JEE Main 2025-26 Mock Test Practice: Organic Compounds Containing Halogens

**Alkyl halides** (also called **haloalkanes**) are organic molecules where a **halogen atom** replaces a hydrogen atom in an alkane. They are classified as **primary (1°)**, **secondary (2°)**, or **tertiary (3°)** depending on the number of carbon atoms attached to the carbon bearing the halogen.

**Aryl halides** are aromatic compounds where a **halogen atom** is directly bonded to an aromatic ring (like benzene), whereas **alkyl halides** have the halogen attached to a saturated carbon atom (alkyl group). Aryl halides generally show lower reactivity in substitution reactions compared to alkyl halides.

The general formula for **haloalkanes** is **CnH2n+1X**, where **X** represents a halogen atom. For example, **chloroethane** (C2H5Cl) is a typical alkyl halide.

**Alkyl halides** can be prepared by:

• Halogenation of alkanes: Substituting hydrogen atoms in alkanes with halogens using ultraviolet light.
• Reaction of alcohols with halogen acids (HX): Converting alcohols to alkyl halides by treating them with concentrated hydrohalic acids like HCl, HBr, or HI.

A **nucleophilic substitution reaction** is a process where the **halogen atom** in an organic halide is replaced by a **nucleophile** (such as OH⁻, CN⁻, or NH3). This reaction is common for alkyl halides and follows two key mechanisms: **SN1** (unimolecular) and **SN2** (bimolecular).

Many **halogenated organic compounds** like **chlorofluorocarbons (CFCs)** and some pesticides are persistent pollutants that can remain for long periods in the environment, leading to problems like **ozone layer depletion** and **bioaccumulation** in food chains. Safe disposal and minimized use are important to reduce their environmental impact.

According to **IUPAC nomenclature**, **haloalkanes** are named by selecting the longest continuous carbon chain, numbering it to give the **halogen** the lowest possible position, and prefixing the halogen name as **fluoro-, chloro-, bromo-,** or **iodo-** to the parent alkane name. Multiple halogens are named in alphabetical order.

In **substitution reactions**, the **halogen atom** is replaced by another group (e.g., a nucleophile), producing a new compound. In **elimination reactions**, the removal of the halogen and a hydrogen atom from adjacent carbon atoms forms a double bond (alkene), with common elimination reactions being **dehydrohalogenation**.

The **carbon–halogen bond** in **alkyl iodides** is weaker compared to that in **alkyl chlorides**, due to the larger atomic size of iodine. This makes it easier to break and results in a **faster nucleophilic substitution** for iodides than for chlorides.

**Halogenated organic compounds** have wide applications, such as:

• Solvents: Used in dry cleaning and as degreasers (e.g., trichloroethylene).
• Pharmaceuticals: Many drugs contain halogen substituents (e.g., chloramphenicol).
• Refrigerants and propellants: CFCs (though now regulated).
• Pesticides: DDT and related compounds.

To detect **halogens** in an organic compound, the **Lassaigne's test** (sodium fusion test) is commonly used. The compound is fused with sodium, and the extract is treated with silver nitrate solution:

• **Chlorine:** White precipitate (AgCl)
• **Bromine:** Pale yellow precipitate (AgBr)
• **Iodine:** Yellow precipitate (AgI)