21.1 Naming Carboxylic Acid Derivatives

Naming and Identifying Carboxylic Acid Derivatives

Introduction to Carboxylic Acid Derivatives

Carboxylic acid derivatives all contain a carbonyl group ($C=O$) bonded to a leaving group. That leaving group is what distinguishes one derivative from another and determines both the name and the reactivity of the compound. The six main types you need to know are acid halides, anhydrides, esters, amides, thioesters, and acyl phosphates.

Naming these derivatives follows a consistent logic: you start from the parent carboxylic acid name and swap out the suffix depending on which leaving group is present. Once you see the pattern, the naming system clicks into place quickly.

IUPAC Nomenclature for Acid Derivatives

Every derivative name starts from the parent carboxylic acid. The suffix changes depending on the functional group attached to the acyl carbon.

• Acid halides: Replace -ic acid with -yl halide, specifying the halogen. Acetic acid becomes acetyl chloride; propanoic acid becomes propanoyl bromide.
• Anhydrides: Replace -ic acid with -ic anhydride. For symmetrical anhydrides, you only name the acid once (acetic anhydride). For unsymmetrical anhydrides, name both acyl groups in alphabetical order (acetic propanoic anhydride).
• Esters: Replace -ic acid with -ate. Name the alkyl group on the oxygen side first, then the carboxylate portion. Acetic acid + ethanol gives ethyl acetate; propanoic acid + methanol gives methyl propanoate.
• Amides: Replace -ic acid with -amide. If the nitrogen carries alkyl substituents, label them with an N- prefix. Acetic acid gives acetamide; with a methyl group on nitrogen, it becomes N-methylacetamide. Two substituents on nitrogen use N,N- (e.g., N,N-dimethylacetamide).
• Thioesters: Replace -ic acid with -thioate. Name the alkyl group on sulfur first, just like you would for esters. Acetic acid + ethanethiol gives S-ethyl thioacetate (or ethyl thioacetate).
• Acyl phosphates: Replace -ic acid with -yl phosphate. Acetic acid becomes acetyl phosphate. These are especially important in biochemistry (ATP hydrolysis involves acyl phosphate chemistry).

Structural Features of Acid Derivatives

Each derivative type has a distinct structural formula built around the acyl group ($R-C=O$).

• Acid halides ($R-COX$, where $X = Cl, Br, I$): A halogen atom is bonded directly to the acyl carbon. Chlorides are by far the most common.
• Anhydrides ($R-CO-O-CO-R'$): Two acyl groups share a central oxygen. You can think of them as two carboxylic acids joined together with loss of water.
• Esters ($R-COOR'$): An alkoxy group ($-OR'$) is bonded to the acyl carbon. The $R'$ group comes from the alcohol component.
• Amides: A nitrogen atom is bonded to the acyl carbon. Amides come in three forms depending on nitrogen substitution:
  • Primary (unsubstituted): $R-CONH_2$
  • Secondary (monosubstituted): $R-CONHR'$
  • Tertiary (disubstituted): $R-CONR'R''$
• Thioesters ($R-COSR'$): A sulfur atom replaces the oxygen of an ester. The alkyl group is attached to sulfur. Acetyl-CoA is a biologically critical thioester.
• Acyl phosphates ($R-CO-O-PO(OH)_2$): A phosphate group is bonded through oxygen to the acyl carbon. These are high-energy intermediates in metabolic pathways.

Drawing Structures from IUPAC Names

When you're given an IUPAC name and need to draw the structure, follow these steps:

1. Identify the parent acid. The root name tells you the carbon chain length (e.g., "acet-" = 2 carbons, "propano-" = 3 carbons).

2. Determine the derivative type from the suffix. The ending tells you which functional group to draw:

   • -yl halide → acid halide
   • -ic anhydride → anhydride
   • -ate → ester
   • -amide → amide
   • -thioate → thioester
   • -yl phosphate → acyl phosphate

3. Draw the acyl group and attach the correct functional group.

   • Acid halide: attach the halogen to the carbonyl carbon
   • Anhydride: draw two acyl groups linked by a bridging oxygen
   • Ester: attach $-OR'$ to the carbonyl carbon (the alkyl group named first in the name is $R'$)
   • Amide: attach $-NH_2$, $-NHR'$, or $-NR'R''$ depending on N- prefixes
   • Thioester: attach $-SR'$ to the carbonyl carbon
   • Acyl phosphate: attach $-O-PO(OH)_2$ to the carbonyl carbon

4. Add any substituents. Number the parent chain starting from the carbonyl carbon (C1) and place substituents at the positions indicated in the name.