Organic Compounds Containing Halogens – JEE Mains Chemistry
1. General Methods of Preparation of Organic Halides
Organic compounds containing halogens (organic halides) can be prepared through various methods, such as:
- From Alkanes: By halogenation (e.g., chlorination or bromination) of alkanes in the presence of heat or light.
- From Alcohols: By reaction with halogenating agents like PCl₃, SOCl₂, or HX.
- From Aromatic Compounds: By electrophilic substitution with halogens (e.g., chlorination or bromination) in the presence of a Lewis acid catalyst like AlCl₃.
2. Properties of Organic Halides
Organic halides exhibit specific chemical and physical properties based on the halogen and the structure of the molecule:
- Physical Properties: Organic halides tend to have higher boiling points compared to their non-halogenated counterparts due to the electronegativity of the halogen atoms.
- Chemical Properties: They are often reactive in substitution and elimination reactions.
3. Reactions of Organic Halides
Organic halides undergo a variety of reactions, including:
- Substitution Reactions: Involves the replacement of the halogen atom with another group or atom. Examples include nucleophilic substitution (SN1 and SN2) and electrophilic substitution in aromatic compounds.
- Elimination Reactions: Involves the loss of a halogen atom and a hydrogen atom from adjacent carbon atoms, leading to the formation of alkenes (e.g., dehydrohalogenation).
4. Nature of the C-X Bond
The bond between carbon and halogen (C-X) in organic halides is polar due to the difference in electronegativity between carbon and halogen. This polarity plays a crucial role in the reactivity of organic halides, making them prone to substitution and elimination reactions. The strength of the C-X bond decreases as follows:
- C-I (weakest)
- C-Br
- C-Cl
- C-F (strongest)
5. Mechanisms of Substitution Reactions
Substitution reactions in organic halides can proceed via two major mechanisms:
- SN1 Mechanism: A two-step process involving the formation of a carbocation intermediate. It is favored by polar solvents and tertiary halides.
- SN2 Mechanism: A single-step process where the nucleophile attacks the carbon atom from the opposite side of the halogen, leading to a concerted displacement of the halogen. It is favored by primary halides and polar aprotic solvents.
6. Uses of Organic Halides
Organic halides have diverse applications in various industries:
- Chloroform (CHCl₃): Used as an anesthetic (historically), solvent, and in the production of Freons.
- Iodoform (CHI₃): Used as a disinfectant and antiseptic.
- Freons: Used as refrigerants and propellants in aerosol cans (though they are now banned due to their contribution to ozone depletion).
- DDT (Dichlorodiphenyltrichloroethane): An insecticide used extensively in the past (now banned in many countries due to environmental and health concerns).
7. Environmental Effects of Organic Halides
The environmental impact of some organic halides can be significant:
- Chloroform: Exposure to chloroform can be harmful to the liver and kidneys, and it is also a suspected carcinogen.
- Iodoform: While generally safe in small amounts, it can cause irritation to the skin and eyes and is toxic in large quantities.
- Freons: The release of Freons into the atmosphere contributes to the depletion of the ozone layer.
- DDT: DDT is highly persistent in the environment and accumulates in the food chain, leading to long-term ecological damage.