FSc Notes Part 2 Chemistry Important Chemicals Alkyl Halide

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Introduction

Alkyl halides are the derivatives of alkanes, they are denoted by RX where R may be any alkyl group and X may be any halogen atom (Cl, Br, I). The general formula of alkyl halide is given by

CnH2n + 1 – X

Where n may be any natural number and X may be halogen atom.

Definition

When one hydrogen atom of alkane is replaced by halogen atom then the substituted alkane is formed that is known as alkyl halide or mono halo alkane.

Classification of Alkyl Halide

On the basis of carbon atom alkyl halides are classified into following three classes.

1. Primary Alkyl Halide
2. Secondary Alkyl Halide
3. Tertiary Alkyl Halide

1. Primary Alkyl Halide (Iº RX)

When one hydrogen atom of methyl group is replaced by an alkyl group, then the carbon of the substituted methyl is called Primary carbon atom.

H-CH2- —-> R-CH2-

Those alkyl halides in which halogen atom is attached directly with primary carbon atom are called Primary alkyl halides.

H-CH2-X —-> R-CH2-X

2. Secondary Alkyl Halide (2º RX)

When two hydrogen atoms of methyl group are replace by any alkyl group, then the carbon atom of substituted methyl is called secondary carbon atom.

H2-CH- —-> R2-CH-

Those alkyl halides in which halogen atom is directly attached with the secondary carbon atom are called secondary alkyl halides. The alkyl group may be similar or different.

H2-CH-X —-> R2-CH-X

3. Tertiary Alkyl Halide (3º RX)

When three hydrogen atoms of methyl groups are replaced by any alkyl group, then the carbon atom of the substituted methyl is called Tertiary carbon atom.

H3-C- —-> R3-C-

Those alkyl halides in which halogen atom is attached directly with the tertiary carbon atom are called Tertiary Alkyl Halide. The alkyl group of tertiary alkyl halide may be different or similar.

H3-C-X —-> R3-C-X

Chemical Reactions of Alkyl Halide

Alkyl Halides are highly reactive compounds and show variety of chemical reactions. Some important chemical reactions are given below.

1. SN Reactions
2. Formation of Grignard’s Reagent
3. Elimination Reactions or E-Reactions

1. SN Reactiosn

In alkyl halide, the electronegativity of halogen atoms is greater than carbon atom of alkyl group. Therefore, the shared pair of electron between R – X (C-X) is shifted towards halogen atom. As a result halogen becomes partial negativity charged and carbon atom of alkyl group becomes partial positively charged ion.

+R – X

+H3C – Cl-

Those atoms/molecules/ions, which are electron deficient or contain positive charge are called Electrophile. Those atoms/molecules/ions, which are electron rich or contain negative charge are called Nucleophile.

In alkyl halide, alkyl group act as electrophile where as halogen atom act as nucleophile. Those reactions in which one nucleophile is replaced by other nucleophile are called Nucleophillic Substitution Reactions or simply SN Reactions.

When alkyl halide reacts with attacking nucleophile or nucleophilic reagent then halogen atom of alkyl halide is replaced is replaced by attacking nucleophile to form a substituted product.

R-X + Nu- —-> R – Nu + X

H3C Br + CN —-> H3C – CN + Br-

H3C – Br + OH- —–> H3C – OH + Br-

H3C – Br + SH- —-> H3C – SH + Br-

H3C – Br + NH2- —-> H3C – NH2 + Br-

H3C – Br + OR- —-> H3C – Or + Br-

H3C – Br + -OOCR —-> H3C – OOCR + Br-

To be an affective nucleophile in Sn reaction, the attacking nucleophile should be stronger base than the leaving group.

Classification of SN Reactions

On the basis of Mechanism, SN reactions are classified into following two classes.

1. SN(1) Reactions
2. SN(2) Reactions

1. SN(1) Reactions

Definition

Those nucleophilic substitution reaction in which rate of reaction and formation of product depends upon the concentration of one specie are known as SN(1) Reactions.

Mechanism

The mechanism of SN(1) Reactions proceeds in two steps.

First Step

It is a reversible and slow step, the alkyl halide dissociates into positively charged carbonium ion and negatively charged halide ion (Leaving Group)

Second Step

It is a irreversible and fast step, the attacking nucleophile reacts with the positively charged carbonium to give a final substituted product.

Rate of Reaction

The slow step of a reaction is a rate determining step. In this mechanism, first step is slow and hence is the rate determining step, which shows that the rate of formation of product depends upon the concentration of one molecule i.e. alkyl halide.

Rate of Reaction = K [R - X]

Since the rate of reaction depends upon the concentration of only one molecule, therefore, it is also known as uni-molecular nucleophilic substitution reaction.

Conclusion

In all tertiary alkyl halide, SN reactions proceed through SN(1) mechanism. In all secondary alkyl halides SN reaction may occur through SN(1) mechanism or SN(2) mechanism depending on the nature of the solvent in which the reaction is carried out. Polar solvents help in ionization so they favor SN(1) Reactions.

2. SN(2) Reactions

Definition

Those nucloephillic reactions in which rate of reaction depends upon the concentration of two species is knows as SN(2) Reactions.

Mechanism

The mechanism of SN(2) Reaction occurs through following mechanism.

The attacking nucleophile reacts with carbon atom of alkyl halide to form an intermediate unstable complex, therefore, the formation of C – Nu bond and cleavage of C – X bond occurs simultaneously to form a substituted product and leaving group.

In this mechanism, the attacking nucleophile attacks the carbon atom from opposite side of the halogen atom.

Rate of Reaction

The slow step of reaction is a rate determining step. In this mechanism, the rate of formation of product depends upon the concentration of two species of molecules i.e. alkyl halide and attacking nculeophile.

Rate of Reaction = K [R - X] [Nu-]

Since the rate of reaction depends upon the concentration of two species therefore, it is also known as bimolecular nucleophilic substitution reaction.

Conclusion

In all Primary alkyl halide, SN reactions proceed through SN(2) mechanism. In all secondary alkyl halides SN reaction may occur through SN(1) mechanism or SN(2) mechanism depending on the nature of the solvent in which the reaction is carried out. Polar solvents help in ionization so they favor SN(1) Reactions, where as non polar solvents favours SN(2) mechanism.

Formation of Grignard’s Reagent

In presence of dry ether, when alkyl halide reacts with magnesium metal, then alkyl magnesium halide is formed. This compound was first synthesized by Grignard therefore it is known as Grignard’s reagent.

Grignard’s reagent plays an important role in synthetic organic chemistry because it is used to prepare a variety of organic compounds.

The reaction of Grignard’s reagent are explained on the basis that due to metal, magnesium act as electrophile, therefore the bond between C – Mg is polarized. As a result the carbon atom bonded with magnesium bears a partial negative charge and hence act as nucleophile.

The carbon atom of Grignard’s reagent (nucleophile) reacts with any electrophillic reagent. As a result the bond between C – Mg is broken and a new bond between carbon and electrophillic reagent is formed.

Elimination Reaction Or E-Reaction Or β Elimination Reactions

Definition

Those reactions in which removal of β hydrogen takes place in an alkyl halide with the formation of double bond are known as β – Eliminates Reaction..

OR

Those reactions in which removal of water molecule takes place with the formation of double bond are also called elimination reactions of simply e-reactions.

Reaction Mechanism

Consider alkyl halide which contains two or more than two carbon atoms. The carbon atoms which is directly bonded with halogen atom is called α-carbon atom. The carbon atom (s) adjacent to α-carbon atom is called β – carbon atom and so on.

The hydrogen atom which is directly attached with α – carbon atom are known as α – hydrogen atom. The hydrogen atom which is directly bonded with β – carbon are known as β – hydrogen atom and so on.

In alkyl halide the electro negativity of halogen atom is more than the carbon of alkyl group. As a result the shared pair of electron between C – X is shifted towards Halogen atom. As a result halogen becomes partial positive ion. Now α – carbon pulled the electron of β – carbon and β – carbon pulled the electron of β – hydrogen atom. Therefore, ultimately the positive charge is shifted to β – hydrogen atom.

Thus, the nucleophile or base i.e. OH- attacks β – hydrogen atom. As a result water molecule is formed with the removal of β – hydrogen atom. The bond between α – carbon and β – carbon takes place simultaneously.

Due to the removal of β – hydrogen atom the elimination is also called β – elimination reaction.

Example

When alkyl is heated with alcoholic potash then dehydrohalogenation takes place. As a result, Alkene is formed with elimination of water molecule.

RC2H4X + KOH —-> RHC = CH2 + H2O + KX

Alkene

HC2H4Cl + KOH —-> H2C = CH2 + H2O + KCl

Ethene

Classification of Elimination Reactions

On the basis of mechanism, elimination reactions are classified into the following two classes.

1. E(1) Reaction.
2. E(2) Reaction.

1. E(1) Reaction

Definition

Those elimination reactions in which the rate of reaction depends upon the concentration of one substance or molecule are known as E(1) Reactions.

Mechanism

The mechanism of E1 Reactions occur through following two steps.

First Step

It is a slow and reversible step. Alkyl halide is dissociated into carbonium ion and halide ion.

Second Step

It is a irreversible and fast step, the attacking (OH-) removes a proton (H+) from the β – carbon atom with the simultaneous formation of double bond between α – carbon atom and β – carbon atom.

Rate of Reaction

The slow step of a reaction is rate determining step. In this mechanism the rate of reaction depends upon the first step or on the concentration of only one molecule, i.e. alkyl halide.

Rate of Reaction = K [R - X]

Since the rate of reaction depends upon the concentration of only one substance or molecule, therefore, it is called uni-molecular elimination reaction or simply E(1) Reaction, where 1 stands for uni-molecular.

Conclusion

In all tertiary halides, elimination reaction occurs through E(1) mechanism. In all secondary alkyl halides elimination reaction may occur through both E(1) and E(2) mechanism, which depends upon the nature of the solvent in which the reaction is carried out. The presence of polar solvent favours E(1) mechanism.

2. E(2) Reactions

Definition

Those elimination reactions in which rate of reaction depends upon the concentration of two substances or molecules is known as E(2) Reactions.

Mechanism

The mechanism of E(2) reaction, occur through the following single step. Due to high electronegativity of halogen atom the shared pair of electron between C – X is shifted towards halogen atom. As a result halogen becomes partial negativity charged and α – carbon atom becomes partial positively charged ion. Ultimately, the positive charge is shifted to β – hydrogen to form unstable intermediate transition stage. Immediately the cleavage of C(β) – H and C(α) – H bond takes place simultaneously. As a result double bond is formed between α – carbon atom and β – carbon with the elimination of water molecule.

OH + H3C-CR2+ —-> Transition Stage —-> H2C=CR2 + H2O

Rate of Reaction

The slow step of reaction is a rate determining step. In this mechanism rate of reaction depends upon the concentration of two species, i.e. alkyl halide and base.

Rate of Reaction = K [R - X] [OH-]

Since the rate of reaction depends upon the concentration of two species therefore it is called bimolecular elimination reaction or simply E(2) reaction where 2 stands for bimolecular.

Conclusion

In all primary alkyl halides, elimination reaction occurs through E(2) mechanism. In all secondary alkyl halides elimination reaction may occur through both E(1) and E(2) mechanism, which depends upon the nature of the solvent in which the reaction is carried out. The presence of polar solvent favours E(1) mechanism, whereas non-polar solvent favours E(2) mechanism