Organic Compounds Containing Halogens Revision Notes for Chemistry NEET

Organic compounds containing halogens, also known as haloalkanes and haloarenes, are a key part of the NEET Chemistry syllabus. These compounds feature one or more halogen atoms (such as fluorine, chlorine, bromine, or iodine) attached to a carbon atom. They are significant due to their widespread uses in industry, pharmacy, and daily life, as well as their diverse chemical reactions.

General Methods of Preparation
Haloalkanes and haloarenes can be synthesized in several ways. The main methods include:

• Direct halogenation of alkanes using halogens under specific conditions (chlorination under UV light or heat).
• Replacement of hydroxyl group in alcohols by halogens (using reagents like PCl₅, PX₃, SOCl₂, or hydrobromic acid).
• Addition of halogens or hydrogen halides to alkenes and alkynes.
• Using diazonium salts for preparation of haloarenes (Sandmeyer’s and Gattermann reactions).

In each method, the choice depends on the substrate and the desired product type. For example, SOCl₂ gives only alkyl chlorides with alcohols, evolved gases are easy to remove. In the preparation of haloarenes, aniline is converted to benzene diazonium chloride, which is then treated with CuCl, CuBr, or KI to get chlorobenzene, bromobenzene, or iodobenzene, respectively.

Properties of Haloalkanes and Haloarenes
Physical properties of haloalkanes vary with the length of the carbon chain and the halogen present. Generally, haloalkanes are colorless, insoluble in water but soluble in organic solvents, and have higher densities than corresponding hydrocarbons. Their boiling points increase with molecular mass and with the presence of heavier halogens.
In contrast, haloarenes such as chlorobenzene have relatively high boiling points and are less reactive than haloalkanes. Most haloalkanes and haloarenes are liquids or solids at room temperature. Their smell is often sweet and distinctive.

Nature of C–X Bond
The carbon-halogen (C–X) bond is polarized due to the difference in electronegativity between carbon and the halogen atom. Halogens are more electronegative, giving the carbon a partial positive charge and the halogen a partial negative charge. Bond strength decreases as the size of the halogen increases; C–F is the strongest, while C–I is the weakest.
A table for C–X bond properties:

Halogen (X)	Bond Length (pm)	Bond Strength (kJ/mol)	Reactivity
F	139	485	Very Low
Cl	178	338	Moderate
Br	193	276	High
I	214	238	Very High

Because of the polarity and variable bond strength, the C–X bond is more susceptible to nucleophilic substitution than typical carbon–carbon or carbon–hydrogen bonds.

Mechanisms of Substitution Reactions
Haloalkanes mainly undergo nucleophilic substitution reactions where the halogen is replaced by a nucleophile. The two major mechanisms are:

• SN1 (Unimolecular Nucleophilic Substitution): Occurs in two steps. First, the halide ion leaves, forming a carbocation; then the nucleophile attacks. This mechanism favors tertiary haloalkanes and produces racemic mixtures due to planar intermediate.
• SN2 (Bimolecular Nucleophilic Substitution): Proceeds in one concerted step, with the nucleophile attacking from the opposite side as the leaving group departs. Favored by primary haloalkanes and leads to inversion of configuration (Walden inversion).

Example:

1. For SN1: $(CH_3)_3CBr\ +\ OH^-\ \rightarrow\ (CH_3)_3COH\ +\ Br^-$
2. For SN2: $CH_3CH_2Br\ +\ OH^-\ \rightarrow\ CH_3CH_2OH\ +\ Br^-$

In haloarenes, nucleophilic substitution is much slower and often requires strong activating groups ortho or para to the halogen.

Other Important Reactions
Beyond substitution, haloalkanes can also undergo elimination reactions (to form alkenes) and coupling reactions (for more complex products). Some typical reactions include:

• Wurtz Reaction: 2 moles of alkyl halide react with sodium in dry ether to give a higher alkane.
• Fittig Reaction: Similar to Wurtz but uses aryl halides to form biaryl compounds.
• Elimination: Dehydrohalogenation with alcoholic KOH gives alkenes.

Haloarenes are more resistant to nucleophilic attack due to resonance stabilization and the partial double bond character of the C–X bond in aryl halides.

Uses of Haloalkanes and Haloarenes
These compounds are highly useful:

• Solvents in industries (e.g., chloroform, carbon tetrachloride).
• Anesthetics (e.g., chloroform; though now rarely used due to toxicity).
• In the manufacture of plastics, dyes, and medicines (e.g., DDT as insecticide; iodoform for antiseptic purposes).
• As refrigerants (Freons) and in fire extinguishers.

Chlorobenzene is used for making phenol, DDT, and other chemicals. Haloalkanes are also key intermediates in organic synthesis.

Environmental Effects of Chloroform, Iodoform, Freons, and DDT
Certain halogenated compounds have severe environmental and health concerns:

• Chloroform: Prolonged exposure can damage the liver and kidneys; in air, it can react to form phosgene, a toxic gas.
• Iodoform: Is an antiseptic but can cause skin irritation and has limited environmental danger due to low persistence.
• Freons (CFCs): Widely used as refrigerants, they persist in the atmosphere, reach the stratosphere, and cause ozone depletion by releasing chlorine atoms.
• DDT: A potent insecticide but non-biodegradable; it accumulates in food chains (biomagnification) and poses risks to wildlife and humans, leading to ban/restriction in many countries.

In summary, while organic compounds containing halogens are very useful, their production, use, and disposal must be carefully managed to minimize health and environmental risks.

NEET Chemistry Notes – Organic Compounds Containing Halogens: Quick Revision for Exam

These revision notes focus on important points about the C–X bond nature, preparation methods, and reactions of halogenated organics. With concise lists and comparisons, they offer easy recall for NEET Chemistry. Key uses and environmental issues of haloalkanes and haloarenes are clearly explained.

Grasping these organic compounds containing halogens concepts helps students tackle tricky reaction mechanisms and MCQs. These notes help connect theory with practical daily life examples, making last-minute exam revision efficient and complete.