Haloalkanes and Haloarenes Class 12 Notes Chemistry Chapter 10

Chapter at a Glance

1. Haloalkanes are classified as fluoro, chloro, bromo or iodo compounds according to the type of halogen present and as mono-, di-, tri-, tetra­ haloalkanes, etc., according to the one, two, three, four, etc., halogen atoms respectively present in their molecule.

2. Alkyl halides are further classified as primary (1°), secondary (2°) and tertiary (3°) according to the halogen atom attached to primary, secondary and tertiary carbon atoms, respectively.

3. Due to electronegativity difference between the carbon and the halogen, the shared pair of electron lies closer to the halogen atom. As a result, the halogen carries a small negative charge, while the carbon carries a small positive charge. Consequently, C-X bond is a polar covalent bond.

4. Methods of Preparation of Haloalkanes:
Haloalkanes can be prepared by displacement of alcoholic group in alkyl alcohols by halogen acid, PCl5 or PCl3. Haloalkanes can also be prepared by addition of halogen acids or halogens on alkene and alkyne. Alkyl halides can also be prepared by free radical halogenation of alkanes.

5. Methods of preparation of Haloarenes. Haloarenes can be prepared by nuclear halogenation of aromatic hydrocarbons.

6. From diazonium salts:
(i) By Sandmeyer reaction
(ii) By Gattermann reaction

7. Chemical reactions of haloalkanes

(i) Nucleophilic substitution reactions:
(a) C -X bond in alkyl halides is polar due to electron attracting nature of the halogen atom and thus they readily undergo nucleophilic substitution reactions. These are of two types:

  1. SN1(Substitution, nucleophilic, unimolecular): In such type of reactions, rate= k[RX], i.e., rate is independent of concentration of nucleophile and occurs in two steps. Such reactions are favoured by polar solvents.
  2. SN2(Substitution, nucleophilic, bimolecular): In such type of reactions, rate= k[RX] [Nu], i.e., rate of reaction depends on concentration of nucleophile as well as alkyl group and take place in one step.

(b) An SN2 reaction proceeds with complete stereochemical inversion, while an SNl reaction proceeds with racemisation.

(ii) Elimination reactions : When a haloalkane is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from P-carbon and a halogen atom from the a-carbon atom. An alkene is formed as a product and reaction is called P-elimination.

(iii) Reaction with metals:

8. Aryl halides are extremely less reactive towards nucleophilic substitution reaction due to following reasons :

  1. In haloarenes, the lone pair of electrons on the halogen atom are delocalised on the benzene ring due to resonance giving C – X a double bond character.
  2. In haloalkanes, the halogen atom is attached to sp3-hybridised carbon while in haloarenes the halogen atom is attached tosp2-hybridised carbon.

9. Electrophilic substitution reaction: Haloarenes undergo the usual electrophilic substitution reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel Crafts reactions. Halogen atom is slightly deactivating and o, p-directing.

10. Trichloromethane (Chloroform):
The major use of chloroform today is in the production of the freon refrigerants. Chloroform is stored in closed dark coloured bottles completely filled so that air is kept out because it is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride (phosgene).

11. Freons are the chlorofluorocarbons of methane and ethane. They are extremely stable, unreactive, nontoxic, noncorrosive and easily liquefiable gases. Freon-12 (CCl2F2) is most common freon in industrial use.

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