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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.

In one line: SOCl2 makes chlorides from alcohols; NaI swaps to iodide; Swarts swaps to fluoride.

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1Prepare chloromethane from methanol.
  2. 2Treat CH3OH with SOCl2; the -OH group is replaced by Cl, forming CH3Cl with gaseous SO2 and HCl as by-products.

Method you can reuse

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.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
NaI in dry acetone converts bromoethane into iodoethane; the reaction is Finkelstein reaction.

Common wrong answer

Using NaI in water for Finkelstein reaction.

Why it loses marks: The reaction is favoured in dry acetone where NaCl or NaBr precipitates.

Don't mix this up

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.

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.
  1. 1

    Enough for 1 mark

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

  2. 2

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    Haloalkanes can be prepared from alcohols using hydrogen halides, phosphorus halides, or thionyl chloride.

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