MCQ Detailed View

Alcohols undergo substitution reactions with phosphorus halides to form alkyl halides. The hydroxyl group (–OH) of the alcohol is replaced by a halogen atom (Cl, Br, or I) supplied by the phosphorus halide. The general reaction can be written as:

3ROH + PX₃ → 3RX + H₃PO₃

Here, R–OH represents an alcohol, PX₃ is a phosphorus trihalide (such as PCl₃, PBr₃, or PI₃), R–X is the alkyl halide, and H₃PO₃ is phosphorous acid as a by-product.

For example:

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  C₂H₅OH + PCl₃ → C₂H₅Cl + H₃PO₃
  Ethanol reacts with phosphorus trichloride to form ethyl chloride (an alkyl halide).

  
  

Similarly, alcohols also react with phosphorus pentachloride (PCl₅), giving alkyl chlorides, phosphorus oxychloride (POCl₃), and hydrogen chloride (HCl).

This reaction is very useful in organic synthesis because alkyl halides are versatile intermediates. They can undergo nucleophilic substitution reactions to form a wide range of other functional groups such as amines, ethers, and nitriles.

Looking at the options:

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  Alkyl amines are obtained by reactions of alkyl halides with ammonia, not directly from alcohols.

  
  


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  Alkanes are produced in reduction reactions, not by phosphorus halides.

  
  


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  Alkynes come from dehydrohalogenation or other synthetic methods, not from alcohol–phosphorus halide reactions.

  
  

Therefore, the correct product of the reaction between alcohols and phosphorus halides is alkyl halides. This transformation is a fundamental step in organic chemistry, especially in laboratory preparations of halogenated compounds.