2-  >  (CH3)2CH-  >  (CH3)3C-

Rate determining step involved in sn2 mechanism is a slow step due to the involvement of two reacting species and thus is called bimolecular nucleophilic substitution reaction.


1.Substituent effect-methyl halide>primary halide>secondary halide>tertiary halide

2.Concentration of nucleophile-By the rate law expression,the rate of reaction is directly proportional to the concentration of nucleophile.Thus if the concentration of nucleophile is more,faster will be the rate of reaction.

3.Strength of nucleophile-strong base are strong nucleophiles and viceversa.The strength of nucleophile decreases due to the presence of electron withdrawing group.when a nucleophile attacks such a molecule, its strength decreases due to presence of electron withdrawing group which takes the electron density from the nucleophile.ex: sn2 reaction in CCl4. Generally the sn2 reaction requires strong nucleophiles.

4.Solvent-sn2 reactions are carried out in polar aprotic solvents. In polar protic solvents like HOH, ROH and RCOOH , all have -OH groups that allow them to form hydrogen bonding with anionic nucleophiles. Solvation forces such as these stabilize the anion and suppress its nucleophilicity. Aprotic solvent does not contain -OH group so they do not involve in solvation of anion, leaving them nekdly to express their better nucleophilic character.

5.Leaving group-the ability of a leaving group depends upon its basicity.The weaker the basicity of it is most reactive.since the weak base cannot hold their electrons tightly,the bond between the leaving group and carbon atom can be broken easily.Thus weaker base facilitates the attack of nucleophile readily as compared to strong bases like fluoride ion which is less reactive and whose bonds cannot be broken easily .(Stronger Base) F- > Cl- > Br- > I- (Weaker Base)

3.Elimination reaction-It is a unimolecular, elimination reaction in which the rate of reaction depends only on the concentration of alkyl halides and its rate law expression is given by


The mechanism pathway is a multistep process ,following two critical steps-

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1Formation of carbocation-loss of leaving group generate a carbocation intermediate.

2.Deprotonation of carbocation-the carbocation formed losses a proton H+ to form a double bond,resulting in the formation of alkene product.A base is required in order to accomplish deprotonation.

Order of reactivity:  (CH3)3 C-  >  (CH3)2CH-   >  CH3CH2-  >  CH3

In a reaction the slowest step is the rate determining step of the reaction.In E1 reaction,the rate determining step is the loss of leaving group to form a carbocation intermediate.The more stable the carbocation formed,faster will be the rate of reaction.

LEAVING GROUP-there is very strong dependence on the nature of leaving group,as the breakage of R-LG bond determaine the rate determining step of the reaction.The better will be the leaving group the fastest will be the rate of reaction.In case of alkyl halides,the leaving group is halide ion X-.The leaving group ability of halide ion depends upon the two factors:-

1.Strength of C-X bond-The weaker bond,the better will be the leaving group.the strength of bond depends on the amount of orbitial overlap between the C and X.The overlap decreases as the size of X increases,F>Cl>>Br>>I.

2.Polarization of C-X bond-The more polarized the bond the better will be the leaving group.The bond polarization decreases with the decreasing electronegativity of X,i.e.F>Cl>>Br>>I.

Thus the order of leaving group ability is I- > Br- > Cl- >> F-.

BASE-As base is not involved in the rate determining step of the reaction,the nature of base is not important in the reaction.

Solvent effects-The solvent taken should be capable of dissolving both the carbocation and the leaving group.E1 reactions requires highly polar protic solvents that solvate ions. Ex: alcohol, water etc.

4.E2 mechanism-It is a bimolecular,elimination reaction whose rate law expression is given by


The rate determining step involves the interaction of both the species ,i.e.base and the organic substrate.The mechanism is a single step process with the involvement of a transition state,where there is a simultaneous removal of proton H+ by the base,loss of leaving group and formation of π-bond.The proton is removed from the carbon atom next to the carbon attached to the leaving group or halogen.

1.Effect of R-In an E2 reaction, the reaction transforms 2 sp3 C atoms into sp2 C atoms. This moves the substituents further apart decreasing any steric interactions. So more highly substituted systems undergo E2 eliminations more rapidly. This is the same reactivity trend as seen in E1 reactions.

2.Leaving group-Good leaving group are required .The better is the leaving group ,the fastest will be the rate of reaction.

Thus the order of leaving group ability is I- > Br- > Cl- >> F-.

3.Base-The base is involved in the rate determining step of the reaction,thus the nature of base is very important.More reactive the base,faster will be the rate of reaction and more readily π-bond will be formed.

4.Solvent effect-polar aprotic solvents favors the rate of reaction. In case of protic solvents,Solvation forces stabilize the anion and suppress the basicity,which decreases the rate of product formation.

5.Reaction with reducing metals

Reaction with magnesium-formation of Grignard reagent:Alkyl halides react with magnesium in the presence of dry ether to produce Grignard reagents.

     Dry ether

RX + Mg ---→ R - Mg - x


-The first step is the rate determining step which involves transfer of one electron from magnesium to the carbon halogen bond.This results in the formation of an alkyl free radical and Mg+ion.

-In the second step the alkyl free radical couples with magnesium halide to form Grignard reagent.

Diethyl ether are good solvents for the preparation of Grignard reagents because they are aprotic.In case of protic solvents like water and alcohol,the Grignard reagent destroys and results in the formation of hydrocarbon.

6.Wurtz reaction-It is a coupling reaction where two alkyl halides react with sodium to form new carbon-carbon bond.

2R-X + 2Na → R-R + 2Na+X−


1.In the first step,an electron is transferred from metal to the halogen to produce a metal halide and an alkyl radical.

R-X + M → R• + M+X−

2.The alkyl radical accepts an electron from other metal atom to form an alkyl anion and a metal cation.

R• + M → R−M+

3. The nucleophilic carbon of the alkyl anion then displaces the halide in an sn2 reaction, forming a new carbon-carbon covalent bond.

R−M+ + R-X → R-R + M+X−

For example:

Wurtz reaction results in symmetric alkanes.

7.Formation of Gilman reagent-

Gilman reagent is a lithium diorganocopper reagent compound,R2CuLi.It can be prepared by treating an alkyl,aryl or alkenyl lithium compound with Cu(I)iodide.

2CH3CH2CH2CH2Li + CuI -diethyl ether---→ (CH3CH2CH2CH2)2Cu- Li+ + LiI

(a gilman reagent)

8.Radical chain mechanism for reaction of alkanes with halides.


1.chain initiation

UV radition or heat causes homolytic cleavage of weak halogen bond to produce two halogen radicles (here bromine radicles).This starts the chain process.

2.Chain propagation

-In this step a bromine radical abstracts a hydrogen from CH4 to form HBr and a methyl radical.

-The methyl radical formed then abstracts a bromine from another Br2 molecule to form methyl bromide and another bromine radical which can react itself with another CH4 molecule to repeat the first step and makes the chain to propagate in cyclic manner.

3.Chain termination

Reaction between the various radicals result in the formation of ethane,Br2 and methyl bromide molecule which removes radicals from the reaction and thus stops further propagation of the chain.


1>Haloalkanes play an important part in synthetic organic chemistry.They are used for the production of polymers.

Monomer-chloroethene monomer- tetrafluroethene

Polymer-polychloroethene or PVC polymer-polytetrafluoroethene or PTFE

Repeating unit: -(CH2-CHCl)n- repeating unit: -(CF2-CF2)n -

PVC is a widly used thermoplastic polymer.It is used in electrical insulation,water pipes,window fames etc.

PTFE -It is used coat the surface of non-stick utensils,as feet for computer mice,as magnetic stirrer coatings etc.


Dichlorofluoromethane-CHFCl2-used as a propellant and refrigerant.

Trichlorofluoromethane-CFCl3-used as a refrigerant.

Bromochlorodifluoromethane-CBrClF2-used as a fire extinguishers.

Trichlorotrifluoroethane-CCl2FCClF2-dry cleaning solvent,degreasing agent.

Trichloromethane-CHCl3-It is used as a solvent and as an anesthetic.

DDT-dichlorodiphenyltrichloroethane-used as an insecticide and pesticide.Effective against diseases such as malaria and typhus.

3>Alkyl halides are the starting reagents for the preparation of alcohols.

4>The Grignard reagents can be used for the production of large no. of organic compounds.

5>Methyl bromide is used as a pesticide and fumigants.