Haloalkanes: Nomenclature, Reactions, Uses & Mechanisms

```mermaid
classDiagram
    class Haloalkane {
        +C-C bond
        +X (halogen)
        +Polar bond
        +Nucleophilic substitution
        +Elimination
    }
    class ReactionMechanism {
        <<abstract>>
        +SN1
        +SN2
        +E1
        +E2
    }
    class SN1 {
        +Unimolecular
        +Carbocation intermediate
        +Rearrangement possible
    }
    class SN2 {
        +Bimolecular
        +Backside attack
        +No rearrangement
    }
    class E1 {
        +Unimolecular elimination
        +Weak base
    }
    class E2 {
        +Bimolecular elimination
        +Strong base
    }
    Haloalkane --> ReactionMechanism
    ReactionMechanism <|-- SN1
    ReactionMechanism <|-- SN2
    ReactionMechanism <|-- E1
    ReactionMechanism <|-- E2

---

What Are Haloalkanes?

Haloalkanes (or alkyl halides) are organic compounds where one or more hydrogen atoms in an alkane are replaced by halogen atoms (F, Cl, Br, I). They are polar because the C–X bond is polar (carbon is less electronegative than halogens).

methanemethaneCH₃Cl (Haloalkane)CH₃Cl (Haloalkane)

Key Features:

• General formula: R–X (R = alkyl group, X = halogen)
• Types:
  • Monohaloalkanes: One halogen (e.g., CH₃Cl)
  • Polyhaloalkanes: Multiple halogens (e.g., CH₂Cl₂, CCl₄)
• Physical properties:
  • Boiling point: Higher than alkanes (due to dipole–dipole forces).
  • Solubility: Insoluble in water (nonpolar), soluble in organic solvents.

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Nomenclature (IUPAC Rules)

Name haloalkanes by:

1. Identifying the longest carbon chain containing the halogen.
2. Numbering the chain to give the halogen the lowest possible number.
3. Prefixing the halogen with its position (e.g., 1-chloropropane, 2-bromobutane).

Parent chain (butane)Parent chain (butane)2-Chlorobutane (correct IUPAC)2-Chlorobutane (correct IUPAC)1-Chlorobutane (incorrect numbering)1-Chlorobutane (incorrect numbering)

Examples:

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Preparation of Haloalkanes

ethaneethane+Cl₂UV lightchloroethanechloroethane+HCl

1. From Alkanes (Free Radical Substitution)

• Reaction: Alkane + Halogen (Cl₂/Br₂) → Haloalkane + HCl/HBr
• Conditions: UV light or heat (initiates free radicals).
• Example: [ \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl} ]

2. From Alcohols (Substitution)

• Reaction: Alcohol + HCl/HBr → Haloalkane + H₂O
• Example: [ \text{CH}_3\text{CH}_2\text{OH} + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} + \text{H}_2\text{O} ]

3. From Alkenes (Addition)

• Reaction: Alkene + HX → Haloalkane
• Example: [ \text{CH}_2=\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} ]

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Reactions of Haloalkanes

Haloalkanes undergo two main types of reactions:

1. Nucleophilic Substitution (SN1/SN2)
2. Elimination (E1/E2)

1. Nucleophilic Substitution (Replacement of Halogen)

A. SN1 (Unimolecular)

• Mechanism: Two-step, forms a carbocation intermediate.
• Conditions: Weak nucleophile, polar protic solvent (e.g., H₂O, alcohol), tertiary haloalkanes.
• Example: [ \text{(CH}_3\text{)}_3\text{C–Br} + \text{H}_2\text{O} \rightarrow \text{(CH}_3\text{)}_3\text{C–OH} + \text{HBr} ]

B. SN2 (Bimolecular)

• Mechanism: One-step, backside attack (inversion of configuration).
• Conditions: Strong nucleophile (e.g., OH⁻, CN⁻), primary haloalkanes, aprotic solvent.
• Example: [ \text{CH}_3\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{OH} + \text{Br}^- ]

2. Elimination (Removal of HX)

A. E1 (Unimolecular Elimination)

• Mechanism: Two-step, forms a carbocation, then loses H⁺.
• Conditions: Weak base, tertiary haloalkanes, heat.
• Example: [ \text{(CH}_3\text{)}_3\text{C–Br} \xrightarrow{\text{heat}} \text{(CH}_3\text{)}_2\text{C=CH}_2 + \text{HBr} ]

B. E2 (Bimolecular Elimination)

• Mechanism: One-step, concerted (base removes H⁺ while Br⁻ leaves).
• Conditions: Strong base (e.g., OH⁻, alkoxide), primary/secondary haloalkanes.
• Example: [ \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} + \text{Br}^- ]

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Factors Affecting SN1/SN2/E1/E2

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Applications of Haloalkanes

1. Solvents: Dichloromethane (CH₂Cl₂) is used as a solvent in labs.
2. Refrigerants: Chlorofluorocarbons (CFCs) like CCl₃F were used in refrigerators (now banned due to ozone depletion).
3. Pesticides: DDT (dichlorodiphenyltrichloroethane) was used as an insecticide.
4. Pharmaceuticals: Some haloalkanes are intermediates in drug synthesis.
5. Plastics: PVC (polyvinyl chloride) is made from chloroethene (CH₂=CHCl).

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Toxicity and Environmental Impact

• Chlorofluorocarbons (CFCs): Deplete the ozone layer (e.g., CCl₂F₂).
• Polyhalogenated compounds: Some are carcinogenic (e.g., chloroform, CHCl₃).
• Biodegradability: Most haloalkanes are non-biodegradable and persist in the environment.

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Solved Examples

Example 1: Naming

Name the compound: CH₃–CH(Cl)–CH₂–CH₃ Solution:

1. Longest chain: 4 carbons (butane).
2. Halogen position: Cl is on carbon-2. Answer: 2-Chlorobutane

Example 2: Reaction Mechanism

Predict the product of: $${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{Br}+{\text{OH}}^{-}\to ?$$ Solution:

• Primary haloalkane + strong nucleophile (OH⁻) → SN2 reaction.
• OH⁻ attacks from the backside, displacing Br⁻. Product: CH₃CH₂CH₂OH (propan-1-ol)

Example 3: Elimination vs. Substitution

What happens when 2-bromopropane reacts with ethanol (C₂H₅OH)? Solution:

• Secondary haloalkane + weak nucleophile (ethanol) → E1 or SN1.
• Heat favors elimination (E1) → forms propene (CH₃–CH=CH₂).

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NEB Board-Style Questions

Short Answer (5 marks)

1. Explain the SN1 mechanism with an example. Draw the intermediate carbocation.
2. How does the nature of the halogen affect the reactivity of haloalkanes?
3. Write the IUPAC names of the following:
   • CH₃–CH₂–CHBr–CH₃
   • CH₂Br–CH₂Br

Long Answer (10 marks)

4. Compare SN2 and E2 reactions with respect to:
   • Mechanism
   • Conditions required
   • Type of haloalkane favored
   • Products formed
5. Describe the preparation of haloalkanes from alkenes. Give one industrial application of haloalkanes.

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Exam Tip

✅ For NEB exams:

• Memorize IUPAC naming rules (common mistakes in numbering).
• Draw mechanisms for SN1/SN2/E1/E2 (examiners check arrow-pushing).
• Know the order of reactivity: I > Br > Cl > F (but F is least reactive due to strong C–F bond).
• Predict products based on substrate type (primary/secondary/tertiary) and reagent strength.
• Environmental impact is a hot topic—mention CFCs and ozone depletion if asked about applications.

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