Haloalkanes and haloarenes notes

For complete details: Haloalkane

 Here are detailed notes on Haloalkanes and 

Haloarenes for Class 12 based on the NCERT syllabus:

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1. Introduction

Haloalkanes and Haloarenes are compounds in which one or more hydrogen atoms in an alkane or arene are replaced by halogen atoms (F, Cl, Br, or I).

The general formula of haloalkanes: where X is a halogen.

Haloalkanes are also known as alkyl halides and haloarenes as aryl halides.

2. Classification of Haloalkanes and Haloarenes

(a) Based on the Number of Halogen Atoms:

1. Monohaloalkanes: Contain one halogen atom (e.g., CH₃Cl – chloromethane).

2. Dihaloalkanes: Contain two halogen atoms (e.g., CH₂Cl₂ – dichloromethane).

3. Trihaloalkanes: Contain three halogen atoms (e.g., CHCl₃ – chloroform).

4. Tetrahaloalkanes: Contain four halogen atoms (e.g., CCl₄ – carbon tetrachloride).

(b) Based on the Nature of the Carbon-Halogen Bond:

1. Primary (1°) Haloalkanes: The halogen is attached to a carbon atom connected to only one other carbon atom (e.g., CH₃CH₂Cl – ethyl chloride).

2. Secondary (2°) Haloalkanes: The halogen is attached to a carbon atom connected to two other carbon atoms (e.g., CH₃CHClCH₃ – isopropyl chloride).

3. Tertiary (3°) Haloalkanes: The halogen is attached to a carbon atom connected to three other carbon atoms (e.g., (CH₃)₃CCl – tert-butyl chloride).

3. Nomenclature of Haloalkanes and Haloarenes

(a) IUPAC Naming for Haloalkanes:

1. Identify the longest carbon chain containing the halogen atom.

2. Number the chain from the end nearer to the halogen atom.

3. Halogen atoms are treated as prefixes, and their position is indicated by the number of the carbon atom to which they are attached.

Example: CH₃CH₂Cl → IUPAC Name: Chloroethane

(b) IUPAC Naming for Haloarenes:

1. The base name is derived from the parent aromatic compound (benzene).

2. Halogen substituents are denoted by prefixes like chloro-, bromo-, fluoro-, or iodo-.

Example: C₆H₅Cl → IUPAC Name: Chlorobenzene

4. Preparation of Haloalkanes and Haloarenes

(a) Methods of Preparation of Haloalkanes:

1. From Alcohols: Alcohols react with halogen acids (HX) to form haloalkanes.

\text{R-OH} + \text{HX} \rightarrow \text{R-X} + \text{H}_2\text{O}

2. From Alkanes: Alkanes undergo halogenation (substitution reaction) in the presence of UV light.

\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl}

3. From Alkenes: Alkenes react with halogens (X₂) in the presence of inert solvents like CCl₄ to give dihaloalkanes.

\text{CH}_2 = \text{CH}_2 + \text{Cl}_2 \rightarrow \text{CH}_2\text{Cl-CH}_2\text{Cl}

(b) Methods of Preparation of Haloarenes:

1. From Arenes: Haloarenes are prepared by electrophilic substitution reactions. For example, benzene reacts with halogens in the presence of a Lewis acid like FeCl₃.

\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}

5. Physical Properties of Haloalkanes and Haloarenes

1. Boiling Points: Haloalkanes and haloarenes have higher boiling points than the corresponding hydrocarbons due to their higher molecular masses and stronger intermolecular forces (dipole-dipole interactions).

2. Solubility: Haloalkanes and haloarenes are insoluble in water due to the lack of hydrogen bonding but are soluble in organic solvents.

3. Density: The density increases as the molecular weight and the number of halogen atoms increase.

6. Chemical Properties of Haloalkanes and Haloarenes

(a) Reactions of Haloalkanes:

1. Nucleophilic Substitution Reaction (SN1 and SN2):

SN1 mechanism (Unimolecular nucleophilic substitution): Favors tertiary haloalkanes.

SN2 mechanism (Bimolecular nucleophilic substitution): Favors primary haloalkanes.

Example: Hydrolysis of an alkyl halide to form an alcohol.

\text{R-X} + \text{KOH} \rightarrow \text{R-OH} + \text{KX}

2. Elimination Reaction: Haloalkanes undergo elimination reactions to form alkenes.

Example: Dehydrohalogenation of alkyl halides in the presence of alcoholic KOH.

\text{R-CH}_2\text{CH}_2\text{X} + \text{KOH (alc.)} \rightarrow \text{R-CH=CH}_2 + \text{KX} + \text{H}_2\text{O}

3. Reaction with Metals (Wurtz Reaction): Alkyl halides react with sodium metal in dry ether to form higher alkanes.

2\text{R-X} + 2\text{Na} \xrightarrow{\text{Dry ether}} \text{R-R} + 2\text{NaX}

(b) Reactions of Haloarenes:

1. Electrophilic Substitution: Haloarenes undergo typical electrophilic substitution reactions (like nitration, sulfonation, halogenation, and Friedel-Crafts reactions) at the ortho and para positions due to the electron-withdrawing effect of the halogen atom.

Example: Chlorobenzene reacts with nitric acid in the presence of sulfuric acid to form 2-nitrochlorobenzene and 4-nitrochlorobenzene.

\text{C}_6\text{H}_5\text{Cl} + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} 2\text{-nitrochlorobenzene} + 4\text{-nitrochlorobenzene}

2. Nucleophilic Substitution: Haloarenes are less reactive towards nucleophilic substitution than haloalkanes due to resonance stabilization of the carbon-halogen bond and the partial double bond character.

7. Environmental Impact of Haloalkanes and Haloarenes

Some haloalkanes, like CFCs (Chlorofluorocarbons), are harmful to the ozone layer. They are responsible for ozone depletion and contribute to global warming.

Haloarenes like DDT (Dichlorodiphenyltrichloroethane) and PCBs (Polychlorinated biphenyls) are non-biodegradable and persist in the environment, causing pollution and harmful effects on living organisms.

8. Uses of Haloalkanes and Haloarenes

Haloalkanes are used as solvents, refrigerants (CFCs), anesthetics (chloroform), and in the production of various chemicals.

Haloarenes are used in the synthesis of pharmaceuticals, agrochemicals, and in organic

 synthesis.

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These notes cover the important aspects of Haloalkanes and Haloarenes for Class 12 NCERT. Let me know if you need further details or explanations!