What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Why is the carbon-halogen bond polar?

Because the halogen is more electronegative than carbon, giving the carbon a partial positive charge.

Compare the \(S_N1\) and \(S_N2\) mechanisms.

\(S_N1\) is two-step, unimolecular, via a carbocation and favours 3° halides; \(S_N2\) is one-step, bimolecular, with inversion, favouring 1° halides.

Why do tertiary halides prefer the \(S_N1\) pathway?

They form a stable, more substituted carbocation, which is the rate-determining intermediate.

What is meant by inversion of configuration in \(S_N2\)?

The nucleophile attacks opposite the leaving group, turning the molecule inside out like an umbrella.

Why are haloarenes less reactive than haloalkanes toward nucleophilic substitution?

Resonance gives the C-X bond partial double bond character and the \(sp^2\) carbon is less prone to attack.

What are enantiomers?

Stereoisomers that are non-superimposable mirror images of each other and rotate plane-polarised light in opposite directions.

How are alkyl halides prepared from alcohols?

By reaction with halogen acids, phosphorus halides (PCl\(_3\), PCl\(_5\)) or thionyl chloride.

What is the Wurtz reaction?

Alkyl halides react with sodium in dry ether to give higher alkanes with an even number of carbon atoms.

Why is chloroform stored in dark coloured bottles filled to the brim?

To prevent its slow oxidation by air and light into the poisonous gas phosgene.

Why is DDT an environmental concern?

It is non-biodegradable, persists in the environment and accumulates in living tissues.

Haloalkanes and Haloarenes

Class 12 Chemistry · Chemistry Part II · Chapter 1

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CBSE Class 12 · Chemistry

Unit 8 carbonyls: 8 marks, biggest unit

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
7 marks in every paper: Biomolecules
6 marks in each 2025 set: Haloalkanes
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Named reactions, reagents, conditions

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Electrochemistry: 9 marks, biggest unit

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Summary

Haloalkanes and haloarenes are formed when hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced by halogen atoms. In haloalkanes the halogen is attached to an \(sp^3\) carbon of an alkyl group, while in haloarenes it is attached to an \(sp^2\) carbon of an aryl ring. They are named by the IUPAC system and classified by the number of halogen atoms and by whether the carbon bearing the halogen is primary, secondary or tertiary. They are prepared from alcohols, hydrocarbons (by free radical halogenation or addition to alkenes), and by halogen exchange. The carbon-halogen bond is polar, making the carbon a site for nucleophilic attack. Haloalkanes undergo nucleophilic substitution by two mechanisms: \(S_N1\), which is unimolecular and proceeds through a carbocation favouring tertiary halides, and \(S_N2\), which is bimolecular with inversion of configuration favouring primary halides. They also undergo elimination to form alkenes. Optical isomerism arises in molecules with a chiral carbon. Haloarenes are far less reactive toward nucleophilic substitution because of resonance and the partial double bond character of the C-X bond. Many organohalogen compounds, such as chloroform, freons and DDT, are industrially and environmentally important, though some are pollutants.

Classification and IUPAC nomenclatureMethods of preparationNature of the C-X bondNucleophilic substitution: SN1 and SN2Optical isomerism and chiralityHaloarenes and uses of organohalogens

Key terms

Haloalkane

A compound with a halogen atom bonded to an \(sp^3\) carbon of an alkyl group.

Haloarene

A compound with a halogen atom bonded to an \(sp^2\) carbon of an aromatic ring.

Nucleophilic substitution

A reaction in which a nucleophile replaces the halogen atom on a carbon.

\(S_N1\) reaction

A two-step unimolecular substitution proceeding through a carbocation intermediate.

\(S_N2\) reaction

A one-step bimolecular substitution with inversion of configuration at carbon.

Chirality

The property of a molecule that is non-superimposable on its mirror image, giving optical isomers.

Extra questions & answers

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In haloalkanes, the halogen is attached to a carbon that is:

Flashcards & interactive practice