HC2H3O2 is the chemical formula for the organic compound acetic acid. Also called ethanoic acid, acetic acid is a colorless liquid compound that plays a vital role in all biological processes. Its chemical formula is sometimes written as CH3COOH or CH3CO2H to emphasize its atomic organization. Acetic acid is the least complex carboxylic acid (aside from formic acid) and is composed out of a single methyl group attached to a carboxyl group.
The presence of an acetyl group in a single molecule of acetic acid is central to its role in the metabolism of living organisms. During cellular respiration, the acetyl group derived from acetic acid binds to coenzyme A which allows the metabolism of carbohydrates and fats. It is also a natural byproduct of the fermentation of produce, such as fruit, grain, rice, and potatoes.
Apart from its crucial role in biology, acetic acid is an important industrial chemical that is used to produce a number of consumer goods. Acetic acid is involved in the production of photography film, glues, fibers, fabrics, and cleaning agents. In the food industry, acetic acid is often used as an additive as it has a distinct sour smell and taste. It is the second main constituent of common household vinegar, after water. Though safe to consume in dilute amounts, concentrated acetic acid can damage skin and internal organs.
Molecular Structure Of Acetic Acid
Acetic acid is composed out of two separate functional groups, a methyl (R[CH3]) and a carboxyl (R[COOH]) group. The two central carbon atoms bond, leaving the three hydrogens from the methyl at one end, and the hydroxyl group from the carboxyl on the other. The organization of the atoms gives acetic acid a pseudo-tetrahedral structure, with three hydrogens serving as the base and the carboxyl group as the tip. Acetic acid has a molar mass of about 60.05 g/mol and is about as dense as water (~1.05g/cm)
The hydroxyl group (R[OH]) at the end of the molecule makes acetic acid slightly polar. When immersed in a liquid solvent, the hydrogen on the hydroxyl group has a tendency to disassociate, releasing a proton (H+) and a conjugate base called acetate (CH3COO−). The release of this single proton explains the acidic character of acetic acid. Acetic acid is a relatively weak acid compared to things like hydrochloric (HCL) acid or sulfuric acid (H2SO4). A 1 molar solution of acetic acid has a pH of about 2.4, meaning that only 0.4% of the molecules have donated a proton.
The presence of the hydroxyl group at the carboxyl end also makes acetic acid slightly polar. As such, it has a higher boiling point than other compounds of an analogous structure. This high boiling point is also explained by the tendency for the hydroxyl group to make hydrogen bonds with other nearby molecules. Likewise, the polarity and hydrogen bonding gives acetic acid a relatively high specific heat capacity of 123.1 J/K⋅mol.
Chemical Properties Of Acetic Acid
The bonding behavior of acetic acid is due to its constituent functional groups. Much of the chemical bonding character of acetic acid is due to its carboxyl group and accompanying OH group. The hydroxyl group allows molecules of acetic acid to engage in hydrogen bonding. The polar hydrogen end of the hydroxyl group will attract the polar negative oxygen atom in the carbonyl group of a neighboring molecule of acetic acid, forming a strong electrostatic attraction. In a solid sample of acetic acid, the action of hydrogen bonding causes the molecules to form long, semi-stable chains.
Acetic acid is an effective solvent and can dissolve not only polar compounds but non-polar compounds as well. Unlike water, it is miscible with oils and like water, it can dissolve most organic compounds.
In terms of chemical reactions, acetic acid will react in the way expected for carboxylic acid. it will form acetates and water when introduced to a basic environment, and it can be reduced by the addition of hydrogen to form ethanol (alcohol).
As it is acidic, acetic acid will react corrosively with metals to make acetate salts. For example, acetic acid will react with magnesium (Mg) to form magnesium acetate (Mg(CH3COOH)2) and hydrogen gas (H2). The oxidation of metals with acidic compounds is sometimes used to create industrial amounts of hydrogen gas.
Acetic Acid In Biology
Acetic acid is one of the integral components in cellular respiration. The acetyl group (CH3CO) which is derived from acetic acid is an important source of energy that drives the biosynthesis of ATP, the fundamental energy currency of life. During cellular respiration, the acetyl group will bind with coenzyme-A, which binds with a molecule of oxaloacetate to form citrate, the central component of the Krebs cycle. Without the acetyl group derived from acetic acid, the biosynthesis of ATP would not be able to continue.
Acetic acid is also the natural byproduct of ethanol fermentation. The fermentation of glucose creates ethanol and minute amounts of acetic acid. In the presence of oxygen, bacteria from the genus Acetobacter are capable of converting ethanol into acetic acid and water. Through human history, the activity of Acetobacter has been the main method of producing vinegar for culinary and industrial use. Acetobacter is still the main method of vinegar production in the food industry.
Other bacteria are capable of directly converting sugars or carbon dioxide into acetic acid without ethanol as an intermediate reactant. Though a more efficient chemical reaction than Acetobacter, most of these kinds of bacteria are ironically not acid tolerant.
Production Of Acetic Acid
Acetic acid can be produced a few ways, natural and synthetic.
Naturally, most acetic acid is formed as the byproduct of biological reactions. Oxygenic respiration in eukaryotes produces acetic acid by the oxidation of pyruvate for use in the Krebs cycle, and various bacteria excrete acetic acid as a byproduct of their metabolisms. Bacterial fermentation remains the primary method of producing acetic acid in the form of vinegar, producing about 10% of the total amount of acetic acid produced in a year. Nowadays, acetic acid production is done in aluminum tanks cultures, where ethanol is fermented to acetic acid.
Most synthetic quantities of acetic acid are created via a process called methanol carbonylation. In this reaction, methanol (CH3OH) and carbon monoxide (CO) undergo a three-step reaction to create a single molecule of acetic acid. Over 70% of the world’s acetic acid is produced by methanol carbonylation. a modified process is used to produce acetates and acetic esters. Other processes involve the oxidation of ethylene (C2H4) and the oxidation of acetaldehyde (CH3CO).
Uses Of Acetic Acid
Due to its chemical flexibility, acetic acid has found a number of applications, both as an important precursor for widely used substances and as a substance in its own right. Many of these applications developed in antiquity. Early human civilizations made acetic acid in the form of vinegar from exposing wine and fermented barley to air. The Greek philosopher Theophrastus described how vinegar could be combined with various metals to produce pigments for art and the Romans boiled vinegar to make a sweet confectionery syrup called sapa.
“If you pour oil and vinegar into the same vessel, you would call them not friends but opponents.” — Aeschylus
Today, most industrial quantities of acetic acid are used to make polyvinyl acetate, a rubbery polymer that is used in adhesive products. Polyvinyl acetate derived glues are used for carpentry, book-binding, envelopes, and as a base in gum chewing products. Polyvinyl acetate is also used in paper products and paint.
Acetic acid also is used as a solvent to facilitate reactions and purify organic compounds. An acetate-based solvent is used in the production of polyethylene terephthalate, a major component in clothing fiber and food packaging products. The solvent properties of acetic acid are used to make a number of polymers.
Acetic acid has also shown use in cleaning agents and sanitizing products. The solvent properties of acetic acid make it good for removing residue and stains on surfaces, hence why it and other acetate-derived compounds are often used in the lab to clean equipment and glassware. Acetic acid also appears to have moderate anti-bacterial and anti-fungal properties. Dilute solutions of acetic acid (~0.3%) have been shown to kill various strains of bacteria, like E. Coli and the MRSA bacteria, and acetic acid can be used to stop the growth of parasitic fungi. The acid creates an oxidizing environment that damages the membranes of bacteria and the cell walls of fungi.
“Red wine vinegar has some personality as well as acidity.” — Alex Guarnaschelli
Acetic acid is also commonly used in the kitchen, in the form of vinegar. All vinegar is typically 4% acetic acid by volume. Vinegar is commonly used as a condiment or as a reagent for pickling vegetables and other food. In small doses and dilute concentrations, the ingestion of acetic acid is harmless and potentially beneficial to humans. However, in large quantities and high concentrations (>25%), acetic acid can damage the skin and internal organs.