Physical and Chemical Properties — Core Principles

Core Principles

Carboxylic acids, defined by the $-COOH$ group, exhibit distinct physical and chemical properties. Physically, they possess unusually high boiling points due to strong intermolecular hydrogen bonding, leading to stable cyclic dimerization.

Lower members are water-soluble due to hydrogen bonding with water, but solubility decreases with increasing alkyl chain length. They have pungent odours at lower molecular masses, becoming odourless waxy solids for higher members.

Chemically, their most prominent feature is their acidic nature, arising from the resonance stabilization of the carboxylate anion after proton donation. They react with active metals, bases, and carbonates/bicarbonates (producing $CO_2$ effervescence).

They undergo nucleophilic acyl substitution reactions, forming derivatives like esters, acid chlorides, anhydrides, and amides, by cleavage of the C-OH bond. The entire carboxyl group can be reduced to a primary alcohol using $LiAlH_4$ or removed via decarboxylation (e.

g., with soda-lime). The $alpha$-carbon can be halogenated via the Hell-Volhard-Zelinsky (HVZ) reaction, while aromatic carboxylic acids undergo meta-directed electrophilic substitution.

Important Differences

vs Alcohols and Phenols (Acidity)

Aspect	This Topic	Alcohols and Phenols (Acidity)
Functional Group	Carboxylic Acids (R-COOH)	Alcohols (R-OH) & Phenols (Ar-OH)
Acidity Strength	Weak acids, stronger than phenols and alcohols.	Alcohols are extremely weak acids (weaker than water). Phenols are weak acids, stronger than alcohols but weaker than carboxylic acids.
Conjugate Base Stability	Carboxylate anion ($R-COO^-$) is resonance stabilized (negative charge delocalized over two oxygen atoms).	Alkoxide ion ($R-O^-$) has localized negative charge. Phenoxide ion ($Ar-O^-$) is resonance stabilized, but the negative charge is on one oxygen and delocalized onto less electronegative carbon atoms of the ring.
Reaction with $NaHCO_3$	Reacts with $NaHCO_3$ to produce $CO_2$ effervescence.	Do not react with $NaHCO_3$ (no $CO_2$ effervescence).
Reaction with NaOH	Reacts with NaOH to form sodium carboxylate salt and water.	Alcohols do not react with NaOH. Phenols react with NaOH to form sodium phenoxide salt and water.

Carboxylic acids are significantly more acidic than both alcohols and phenols. This enhanced acidity is primarily due to the superior resonance stabilization of the carboxylate anion, where the negative charge is delocalized symmetrically over two highly electronegative oxygen atoms. In contrast, the alkoxide ion from an alcohol has a localized negative charge, making it highly unstable. While the phenoxide ion from a phenol is resonance stabilized, the negative charge is delocalized onto the less electronegative carbon atoms of the benzene ring, making it less stable than the carboxylate anion. This difference in acidity is evident in their reactions: carboxylic acids react with bicarbonates to produce $CO_2$, a test not shown by phenols or alcohols.