MCQ Detailed View

Ethanol and methanol are both primary alcohols, but they can be distinguished by specific chemical tests. The most reliable test for this purpose is the iodoform test.

The iodoform test detects the presence of a methyl group (-CH₃) directly attached to a carbon atom bearing a hydroxyl group (-OH). This structural arrangement is called a methyl carbinol group (-CHOH–CH₃). Ethanol (CH₃–CH₂OH), when oxidized under alkaline iodine conditions, forms acetaldehyde (CH₃CHO), which further reacts to give a yellow precipitate of iodoform (CHI₃). The yellow crystals and characteristic antiseptic smell confirm a positive test.

On the other hand, methanol (CH₃OH) does not have this structural requirement. Oxidation of methanol produces formaldehyde (HCHO), which cannot form iodoform. Therefore, methanol gives a negative iodoform test. This clear difference allows ethanol and methanol to be distinguished easily.

The other tests are not suitable for distinguishing ethanol from methanol:

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  Lucas test differentiates primary, secondary, and tertiary alcohols based on reaction with HCl and ZnCl₂. Since both ethanol and methanol are primary alcohols, they react very slowly and are not easily distinguished by this method.

  
  


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  Benedict’s test is used to identify reducing sugars, not alcohols.

  
  


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  Tollens’ test is used to detect aldehydes, not to differentiate simple alcohols.

  
  

The iodoform test is therefore the correct method to distinguish ethanol from methanol. In laboratory practice, the yellow precipitate of CHI₃ is the key observation.

This principle is important in organic chemistry for identifying alcohols and carbonyl compounds that contain the methyl carbinol group.