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Preparation of Haloalkanes and Haloarenes

Preparation methods are chemical routes used to introduce halogen into organic compounds, commonly from alcohols, hydrocarbons, or by halogen exchange reactions.

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Student-friendly explanation

Haloalkanes can be prepared from alcohols using hydrogen halides, phosphorus halides, or thionyl chloride. SOCl2 is often preferred for alkyl chlorides because the by-products are gases. Alkanes give haloalkanes by free radical halogenation, usually under light or heat, but mixtures may form. Finkelstein reaction converts alkyl chlorides or bromides to alkyl iodides using NaI in dry acetone. Swarts reaction is used to prepare alkyl fluorides from alkyl chlorides or bromides using metallic fluorides.

How to write this in exams

  1. 1

    Start with the exact idea

    Preparation methods are chemical routes used to introduce halogen into organic compounds, commonly from alcohols, hydrocarbons, or by halogen exchange reactions.

  2. 2

    Then show how to use it

    Identify the starting functional group, decide whether replacement of -OH or halogen exchange is needed, choose the reagent, write the product by replacing the leaving group, then mention the condition if it controls the reaction.

  3. 3

    Add one concrete example

    CH3CH2OH treated with SOCl2 gives CH3CH2Cl. CH3CH2Br treated with NaI in dry acetone gives CH3CH2I by halogen exchange.

  4. 4

    Avoid this incomplete answer

    Using NaI in water for Finkelstein reaction is wrong because the reaction is favoured in dry acetone where NaCl or NaBr precipitates.

Definition

Preparation methods are chemical routes used to introduce halogen into organic compounds, commonly from alcohols, hydrocarbons, or by halogen exchange reactions.

Example

CH3CH2OH treated with SOCl2 gives CH3CH2Cl. CH3CH2Br treated with NaI in dry acetone gives CH3CH2I by halogen exchange.

Rule to remember

Alcohol conversion: R-OH + SOCl2 → R-Cl + SO2 + HCl. Finkelstein: R-Cl or R-Br + NaI in dry acetone → R-I + NaCl or NaBr. Swarts: R-Cl or R-Br + metallic fluoride → R-F.

Sequence to remember

Alcohol conversion: R-OH + SOCl2>
R-Cl + SO2 + HCl. Finkelstein: R-Cl or R-Br + NaI in dry acetone>
R-I + NaCl or NaBr. Swarts: R-Cl or R-Br + metallic fluoride>
R-F. A flow chart from alcohols, hydrocarbons, and halogen exchange to haloalkanes is helpful for revision but the reactions can be learned through equations. Identify the starting functional group, decide whether replacement of -OH or halogen exchange is needed, choose the reagent, write the product by replacing the leaving group, then mention the condition if it controls the reaction

Memory hook

SOCl2 makes chlorides from alcohols; NaI swaps to iodide; Swarts swaps to fluoride.

Examples and method

Worked example

Prepare chloromethane from methanol. Treat CH3OH with SOCl2; the -OH group is replaced by Cl, forming CH3Cl with gaseous SO2 and HCl as by-products.

Method to apply

Identify the starting functional group, decide whether replacement of -OH or halogen exchange is needed, choose the reagent, write the product by replacing the leaving group, then mention the condition if it controls the reaction.

Diagram support

A flow chart from alcohols, hydrocarbons, and halogen exchange to haloalkanes is helpful for revision but the reactions can be learned through equations.

How CBSE asks it

Exams ask for reagent selection, named reaction identification, completion of reaction equations, and reasons for preferred conditions such as dry acetone in Finkelstein reaction.

Avoid common mistakes

Common confusion

Students often forget that Finkelstein reaction is driven by precipitation of NaCl or NaBr in acetone and is not a general method for aryl halides.

Common wrong answer

Using NaI in water for Finkelstein reaction is wrong because the reaction is favoured in dry acetone where NaCl or NaBr precipitates.

Exam tip

Always attach the reagent to its role: SOCl2 replaces -OH by -Cl, NaI replaces Cl or Br by I, and metallic fluorides replace Cl or Br by F.

Quick check

Which reagent converts bromoethane into iodoethane, and what is the reaction called?

NaI in dry acetone converts bromoethane into iodoethane; the reaction is Finkelstein reaction.

Answer writing and exam use

1-mark answer

Preparation methods are chemical routes used to introduce halogen into organic compounds, commonly from alcohols, hydrocarbons, or by halogen exchange reactions.

2-mark answer

Preparation methods are chemical routes used to introduce halogen into organic compounds, commonly from alcohols, hydrocarbons, or by halogen exchange reactions. Alcohol conversion: R-OH + SOCl2 → R-Cl + SO2 + HCl. Finkelstein: R-Cl or R-Br + NaI in dry acetone → R-I + NaCl or NaBr. Swarts: R-Cl or R-Br + metallic fluoride → R-F. CH3CH2OH treated with SOCl2 gives CH3CH2Cl. CH3CH2Br treated with NaI in dry acetone gives CH3CH2I by halogen exchange.

3-mark answer

Haloalkanes can be prepared from alcohols using hydrogen halides, phosphorus halides, or thionyl chloride. SOCl2 is often preferred for alkyl chlorides because the by-products are gases. Alkanes give haloalkanes by free radical halogenation, usually under light or heat, but mixtures may form. Finkelstein reaction converts alkyl chlorides or bromides to alkyl iodides using NaI in dry acetone. Swarts reaction is used to prepare alkyl fluorides from alkyl chlorides or bromides using metallic fluorides. Alcohol conversion: R-OH + SOCl2 → R-Cl + SO2 + HCl. Finkelstein: R-Cl or R-Br + NaI in dry acetone → R-I + NaCl or NaBr. Swarts: R-Cl or R-Br + metallic fluoride → R-F. Prepare chloromethane from methanol. Treat CH3OH with SOCl2; the -OH group is replaced by Cl, forming CH3Cl with gaseous SO2 and HCl as by-products. Exams ask for reagent selection, named reaction identification, completion of reaction equations, and reasons for preferred conditions such as dry acetone in Finkelstein reaction. Using NaI in water for Finkelstein reaction is wrong because the reaction is favoured in dry acetone where NaCl or NaBr precipitates.
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