Properties of Acetic acid

Acidity

The hydrogen center in the carboxyl group (-COOH) in carboxylic acids (e.g. acetic acid) can be dissociated from the molecule by ionization:

CH3COOH ⇌ CH3CO−2 + H+

Acetic acid is acidic due to the release of protons (H+). Acetic acid is a weak monoprotic acid. In aqueous solution,it has a pKa of 4.76.Its conjugate base is acetate (CH3COO−).A 1.0 M solution (approximately the concentration of domestic vinegar) had a pH of 2.4, indicating that only 0.4% of the acetic acid molecules were dissociated.However, in very dilute (< 10−6 M) solutions, acetic acid is >90% dissociated.

Structure 

In solid acetic acid, the molecules form chains, with individual molecules linked to each other by hydrogen bonds.In steam at 120 °C (248 °F),dimers can be detected.Dimers also occur in the liquid phase in dilute solutions of non-hydrogen-bonding solvents, and to some extent in pure acetic acid,but are disrupted by hydrogen-bonding solvents.The dissociation enthalpy of the dimer is estimated to be 65.0–66.0 kJ/mol, and the dissociation entropy is 154–157 J mol−1 K−1.Other carboxylic acids participate in similar intermolecular hydrogen-bonding interactions.

Solvent properties

Liquid acetic acid is a hydrophilic (very) protic solvent, similar to ethanol and water.With a relative permittivity (dielectric constant) of 6.2, it can dissolve not only polar compounds such as inorganic salts and sugars, but also non-polar compounds such as oils and polar solutes.It is miscible with polar and non-polar solvents such as water, chloroform and hexane.For higher alkanes (starting with octane), acetic acid is immiscible in all components, and the solubility of acetic acid in alkanes decreases with increasing n-alkanes.The solvent and miscibility of acetic acid make it a useful industrial chemical, for example, as a solvent in the production of dimethyl terephthalate.

Biochemistry:

At physiological pH, acetic acid is usually fully ionized to acetate.The acetyl group, formally derived from acetic acid, is the basis of all life forms.When combined with Coenzyme A,it is central to carbohydrate and fat metabolism.Unlike long-chain carboxylic acids (fatty acids),acetic acid does not occur in natural triglycerides.However, the artificial triglyceride triacetin (triacetin) is a common food additive found in cosmetics and topical medications.Acetic acid is produced and excreted by acetic acid bacteria, particularly Acetobacter and Clostridium acetobutylicum.These bacteria are ubiquitous in food, water and soil, and acetic acid is naturally produced as fruit and other foods spoil.Acetic acid, also an ingredient in vaginal lubricants in humans and other primates,appears to be a mild antimicrobial.