MCQ Detailed View

Lithium aluminium hydride (LiAlH₄) is a powerful reducing agent widely used in organic synthesis. It is capable of reducing a variety of functional groups, such as aldehydes, ketones, carboxylic acids, and esters, into their corresponding alcohols. Acetaldehyde (CH₃CHO) is a simple aldehyde with a carbonyl functional group (C=O) bonded to a hydrogen atom and a methyl group.

When acetaldehyde is treated with LiAlH₄ in a dry ether solvent, it undergoes a reduction reaction where the carbonyl group is converted into a primary alcohol. The reaction proceeds as follows:

CH3CHO  +  2[H]  →LiAlH4  CH3CH2OHCH₃CHO \; + \; 2[H] \;\xrightarrow{LiAlH₄}\; CH₃CH₂OHCH3CHO+2[H]LiAlH4CH3CH2OH

Here, the hydride ions (H⁻) from LiAlH₄ attack the electrophilic carbon atom of the carbonyl group, breaking the double bond and forming an alkoxide intermediate. After hydrolysis, the product obtained is ethyl alcohol (ethanol).

The other options given are incorrect:

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  Acetic acid: This is formed by oxidation of acetaldehyde, not reduction.

  
  


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  Methane: Complete hydrogenation of carbonyl to alkane does not occur under LiAlH₄ conditions.

  
  


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  Methyl alcohol: Reduction of formaldehyde gives methyl alcohol, not acetaldehyde.

  
  

LiAlH₄ is highly reactive and reacts violently with water and alcohols, so it must be used in anhydrous conditions. Its strong reducing ability makes it a key reagent in organic chemistry for converting aldehydes and other carbonyl compounds into alcohols.

This reaction demonstrates a functional group transformation where an aldehyde is reduced to a primary alcohol, which is one of the most fundamental and widely used synthetic methods in organic chemistry.