The naming of carboxylic acids and their derivatives follows systematic rules, primarily established by the International Union of Pure and Applied Chemistry (IUPAC), alongside some common names that are widely accepted. Understanding these rules is crucial for accurately identifying and communicating about these fundamental organic compounds.
Naming Carboxylic Acids
Carboxylic acids are organic compounds characterized by a carboxyl group (–COOH). Their nomenclature is foundational for understanding their derivatives.
IUPAC Nomenclature
For acyclic carboxylic acids, the IUPAC system involves replacing the "-e" ending of the corresponding alkane name with "-oic acid". The carbon atom of the carboxyl group is always assigned position 1.
- Rule 1: Identify the Longest Carbon Chain: Find the longest continuous carbon chain that includes the carboxyl group.
- Rule 2: Number the Chain: Number the chain starting from the carboxyl carbon (C-1).
- Rule 3: Name Substituents: Identify and name any substituents on the parent chain, indicating their position by number.
- Rule 4: Apply the Suffix: Replace the "-e" from the corresponding alkane name with "-oic acid". If the suffix "-oic acid" begins with a vowel sound, the final "e" of the parent alkane name is dropped before adding the suffix. For example, the three-carbon alkane propane becomes propanoic acid.
Examples:
- $\text{CH}_3\text{COOH}$ (2 carbons) → Ethane → Ethanoic acid (Common name: Acetic acid)
- $\text{CH}_3\text{CH}_2\text{COOH}$ (3 carbons) → Propane → Propanoic acid (Common name: Propionic acid)
- $\text{HOOC}-\text{COOH}$ (2 carbons, dicarboxylic acid) → Ethane → Ethanedioic acid (Common name: Oxalic acid)
Common Nomenclature
Many simple carboxylic acids have widely used common names that do not follow the systematic IUPAC rules but are derived from their historical sources. These often use Greek letters ($\alpha$, $\beta$, $\gamma$, etc.) to indicate the position of substituents, starting from the carbon adjacent to the carboxyl group ($\alpha$-carbon).
Common Carboxylic Acids:
- Formic acid ($\text{HCOOH}$) - from ants (Latin: formica)
- Acetic acid ($\text{CH}_3\text{COOH}$) - from vinegar (Latin: acetum)
- Propionic acid ($\text{CH}_3\text{CH}_2\text{COOH}$) - "first fat" acid
- Butyric acid ($\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$) - from butter (Latin: butyrum)
Carboxylic Acids Attached to Rings
When the carboxyl group is directly attached to a ring, the suffix "-carboxylic acid" is added to the name of the parent ring system. The carbon atom of the carboxyl group is not included in the numbering of the ring.
Example:
- A benzene ring with a -COOH group → Benzoic acid (common and accepted IUPAC name)
- A cyclohexane ring with a -COOH group → Cyclohexanecarboxylic acid
Naming Carboxylic Acid Derivatives
Carboxylic acid derivatives are compounds in which the -OH group of a carboxylic acid is replaced by another group. These include esters, acid halides, acid anhydrides, amides, and nitriles.
Esters
Esters are derived from a carboxylic acid and an alcohol. They are named as alkyl alkanoates.
- Rule 1: Name the Alkyl Group (from the alcohol): This is the group attached to the oxygen atom that was originally part of the alcohol.
- Rule 2: Name the Carboxylate Group (from the carboxylic acid): Replace the "-ic acid" or "-oic acid" suffix of the parent carboxylic acid with "-oate".
Examples:
- $\text{CH}_3\text{COOCH}_2\text{CH}_3$ → Ethyl from ethanol + Acetate from acetic acid → Ethyl acetate (IUPAC: Ethyl ethanoate)
- $\text{CH}_3\text{CH}_2\text{COOCH}_3$ → Methyl from methanol + Propanoate from propanoic acid → Methyl propanoate
Acid Halides
Acid halides (or acyl halides) have a halogen atom (e.g., Cl, Br) replacing the -OH group of a carboxylic acid.
- Rule: Replace Suffix: Replace the "-ic acid" or "-oic acid" suffix of the parent carboxylic acid with "-yl halide" (e.g., "-yl chloride", "-yl bromide").
Examples:
- $\text{CH}_3\text{COCl}$ → Acetic acid → Acetyl chloride (IUPAC: Ethanoyl chloride)
- $\text{CH}_3\text{CH}_2\text{COBr}$ → Propanoic acid → Propanoyl bromide
Acid Anhydrides
Acid anhydrides are formed by the condensation of two carboxylic acid molecules with the loss of a water molecule.
- Symmetrical Anhydrides: If derived from two identical carboxylic acids, replace "acid" in the carboxylic acid name with "anhydride".
- Unsymmetrical (Mixed) Anhydrides: If derived from two different carboxylic acids, list the names of both parent acids (alphabetically) followed by "anhydride".
Examples:
- $(\text{CH}_3\text{CO})_2\text{O}$ → Acetic acid → Acetic anhydride (IUPAC: Ethanoic anhydride)
- $\text{CH}_3\text{CO-O-COCH}_2\text{CH}_3$ → Acetic acid and Propanoic acid → Acetic propanoic anhydride (IUPAC: Ethanoic propanoic anhydride)
Amides
Amides are derivatives where the -OH group of a carboxylic acid is replaced by an amino group ($\text{-NH}_2$, $\text{-NHR}$, or $\text{-NR}_2$).
- Rule: Replace Suffix: Replace the "-oic acid" or "-ic acid" suffix of the parent carboxylic acid with "-amide".
- N-Substituted Amides: If there are alkyl groups attached to the nitrogen atom, they are named as "N-alkyl" or "N,N-dialkyl" prefixes before the amide name.
Examples:
- $\text{CH}_3\text{CONH}_2$ → Acetic acid → Acetamide (IUPAC: Ethanamide)
- $\text{CH}_3\text{CH}_2\text{CONHCH}_3$ → Propanoic acid with a methyl group on nitrogen → N-Methylpropanamide
- $\text{CH}_3\text{CON(CH}_3)_2$ → Acetic acid with two methyl groups on nitrogen → N,N-Dimethylacetamide (IUPAC: N,N-Dimethylethanamide)
Nitriles
Nitriles contain a cyano group ($\text{-C}\equiv\text{N}$). Although not strictly a carbonyl derivative, they are often grouped with carboxylic acid derivatives due to their hydrolysis to carboxylic acids.
- Rule 1: Name as Alkanenitrile: Add the suffix "-nitrile" to the alkane name corresponding to the longest carbon chain, including the carbon atom of the cyano group. The nitrile carbon is C-1.
- Rule 2: Cyclic Systems: If the -CN group is attached to a ring, add "-carbonitrile" to the ring name.
Examples:
- $\text{CH}_3\text{C}\equiv\text{N}$ (2 carbons) → Ethane → Ethanenitrile (Common name: Acetonitrile)
- $\text{CH}_3\text{CH}_2\text{C}\equiv\text{N}$ (3 carbons) → Propane → Propanenitrile
- A benzene ring with a -CN group → Benzonitrile
Summary Table of Carboxylic Acid Derivative Nomenclature
| Derivative Type | General Structure | Suffix/Prefix (IUPAC) | Example (IUPAC) | Common Example (if applicable) |
|---|---|---|---|---|
| Carboxylic Acid | R-COOH | -oic acid | Propanoic acid | Acetic acid |
| Ester | R-COOR' | Alkyl alkanoate | Methyl propanoate | Ethyl acetate |
| Acid Halide | R-COX (X=halogen) | -oyl halide | Ethanoyl chloride | Acetyl chloride |
| Acid Anhydride | R-CO-O-CO-R' | -oic anhydride (symmetrical); -oic -oic anhydride (mixed) | Ethanoic anhydride | Acetic anhydride |
| Amide | R-CONH2, R-CONHR', R-CONR'R" | -amide; N-alkyl-amide | Ethanamide; N-Methylpropanamide | Acetamide; N-Methylacetamide |
| Nitrile | R-C$\equiv$N | -nitrile | Ethanenitrile | Acetonitrile |
