The attack of nucleophile occurs from the back side (i.e opposite to the side in which the halogen is attacked). The carbon at which substitution occurs has inverted configuration during the course of reaction just as an umbrella has tendency to invert in a wind storm. This inversion of configuration is called Walden inversion.

i) DDT: DDT can be prepared by heating a mixture of chlorobenzene with chloral (Trichloro acetaldehyde) in the presence of con. H₂SO₄,

ii) Chloroform :The reaction of methane with excess of chlorine in the presence of sunlight will give carbon tetrachloride as the major product.
\(\underset { Methane }{ { CH }_{ 4 }+4C{ l }_{ 2 }\overset { h\gamma }{ \longrightarrow } } +\underset { Carbon \ tetrachloride }{ 4HCl } \)
iii) Bipheenyl :
\(\underset { Chlorobenzene }{ { C }_{ 6 }{ H }_{ 5 }Cl } +2Na+Cl-{ C }_{ 6 }{ H }_{ 5 }\overset { Ether }{ \longrightarrow } \underset { Biphenyl }{ { C }_{ 6 }{ H }_{ 5 }-{ C }_{ 6 }{ H }_{ 5 }+2NaCl } \)
iv) Chloropicrin : Chloroform reacts with nitric acid to form chloropicrin.(Trichloro nitro methane)
\(\underset { Chloroform }{ { CH }_{ 3 }+HN{ O }_{ 3 }\overset { \triangle }{ \longrightarrow } } \underset { Chloropicrin }{ C{ Cl }_{ 3 }N{ O }_{ 2 }+{ H }_{ 2 }O } \)
v) Freon-12 :Freon - 12 is prepared by the action of hydrogen fluoride on carbon tetrachloride in . the presence of catalylic amount of antimony pentachloride. is is called swartz reaction
\(\underset { Carbontetrachloridew }{ { CCl }_{ 3 }+2HF\overset { SbC{ l }_{ 5 } }{ \longrightarrow } } \underset { Freon-12 }{ 2HC{ l }+CCl_{ 2 }F_{ 2 } } \)

Freons (CFC): The chloro uoro derivatives of methane and ethane are called freons.
Uses:
(i) Freons are a used as refrigerants in refrigerators and air conditioners. 
(ii) It is used as a propellant for aerosols and foams
(iii) It is used as propellant for foams to spray out deodorants, shaving creams, and insecticides

Alcoholscanbeconvertedintohaloalkenesbyreactingit withanyoneofthefollowingreagent
1.hydrogen halide
2. Phosphorous halides
3. Thionyl chloride
a) Reaction with hydrogen halide:
\(\underset { Ethanol }{ { CH }_{ 3 }{ CH }_{ 2 }OH+HCl } \overset { Anhydrous\\ ZN{ Cl }_{ 2 } }{ \underset { \triangle }{ \longrightarrow } } \underset { Chloroethane }{ { CH }_{ 3 }{ CH }_{ 2 }Cl+{ H }_{ 2 }O } \)
Mixture of con.HCI and anhydrous ZnCl₂ is called Lucas reagent
b) Reaction with phosphorous halides :
Alcohols react with PX₅ or PX₃ to form haloalkane. PBr₃ and PI₃ are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine, respectively.

c)Reaction with thionyl chloride :

This reaction is known as Darzen's halogenation

Carbon halogen bond is a polar bond as halogens are more electro negative than carbon. The carbon atom exhibits a partial positive charge (\(\delta \)+) and halogen atom a partial negative charge (\(\delta \)-)
The C -X bond is formed by overlap of sp³ orbital of carbon atom I with half filled p-orbitat of the halogen atom. The atomic size of halogen 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. The bond strength of C - X decreases from C - F to C - I in CH₃X

The Chlorination of methane is carried out by free radical mechanism. The initiation step to form free radical needs high energy which is supplied by light energy.
\(\mathrm{Cl}-\mathrm{Cl} \stackrel{h \nu}{\longrightarrow} 2 \mathrm{Cl}\)
So this reaction is not possible in dark.

Uses of haloalkane :
Chloroform : As a solvent in pharmaceutical industry and for producing pesticides and drugs As an anaesthetic.
As a preservative for anatomical specimens.
Iodoform : Iodoform is used as an antiseptic for dressing wounds.
Carbon tetrachloride : Carbon tetrachloride is used as dry cleaning agent. It is used as a solvent for oils, fats and waxes. As the vapour of CCl4 is non - combustible, it is used under the name pyrene for extinguishing the fire in oil or petrol.

CH₃I < CH₃Br < CH₃CI < CH₃F

(i) SN₂ reaction means bimolecular nucleophilic substitution reaction of second order.
(ii) The rate of SN₂ reaction depends upon the concentration of both alkyl halides and the nucleophile.
(iii) This reaction involves the formation of a transition state in which both the reactant molecules are partially bonded to each other. The attack of nucleophile occurs from the back side.
(iv) The carbon at which substitution occurs has inverted configuration during the course of reaction just as an umbrella has the tendency to invert in a wind-storm. This inversion of configuration is called walden inversion.

S_N2 Mechanism :
(i) The rate of S_N2 reaction depends upon the . concentration of both alkyl halide and the nucleophile.
(ii) Rate of reaction = is [alkylhalide] [nucleophile]. It follows second order kinetics and occurs in one step. 
(iii) This reaction involves the formation of a transition state in which both the reactant molecules are partially bonded to each other. The attack of nucleophile occurs from the back side (i.e opposite to the side in which the halogen is attacked).
(iv) The carbon at which substitution occurs has inverted configuration during the course of reaction just as an umbrella has tendency to invert in a wind stonri. is inversion of configuration is called Walden inversion; after paul walden who 1st discovered the inversion of configuration of a compound in S_N2reaction.
(v) S_N2reaction of an optically active haloalkane . is always accompanied by inversion of configuration at the asymmetric centre.
(vi) When 2 - Bromooctane is heated with sodium hydroxide, 2 - octanol is formed with invesion of configuration. (-) - 2 - Bromo octane is heated with sodium hydroxide (+) - 2 - Octanol is formed in which - OR group occupies a position opposite to what bromine had occupied,

(a) (-).2 - Bromo octane
(b) Transition State
(c) (+) 2 - Octanol (product)