NCERT Books for Class 12 Chemistry Chapter 10 - Haloalkanes and Haloarenes Free PDF Download

Several name reactions from this chapter are frequently asked in board exams. For the 2025-26 session, focus on:

• Swarts Reaction: For the synthesis of alkyl fluorides.

• Finkelstein Reaction: For preparing iodoalkanes from other alkyl halides.

• Wurtz-Fittig Reaction: For coupling an alkyl halide with an aryl halide to form an alkyl-substituted aromatic compound.

• Sandmeyer's Reaction: For the preparation of aryl halides from diazonium salts. Questions often require you to write the reaction and state the reagents used. A good way to master these is to check the list of important name reactions in organic chemistry.

Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions due to several factors. This is a classic high-order thinking skills (HOTS) topic. The key reasons are:

• Resonance Effect: The C-X bond in haloarenes develops a partial double-bond character due to resonance, making it stronger and harder to break.

• Hybridisation of Carbon: The carbon atom in the C-X bond of a haloarene is sp² hybridised (more s-character), which makes the bond shorter and stronger compared to the sp³ hybridised carbon in a haloalkane.

• Instability of Phenyl Cation: Nucleophilic substitution via the S_N1 route is ruled out as the intermediate phenyl cation is highly unstable.

• Electronic Repulsion: The electron-rich benzene ring repels the incoming electron-rich nucleophile.

A typical HOTS question might ask you to arrange different halo-compounds (e.g., C₆H₅Cl, CH₃Cl, CH₂=CHCl) in increasing order of reactivity towards a nucleophile, with justification.

Questions on stereochemistry from this chapter are designed to test your understanding of 3D structures and reaction outcomes. Important question types include:

• Identifying Chirality: You may be given structures and asked to identify the chiral carbon atom (or asymmetric carbon).

• Drawing Enantiomers: You might have to draw the non-superimposable mirror images (enantiomers) of a given chiral molecule.

• Predicting Reaction Products: A key question is to predict the stereochemical outcome of S_N1 (racemisation) and S_N2 (inversion of configuration or Walden Inversion) reactions.

• Definitions: 1-mark questions can ask for definitions of terms like chiral, achiral, racemic mixture, and enantiomers.

This is a frequently asked conceptual question. Although haloalkanes are polar molecules, they are immiscible in water. The reason is that haloalkanes cannot form hydrogen bonds with water molecules. To dissolve a haloalkane in water, energy is required to overcome the attractions between haloalkane molecules and to break the hydrogen bonds between water molecules. The new attractions formed between haloalkane and water molecules are not strong enough to release sufficient energy to compensate for this. This makes it an important question as it tests the application of the 'like dissolves like' principle and the concept of intermolecular forces.

Distinction test questions are a practical application of the chapter's chemical properties and are very common in board exams, usually for 2 marks. A typical question asks you to give a simple chemical test to distinguish between a pair of compounds, for example:

• Chlorobenzene and Benzyl Chloride: This can be done using the silver nitrate (AgNO₃) test. Benzyl chloride, being a haloalkane derivative, will give a white precipitate of AgCl with aqueous AgNO₃, while chlorobenzene will not react under these conditions.

• Chloroform and Carbon Tetrachloride: Chloroform gives a positive carbylamine test with alcoholic KOH and aniline, producing an offensive smell, whereas carbon tetrachloride does not.

For more practice on such problems, solving questions from the NCERT Solutions for Haloalkanes and Haloarenes is highly recommended.

The halogen atom in a haloarene exhibits two opposing effects:

• -I Effect (Inductive Effect): Being more electronegative than carbon, the halogen atom withdraws electron density from the benzene ring, deactivating it towards electrophilic substitution.

• +R Effect (Resonance Effect): Through its lone pairs, the halogen donates electron density to the ring, particularly at the ortho and para positions.

The -I effect is stronger than the +R effect, making haloarenes less reactive than benzene. However, the resonance effect directs the incoming electrophile to the ortho and para positions. This dual nature makes it a common trap. Students often get confused, thinking that because it is deactivating, it should be meta-directing. A key takeaway for exams is: Halogens are deactivating but ortho, para-directing.

From this section, expect 1 or 2-mark questions, often in the MCQ or short answer format. Important questions include:

• Uses of Specific Compounds: For example, the use of iodoform as an antiseptic or carbon tetrachloride as a fire extinguisher under the name Pyrene.

• Environmental Impact: The primary question here relates to the role of Freons (CFCs) in causing ozone layer depletion. You may be asked to write the chemical reactions involved.

• DDT: Questions on why the use of DDT was banned in many countries, focusing on its high toxicity and non-biodegradable nature.

Reviewing Haloalkanes and Haloarenes Class 12 notes can help quickly cover these factual topics before the exam.