Asked in PMC National MDCAT 2020 2020Moderate

The exact reactivity order for carboxylic acid derivatives is:

Correct answer: D. Acylchloride > Anhydride > ester

  • A. Anhydride > Acylchloride > ester
  • B. Ester > Anhydride > Acylchloride
  • C. Amide > Acylchloride > ester
  • D. Acylchloride > Anhydride > ester

Explanation

Acyl chlorides are the most reactive carboxylic acid derivatives. The electronegative chlorine atom pulls electrons toward it in the C-Cl bond, which makes the carbonyl carbon more electrophilic. This makes nucleophilic attack easier. Also, the Cl- is an excellent leaving group, so that step is also fast. Electronically, polarized acid derivatives are attacked more readily than less polar ones. Thus, acid chlorides are more reactive than anhydrides, which are more reactive than esters, which are more reactive than amides.

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About Carboxylic Acid Derivatives

Carboxylic acid derivatives include acid halides, acid anhydrides, esters and amides, formed by replacing the hydroxyl group of a carboxylic acid. Their nomenclature, structures, preparation, hydrolysis and relative reactivity are linked through nucleophilic acyl substitution, which differs from the nucleophilic addition reactions of aldehydes and ketones.

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