HALOGEN DERIVATIVES

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HALOGEN DERIVATIVES OF ALKANES AND ARENES

Introduction:

Saturated aliphatic hydrocarbons are called alkanes, represented by general formula CnH2n+2. 

Aromatic hydrocarbons are called Arenes. 

The compounds which are obtained by replacing one or more hydrogen atoms of aliphatic or aromatic 

hydrocarbon by corresponding number of halogen atoms (Chlorine, Bromine or Iodine),are called as 

halogen derivatives of alkanes or of Arenes.

Give the Classification of Halogen Derivatives of Alkanes

Halogen derivatives of alkane are classified as monohalogen derivatives and polyhalogen derivatives.

1] Monohalogen derivatives (Gen. Formula- CnH2n+1X)

When one hydrogen atom of alkanes is substituted by one halogen atom, the compounds formed are 

called as Monohalogen derivatives of alkanes.

e.g. CH3Cl (methyl chloride)  CH3CH2Br (ethyl bromide)

2] Polyhalogen derivatives

When more than one hydrogen atoms of alkanes are substituted by corresponding number of halogen 

atoms, the compounds formed are called polyhalogen derivatives of alkanes. 

On the basis of number of halogen atoms present in a molecule, polyhalogen derivatives are 

further classified as dihalogen derivatives, trihalogen derivatives, tetrahalogen derivatives, etc.

a] Dihalogen derivatives(CnH2nX2)

When two halogen atoms substitute two hydrogen atoms of alkanes, the compounds formed are called dihalogen derivatives of alkanes.

e.g. CH2C12 (methylene dichloride) C2H4C12 (ethylene dichloride)

b] Trihalogen derivatives (CnH2n-1X3)

When three hydrogen atoms of alkanes are substituted by three halogen atoms, the compounds 

formed are called trihalogen derivatives of alkanes.

e.g. CHC13 (chloroform) CHI3 (iodoform)

Define Alkyl Halide and explain its classification.

Definition:- 

Monohalogen derivatives of alkanes are called alkyl halides.

These are represented by the general formula CnH2n+1X or R-X (R = alkyl group, X = halogen atom).

Classification of alkyl halides: 

Alkyl halides are further classified on the basis of the type of the carbon atom to which the halogen 

atom is bonded.

Primary alkyl halides (1°)

Secondary alkyl halides (2°)

Tertiary alkyl halides (30)

Primary alkyl halides (1°)

Alkyl halides in which a halogen atom is bonded to a primary carbon atom (i.e. 1°Carbon

atom is the carbon which is attached to only one other carbon atom) are called primary alkyl halides.

They are represented by the

Secondary alkyl halides (2°)                              Alkyl halides in which a halogen atom is bonded to a secondary carbon atom (i.e. 2° carbon is the carbon which is attached to two other carbon atoms) are called secondary alkyl halides

Tertiary alkyl halides (3o)

Alkyl halides in which a halogen atom is bonded to a tertiary carbon atom (i.e. 3° carbon is the      carbon, which is attached to three other carbon atoms) are called tertiary alkyl halides.

Explain the Nature of C-X bond in alkyl halides?

Carbon halogen bond is a polar bond as halogens are more electronegative than carbon. 

The carbon atom exhibits a partial positive charge and halogen atom a partial negative charge.

The C-X bond is formed by overlap of sp3 orbital of carbon atom with half filled P orbital of halogen atom. 

The size of halogen atom increases from fluorine to iodine, which increases the C-X bond length.

Larger the size, greater is the bond length and weaker is the bond formed.

Bond strength of C-X decreases from C-F to C-I in CH3X.

 Strongest bond is formed by overlap of orbitals of the same principle quantum number.

Halogen                  F Cl Br        I

Overlapping orbital 2pz 3pz 4pz 5pz

The 2sp3 orbital of carbon cannot penetrate into larger p orbitals (3rd,4th and 5th main energy levels) 

sufficiently to form strong bonds. Hence the order of C-X bond strength in CH3X is,

CH3F > CH3Cl > CH3Br > CH3I

Give the Different Methods of preparation of Alkyl halides?

1] From Halogenation of alkanes:-

General Reaction: 

  

The process in which hydrogen atom of alkane is replaced by halogen atom is called as Halogenation of Alkane.

Order of reactivity of Halogen 

a) Chlorination and Bromination:  

Alkyl chlorides are formed by the action of chlorine on alkanes in presence of diffused sunlight or 

Ultraviolet light or at high temperature. 

E.g. Ethane on chlorination forms ethyl chloride.

Similarly, alkyl bromides are formed when alkanes are heated with bromine in the presence of 

anhydrous aluminum tribromide.

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