Step 1: Understanding the Concept:
The acidity of carboxylic acids is influenced by electronic effects. Electron-withdrawing groups (EWG) like \(-NO_{2}\) increase acidity, while electron-releasing groups (ERG) like \(-CH_{3}\) or \(-OCH_{3}\) decrease acidity. Aliphatic acids are generally weaker than aromatic acids. Step 2: Detailed Explanation:
We compare the given compounds with benzoic acid:
1. Nitro-substituted benzoic acids: \(NO_{2}\) is an EWG, making them stronger than benzoic acid.
2. Salicylic acid: Ortho-hydroxy group increases acidity due to intramolecular H-bonding and the ortho effect, making it stronger.
3. \(o\)-Methylbenzoic acid: Increases acidity due to the ortho effect, making it stronger.
4. \(p\)-Methylbenzoic acid: \(CH_{3}\) is an ERG (+I and hyperconjugation), making it weaker.
5. \(p\)-Methoxybenzoic acid: \(OCH_{3}\) is a strong ERG (+M effect), making it weaker.
6. Acetic acid (\(CH_{3}COOH\)): Aliphatic acids (except formic) are generally weaker than benzoic acid because the phenyl group is slightly electron-withdrawing.
From the list, the 3 weaker acids are \(p\)-methylbenzoic acid, \(p\)-methoxybenzoic acid, and acetic acid. Step 3: Final Answer:
The number of compounds weaker than benzoic acid is 3.
