krit.club logo

Haloalkanes and Haloarenes - Classification and Nomenclature

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Haloalkanes are compounds where halogen atoms are attached to the sp3sp^3 hybridized carbon atom of an alkyl group (R−XR-X), whereas Haloarenes contain halogen atoms attached to sp2sp^2 hybridized carbon atoms of an aryl group (Ar−XAr-X).

•

Classification based on the number of halogen atoms: Monohaloalkanes (one XX), Dihaloalkanes (two XX), and Polyhaloalkanes (multiple XX atoms).

•

Classification of Csp3−XC_{sp^3}-X compounds: 1. Alkyl halides (R−CH2XR-CH_2X is 1∘1^\circ, R2CHXR_2CHX is 2∘2^\circ, R3CXR_3CX is 3∘3^\circ); 2. Allylic halides (halogen attached to sp3sp^3 C adjacent to a C=CC=C double bond); 3. Benzylic halides (halogen attached to sp3sp^3 C adjacent to an aromatic ring).

•

Classification of Csp2−XC_{sp^2}-X compounds: 1. Vinylic halides (halogen attached directly to a carbon of a C=CC=C double bond); 2. Aryl halides (halogen attached directly to the aromatic ring).

•

Dihaloalkanes can be classified as Geminal dihalides (two halogen atoms on the same carbon atom, also called alkylidene halides) and Vicinal dihalides (two halogen atoms on adjacent carbon atoms, also called alkylene dihalides).

•

IUPAC Nomenclature Rules: Select the longest carbon chain containing the halogen; number the chain to give the halogen the lowest possible locant; if multiple halogens are present, list them alphabetically (e.g., 'bromo' before 'chloro').

📐Formulae

CnH2n+1X (General formula for Monohaloalkanes)C_nH_{2n+1}X \text{ (General formula for Monohaloalkanes)}

R−CH2−X (Primary - 1∘ halide)R-CH_2-X \text{ (Primary - } 1^\circ\text{ halide)}

R2CH−X (Secondary - 2∘ halide)R_2CH-X \text{ (Secondary - } 2^\circ\text{ halide)}

R3C−X (Tertiary - 3∘ halide)R_3C-X \text{ (Tertiary - } 3^\circ\text{ halide)}

CH2=CH−CH2−X (Allylic halide structure)CH_2=CH-CH_2-X \text{ (Allylic halide structure)}

C6H5−CH2−X (Benzylic halide structure)C_6H_5-CH_2-X \text{ (Benzylic halide structure)}

💡Examples

Problem 1:

Give the IUPAC name of the following compound: CH3−CH(Cl)−CH(Br)−CH3CH_3-CH(Cl)-CH(Br)-CH_3

Solution:

2-Bromo-3-chlorobutane

Explanation:

The longest chain has 4 carbons (butane). Numbering starts from the right to give the substituents the lowest set of locants (2,3). Since 'bromo' comes alphabetically before 'chloro', it is written first: 2-Bromo-3-chlorobutane.

Problem 2:

Classify CH2=CH−CH2−ClCH_2=CH-CH_2-Cl as vinylic, allylic, or alkyl halide.

Solution:

Allylic halide

Explanation:

In CH2=CH−CH2−ClCH_2=CH-CH_2-Cl, the chlorine atom is attached to an sp3sp^3 hybridized carbon atom which is adjacent to a carbon-carbon double bond (C=CC=C). This is the characteristic structure of an allylic halide.

Problem 3:

Write the IUPAC name for C6H5BrC_6H_5Br.

Solution:

Bromobenzene

Explanation:

The halogen (Bromine) is directly attached to the benzene ring. According to IUPAC nomenclature for haloarenes, the prefix 'halo' is added to 'benzene'. Thus, it is Bromobenzene.

Classification and Nomenclature - Revision Notes & Key Formulas | CBSE Class 12 Chemistry