MCQ Detailed View

Grignard reagents are highly reactive organometallic compounds of the form R–MgX, where R is an alkyl or aryl group and X is a halogen. They are widely used in organic synthesis for building carbon–carbon bonds. A common application of Grignard reagents is their reaction with carbonyl compounds such as aldehydes and ketones to produce alcohols after hydrolysis.

The mechanism involves nucleophilic attack by the carbanion-like R group on the electrophilic carbon of the carbonyl group. Depending on the type of carbonyl compound, different classes of alcohols are formed. For example:

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  Formaldehyde (H–CHO) reacts with a Grignard reagent to give a primary alcohol.

  
  


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  Acetaldehyde (CH₃–CHO) reacts to form a secondary alcohol.

  
  


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  Propanone (CH₃–CO–CH₃) reacts to form a tertiary alcohol.

  
  

However, when carbon dioxide (CO₂) reacts with a Grignard reagent, the product is not an alcohol. Instead, the reaction leads to the formation of a carboxylate salt, which upon acid hydrolysis gives a carboxylic acid. For example, ethyl magnesium bromide reacts with CO₂ to form ethanoic acid after hydrolysis. This reaction is an important method for the synthesis of carboxylic acids using Grignard reagents.

This distinction is crucial for students preparing for chemistry exams because it highlights that not all electrophilic compounds reacting with Grignard reagents yield alcohols. Carbonyl compounds (aldehydes and ketones) form alcohols, while carbon dioxide leads to the formation of carboxylic acids.

Thus, among the given options, carbon dioxide does not form an alcohol with a Grignard reagent, making it the correct answer.