CHAPTER 16: Unit 2. Nomenclature and Classification
Nomenclature and Classification
Learning Objectives
- Name carboxylic acids with common names.
- Name carboxylic acids according to IUPAC nomenclature.
The IUPAC name for a monocarboxylic acid is formed by replacing the final -e of the hydrocarbon present name with -oic acid. Following the nomenclature in previous chapters, the longest carbon chain containing the functional group is identified and it is numbered starting with the carboxyl carbon atom. Common names are more widely used for carboxylic acids than any other organic compounds.
Monocarboxylic acids:
- Select as the parent carbon chain the longest carbon chain that includes the carbon atom of the carboxyl group.
- name the parent chain by changing the -e ending of the corresponding alkane to -oic acid. Note that there is a space between -oic and acid.
- Number the parent chain by assigning the number 1 to the carboxyl carbon atom, but omit the number from the name.
- determine the identity and location of any substituent in the usual manner and append this information to the front of the parent chain name.
- If the carboxylic acid is bonded to a carbon ring, name the ring and add the words “carboxylic acid”. the carbon bearing carboxylic acid group is always carbon 1. Locate any other ring substituents in the usual manner.
In a carboxylic acid, the carbon atoms are named relative to the carboxyl group (-COOH), which is the functional group responsible for its acidic properties. Here’s how the naming works:
Carbon Nomenclature in Carboxylic Acids
- Alpha (α) carbon: This is the first carbon atom adjacent to the carboxyl carbon. It’s directly bonded to the carbon of the -COOH group.
- Beta (β) carbon: This is the second carbon atom away from the carboxyl carbon, following the alpha carbon.
- Carboxyl carbon: This is the carbon atom within the -COOH group itself. It is not considered alpha or beta—it’s the reference point.
Example: Propanoic Acid (CH₃CH₂COOH)
- COOH group: The carboxyl carbon is the one double-bonded to oxygen and single-bonded to OH.
- Alpha carbon: The CH₂ group directly attached to the carboxyl carbon.
- Beta carbon: The CH₃ group attached to the alpha carbon.
CH₃ — CH₂ — COOH
↑ ↑ ↑
β α carboxyl
Carboxylic acids are classified by the number of carboxyl groups present monocarboxylic, dicarboxylic etc) and degree of unsaturation ( saturated, unsaturated, aromatic) and by additional functional groups present ( hydroxy, keto etc.)
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| Ethanoic Acid |
 |
| 2-bromobutanoic acid |
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| Benzoic Acid |
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| Phthalic Acid(dicarboxylic Acid) |
| Some are long chain ( number of carbon between 12-20) saturated or unsaturated carboxylic acids called fatty acids. They are ingredients of soap and many other organic compounds. |
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| Some carboxylic acids are very popular with their common names. Here some of the examples and nomenclature |
| Carbon atoms | Common name | IUPAC name |
| 2 | Acetic acid | Ethanoic acid |
| 3 | Propionic acid | Propanoic acid |
| 4 | Butyric acid | Butanoic acid |
| 5 | Valeric acid | Pentanoic acid |
Polyfunctional carboxylic acid: A carboxylic acid that contains one or more additional functional groups besides one or more carboxylic acid groups.
example: keto acid, hydroxy acid or unsaturated acid.
Citric Acid
Example
Write the condensed structural formula for β-chloropropionic acid.
Solution
Propionic acid has three carbon atoms: C–C–COOH. Attach a chlorine (Cl) atom to the parent chain at the beta carbon atom, the second one from the carboxyl group: Cl–C–C–COOH. Then add enough hydrogen atoms to give each carbon atom four bonds: ClCH2CH2COOH.
IN CLASS PRACTICE PROBLEMS:
Draw the structure of aspirin.
Write the name of the following compounds
Homework Exercise
1. Draw the structure for each compound.
a. heptanoic acid
b. 3-methylbutanoic acid
c. 2,3-dibromobenzoic acid
d. β-hydroxybutyric acid (3-Hydroxy Butanoic Acid)
2. Draw the structure for each compound.
a. o-nitrobenzoic acid
b. p-chlorobenzoic acid
c. 3-chloropentanoic acid
d. α-chloropropionic acid ( 2-Chloropropanoic Acid)
3. Name each compound with either the IUPAC name, the common name, or both.
a. (CH3)2CHCH2COOH
b. (CH3)3CCH(CH3)CH2COOH
c. CH2OHCH2CH2COOH
4. Name each compound with its IUPAC name.
a. CH3(CH2)8COOH
b. (CH3)2CHCCl2CH2CH2COOH
c. CH3CHOHCH(CH2CH3)CHICOOH
Answers
1.
a. CH3CH2CH2CH2CH2CH2COOH
b.
c.
d.
2. a. 
b. 
c. 
d. 
a. 3-methylbutanoic acid; β-methylbutyric acid
b. 3,4,4-trimethylpentanoic acid
c. 4-hydroxybutanoic acid; γ- hydroxybutyric acid