Acetic acid

Acetic acid, systematically called ethanoic acid, is a colorless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2 or HC2H3O2). When it is undiluted it is sometimes referred to as glacial acetic acid. Vinegar is no less than 4% vol. Acetic acid, which makes vinegar the main ingredient of vinegar besides water. Acetic acid has a distinctive sour taste and pungent odor. In addition to household vinegar, it is mainly produced as a precursor to polyvinyl acetate and cellulose acetate. It is classified as a weak acid because it dissociates only partially in solution, but concentrated acetic acid is caustic and can damage the skin.

Acetic acid is the second simplest carboxylic acid (after formic acid). It consists of a methyl group attached to a carboxyl group. It is an important chemical reagent and industrial chemical, mainly used in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue and synthetic fibers and fabrics. In households, dilute acetic acid is often used in descaling agents. In the food industry, acetic acid is regulated by the food additive code E260 as an acidity regulator and as a condiment. In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all life forms. When bound to Coenzyme A, it is central to carbohydrate and fat metabolism.

The worldwide demand for acetic acid is about 6.5 million tons per year (Mt / a), of which about 1.5 Mt / a is collected through recycling; the rest is made from methanol. Vinegar is usually dilute acetic acid, often produced by fermentation and subsequent oxidation of ethanol.

Applications

Acetic acid is a chemical reagent for the production of chemical compounds. The largest single use of acetic acid is vinyl acetate monomer production, followed closely by acetic anhydride and ester production. The volume of acetic acid used in vinegar is relatively small.

-Monomer of vinyl acetate

The main use of acetic acid is the production of vinyl acetate monomer (VAM). In 2008, this application would consume an estimated one third of the world's acetic acid production. The reaction consists of ethylene and acetic acid with oxygen over a palladium catalyst, carried out in the gas phase.

Vinyl acetate can be polymerized to polyvinyl acetate or other polymers, which are components in paints and adhesives.

-Ester production

The major esters of acetic acid are often used as solvents for inks, paints and coatings. The esters include ethyl acetate, n-butyl acetate, isobutyl acetate and propyl acetate. They are usually produced by a catalyzed reaction of acetic acid and the accompanying alcohol.

However, most acetate esters are produced from acetaldehyde using the Tishchenko reaction. In addition, ether acetates are used as solvents for nitrocellulose, acrylic lacquers, lacquer removers and wood stains. First, glycol monoethers are produced from ethylene oxide or propylene oxide with alcohol, which are then esterified with acetic acid. The three main products are ethylene glycol monoethyl ether acetate (EER), ethylene glycol monobutyl ether acetate (EBA) and propylene glycol monomethyl ether acetate (PMA, more commonly known as PGMEA in semiconductor manufacturing processes, where it is used as a resist solvent). This application consumes about 15% to 20% acetic acid worldwide. Ether acetates, for example EER, have been shown to be harmful to human reproduction.

-Use as a solvent

As noted above, glacial acetic acid is an excellent polar protic solvent. It is often used as a recrystallization solvent to purify organic compounds. Acetic acid is used as a solvent in the production of terephthalic acid (TPA), the raw material for polyethylene terephthalate (PET). In 2006 about 20% acetic acid was used for the production of TPA.

Acetic acid is often used as a solvent for reactions involving carbocations, such as Friedel-Crafts alkylation. For example, one stage in the commercial manufacture of synthetic camphor includes a Wagner-Meerwein rearrangement of camphene to isobornyl acetate; here acetic acid acts both as a solvent and as a nucleophile to trap the rearranged carbocation.

Glacial acetic acid is used in analytical chemistry to estimate weakly alkaline substances such as organic amides. Glacial acetic acid is a much weaker base than water, so the amide behaves like a strong base in this medium. It can then be titrated with an glacial acetic acid solution of a very strong acid such as perchloric acid.

-Medical use

Acetic acid is used in many areas of the developing world as part of cervical cancer screening. The acid is applied to the cervix, and if a white area appears after about a minute, the test is positive.

Acetic acid is an effective antiseptic when used as a 1% solution, with a wide spectrum of action against streptococci, staphylococci, pseudomonas, enterococci and others. It can be used to treat skin infections caused by strains of pseudomonas resistant to typical antibiotics.

Although dilute acetic acid is used in iontophoresis, no high-quality evidence supports this treatment for rotator cuff disease.

As a treatment for otitis externa, it is on the World Health Organization's list of essential medicines, the safest and most effective medicines needed in a health system.

-Foods

Acetic acid has 349 kcal per 100 g. Vinegar is typically no less than 4% acetic acid by mass. Legal limits for acetic acid content vary by jurisdiction. Vinegar is used directly as a condiment and in canning vegetables and other foods. Table vinegar tends to be more dilute (4% to 8% acetic acid), while commercial canning with food uses more concentrated solutions. The proportion of acetic acid used worldwide as vinegar is not as large as commercial use, but it is by far the oldest and best known use.

Other derivatives

Organic or inorganic salts are produced from acetic acid. Some commercially important derivatives:

-Sodium acetate, used in the textile industry and as a food preservative (E262).
Copper (II) acetate, used as a pigment and fungicide.
Aluminum acetate and iron (II) acetate - used as stains for dyes.
Palladium (II) acetate, used as a catalyst for organic coupling reactions such as the Heck reaction.

Halogenated acetic acids are produced from acetic acid. Some commercially important derivatives:

Chloroacetic acid (monochloroacetic acid, MCA), dichloroacetic acid (considered a by-product) and trichloroacetic acid. MCA is used in the manufacture of indigo dye.
Bromoacetic acid, which has been esterified to produce the reagent ethyl bromoacetate.
-Trifluoroacetic acid, a commonly used reagent in organic synthesis.

The amounts of acetic acid used in these other applications together account for an additional 5-10% of the world's acetic acid use.