Haloalkanes and Haloarenes

 Chapter Summary: Haloalkanes and Haloarenes (CBSE Class XII)

The chapter "Haloalkanes and Haloarenes" is a fundamental topic in organic chemistry for the CBSE Class XII curriculum, introducing students to a crucial class of organic compounds. This chapter typically has a weightage of around 6 marks in the board exams.

Key Concepts

1. Introduction and Classification:

 * Haloalkanes: These are organic compounds where a hydrogen atom in an alkane is replaced by a halogen atom. The halogen is attached to an sp^3 hybridized carbon atom.

 * Haloarenes: These are aromatic compounds where a hydrogen atom in a benzene ring is replaced by a halogen atom. The halogen is attached to an sp^2 hybridized carbon atom.

 * Classification: Compounds are classified as mono-, di-, or poly-halogen derivatives based on the number of halogen atoms. Haloalkanes are further classified as primary (1^\circ), secondary (2^\circ), or tertiary (3^\circ) based on the nature of the carbon atom to which the halogen is bonded.

2. Nomenclature and Nature of the C-X Bond:

 * Nomenclature: The chapter covers the IUPAC naming of haloalkanes and haloarenes.

 * Nature of C-X Bond: The carbon-halogen bond is polar due to the higher electronegativity of the halogen atom. This makes the carbon atom partially positive (\delta+) and the halogen atom partially negative (\delta-), influencing the reactivity of the molecule. The bond length increases and bond strength decreases as you move down the halogen group (C-F < C-Cl < C-Br < C-I).

3. Preparation of Haloalkanes and Haloarenes:

 * Haloalkanes: Common methods include:

   * From alcohols, using halogen acids, phosphorus halides, or thionyl chloride.

   * From alkanes, via free radical halogenation.

   * From alkenes, by addition of hydrogen halides (following Markovnikov's rule or anti-Markovnikov's rule).

   * Halogen exchange reactions like Finkelstein and Swarts reactions.

 * Haloarenes: Preparation methods include:

   * Halogenation of aromatic hydrocarbons in the presence of a Lewis acid.

   * Sandmeyer and Gattermann reactions from diazonium salts.

4. Physical Properties:

 * Boiling Points: Boiling points of haloalkanes are higher than their parent alkanes due to their polarity and higher molecular mass. The boiling point increases with the size of the halogen and the length of the carbon chain.

 * Density: The density of haloalkanes is generally higher than that of water.

 * Solubility: Despite their polarity, haloalkanes are only slightly soluble in water because they cannot form hydrogen bonds with water molecules.

5. Chemical Reactions:

 * Nucleophilic Substitution Reactions (S_N1 and S_N2): This is a key section of the chapter.

   * S_N2 (bimolecular): A single-step reaction where the nucleophile attacks from the side opposite to the leaving group. It is favored by primary (1^\circ) haloalkanes. The rate depends on both the concentration of the substrate and the nucleophile.

   * S_N1 (unimolecular): A two-step reaction involving the formation of a carbocation intermediate. It is favored by tertiary (3^\circ) haloalkanes. The rate depends only on the concentration of the substrate.

 * Elimination Reactions: Haloalkanes can undergo elimination reactions (dehydrohalogenation) to form alkenes, following Saytzeff's rule (the major product is the more substituted alkene).

 * Reaction with Metals: Haloalkanes react with metals like magnesium to form Grignard reagents and with sodium (Wurtz reaction).

 * Reactions of Haloarenes: Haloarenes are less reactive towards nucleophilic substitution due to factors like resonance stabilization, partial double bond character of the C-X bond, and the sp^2 hybridized carbon. They undergo electrophilic substitution reactions, where the halogen group is deactivating but ortho- and para-directing.

6. Polyhalogen Compounds:

 * The chapter also briefly covers the uses and environmental effects of important polyhalogen compounds like dichloromethane, chloroform, iodoform, freons, and DDT.