To determine the most acidic compound among the given options, we need to understand acidity in organic compounds. Acidity is primarily determined by the stability of the conjugate base. A more stable conjugate base means the acid is stronger. Key factors affecting stability include resonance, inductive effects, and hydrogen bonding.

The compounds are:

• p-toluic acid (p-methylbenzoic acid)
• p-nitrophenol
• p-hydroxybenzoic acid
• o-hydroxybenzoic acid (salicylic acid)

Step 1: Compare Acidic Groups
p-toluic acid and both hydroxybenzoic acids contain a carboxylic acid group (–COOH), which is acidic. p-Nitrophenol contains a phenolic –OH group. Generally, carboxylic acids are stronger acids than phenols because the carboxylate anion is stabilized by resonance over two oxygen atoms, making it more stable than a phenoxide ion.

Step 2: Evaluate Substituent Effects
Substituents on the aromatic ring affect acidity through electronic effects:

• Electron-withdrawing groups (EWG): Increase acidity by stabilizing the conjugate base through resonance or inductive effect.
• Electron-donating groups (EDG): Decrease acidity by destabilizing the conjugate base.

For p-toluic acid, the –CH₃ group is electron-donating (+I effect), which destabilizes the carboxylate anion, making it less acidic than unsubstituted benzoic acid.

For p-nitrophenol, the –NO₂ group is strongly electron-withdrawing (–I and –R effects), which stabilizes the phenoxide ion. However, even with this stabilization, it is still weaker than carboxylic acids.

For p-hydroxybenzoic acid, the –OH group at para position is electron-withdrawing by resonance (–R effect) but electron-donating by inductive effect (+I). The net effect is electron-withdrawing, but it is less effective than –NO₂. The carboxylate anion is stabilized, but intramolecular hydrogen bonding is not possible due to the distance.

For o-hydroxybenzoic acid, the –OH group is ortho to the –COOH group. This allows for intramolecular hydrogen bonding in the conjugate base between the carboxylate oxygen and the hydrogen of the –OH group. This hydrogen bonding stabilizes the conjugate base significantly, making it much more acidic than the para isomer and other compounds. The effect is specific to ortho substitution.

Step 3: Order of Acidity
The order of acidity is: o-hydroxybenzoic acid > p-hydroxybenzoic acid > p-toluic acid > p-nitrophenol.
Thus, o-hydroxybenzoic acid is the most acidic due to intramolecular hydrogen bonding in its conjugate base.

Final Answer: o-hydroxybenzoic acid

Related Topics

Acidity and Basicity in Organic Chemistry: Acidity is influenced by factors such as atom electronegativity, resonance, inductive effects, and hydrogen bonding. For aromatic acids, the position and nature of substituents play a crucial role. Electron-withdrawing groups enhance acidity, while electron-donating groups reduce it. Ortho effect can lead to exceptional behavior due to steric or bonding interactions.

Formulae

Acidity constant: $K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[\mathrm{HA}\right]}$, lower pK_a (pK_a = -log K_a) means stronger acid.