Acetic acid

Acetic acid (also called methylcarboxylic acid or ethanoic acid) can be in the form of the acetate ion. It is found in vinegar, and is primarily responsible for its sour taste and smell. Its formula is CH₃-COOH (C₂H₄O₂). According to IUPAC, it is systematically called ethanoic acid.

Chemical formula; the carboxyl group, which gives it acidity, is in blue.

It is the second simplest carboxylic acid after formic or methanoic acid, which only has one carbon, and before propanoic acid, which already has a three-carbon chain.

The melting point is 16.6 °C and the boiling point is 117.9 °C.

In aqueous solution, it can lose the proton from the carboxyl group to give its conjugate base, acetate. Its pKa is 4.8 at 25 °C, which means that at a moderately acidic pH of 4.8, half of its molecules will have detached from the proton. This makes it a weak acid and, in suitable concentrations, it can form buffer solutions with its conjugate base. The dissociation constant at 20 °C is Ka = 1.75 10⁻⁵.

It is of interest for organic chemistry as a reagent, for inorganic chemistry as a ligand, and for biochemistry as a metabolite (activated as acetyl-coenzyme A). It is also used as a substrate, in its activated form, in reactions catalyzed by enzymes known as acetyltransferases and, specifically, histone acetyltransferases.

Today, the natural way to obtain it is through the carbonylation (reaction with CO) of methanol. In the past it was produced by oxidation of ethylene into acetaldehyde, which was subsequently oxidized to finally obtain acetic acid.

Production

It is produced by synthesis and by bacterial fermentation. Today, the biological route provides about 10% of world production, but it remains important in vinegar production, as global food purity laws stipulate that vinegar for use in food must be of biological origin. About 75% of the acetic acid made in the chemical industry is prepared by carbonylation of methanol, explained later. Alternative methods (such as ethyl formate isomerization, syngas conversion, ethylene and ethanol oxidation) provide the rest.

In different reactions acetic acid is a by-product. For example, in the synthesis of acrylic acid from propane, propylene, and acrolein, acetic acid is also produced with selectivities between 1 and 15%. Fermentative production of lactic acid also produces acetic acid. Total world production of virgin acetic acid is estimated at 5 Mt/y (million tons per year), and approximately half is produced in the United States. Europe's production is approximately 1 Mt/y and is declining, and Japan produces 0.7 Mt/y. Another 1.5 Mt is recycled each year, giving, on the world market, a total of 6.5 Mt/a. The two largest producers of virgin acetic acid are Celanese and BP. Other major producers include Millennium Chemicals, Sterling Chemicals, Samsung, Eastman Chemical Company, and Svensk Etanolkemi.

Carbonylation of methanol

Most of the acetic acid is produced by carbonylation of methanol. In this process, methanol and carbon monoxide react to produce acetic acid, according to the chemical equation:

CH3OH+COΔ Δ CH3COOH{displaystyle CH_{3}OH+COlongrightarrow CH_{3}COOH}

The process involves iodomethane as an intermediate, and it happens in three steps. A catalyst, usually a metal complex, is needed for the carbonylation (step 2).

(1) CH3OH+HIΔ Δ CH3I+H2O{displaystyle CH_{3}OH+HIlongrightarrow CH_{3}I+H_{2}O}

(2) CH3I+COΔ Δ CH3COI{displaystyle CH_{3}I+COlongrightarrow CH_{3}COI}

(3) CH3COI+H2OΔ Δ CH3COOH+HI{displaystyle CH_{3}COI+H_{2}Olongrightarrow CH_{3}COOH+}HI

By changing the process conditions, acetic anhydride can also be produced in the same plant. Since both methanol and carbon monoxide are cheap raw materials, methanol carbonylation appeared to be an attractive method for the production of acetic acid.

Acetic acid purification plant. Photo of 1884.

Henry Dreyfus, at the British Celanese company, developed a pilot plant for the carbonylation of methanol as early as 1925. However, the lack of practical materials that could contain the corrosive reaction at the high pressure (200 atm) required discouraged the marketing of these routes. The first commercial methanol carbonylation process, using a cobalt catalyst, was developed by the German chemical company BASF in 1963.

In 1968, a catalyst based on rhodium (cis-[Rh (CO)₂I₂]^- showed that it could work efficiently at lower temperatures, and with almost no by-products.The first plant to use this catalyst was built by the North American chemical company Monsanto in 1970, and rhodium-catalyzed carbonylation of methanol became the dominant method of acetic acid production (see Monsanto process) In the late 1990s, BP chemical companies commercialized the Cativa process catalyst (Ir(CO)₂I₂]⁻), which is promoted by ruthenium.This iridium-catalyzed process is greener and more efficient and has widely replaced the Monsanto process, often in the same production plants.

Oxidation of acetaldehyde

Prior to the commercialization of the Monsanto process, most acetic acid was produced by oxidation of acetaldehyde. This remains the second most important manufacturing method, although it is not competitive with methanol carbonylation.

Acetaldehyde can be produced by oxidation of butane or light naphtha, or by hydration of ethylene. When butane or light naphtha is heated with air in the presence of various metal ions, including manganese, cobalt, and chromium; peroxide is formed and then decomposes to produce acetic acid according to the chemical equation:

2C4H10+5O2Δ Δ 4CH3COOH+2H2O{displaystyle 2C_{4}H_{10}+5O_{2}longrightarrow 4CH_{3}COOH+2H_{2}O}

Generally, the reaction is carried out at a combination of temperature and pressure designed to be as hot as possible while keeping the butane in the liquid phase. Typical reaction conditions are 150 °C and 55 atm. By-products can be formed, including butanone, ethyl acetate, formic acid, and propionic acid. These by-products are also of commercial value, and reaction conditions can be modified to produce more of them if they are economically useful. However, the separation of acetic acid from the by-products adds cost to the process.

Under similar conditions and using catalysts similar to those used for the oxidation of butane, acetaldehyde can be oxidized by oxygen in air to produce acetic acid.

2CH3CHO+O2Δ Δ 2CH3COOH{displaystyle 2CH_{3}CHO+O_{2}longrightarrow 2CH_{3CO}OH}

Using modern catalysts, this reaction can have a yield of acetic acid greater than 95%.

The main by-products are ethyl acetate, formic acid and formaldehyde, all of which have a lower boiling point than acetic acid, and can be easily separated by distillation.

Oxidation of ethylene

Acetaldehyde can be prepared from ethylene by the Wacker process, but it should be noted that it cannot be oxidized. More recently, a cheaper, single-stage conversion of ethylene to acetic acid has been commercialized by the chemical company Showa Denko, which opened an ethylene oxidation plant in Ōita, Japan, in 1997. The process is triggered by a catalyst. palladium metal on a heteropolyacid support, such as tungstosilicic acid. This method is believed to be competitive with methanol carbonylation in small plants (100–250 kt/y), depending on the local ethylene price.

Oxidative fermentation

For most of human history, acetic acid, in the form of vinegar, has been prepared by bacteria of the genus Acetobacter. In the presence of sufficient oxygen, these bacteria can produce vinegar from a wide variety of alcoholic foods. Some common inputs are cider, wine, fermented cereal, malt, rice, or potatoes. The general chemical reaction facilitated by these bacteria is:

C2H5OH+O2Δ Δ CH3COOH+H2O{displaystyle C_{2}H_{5}OH+O_{2}longrightarrow CH_{3}COOH+H_{2}O}

A dilute solution of alcohol, inoculated with Acetobacter and kept in a warm and airy place will become vinegar in the course of a few months. Industrial vinegar preparation methods speed up this process by improving the oxygen supply to the bacteria.

Probably, the first vinegar production was a consequence of fermentation errors during the winemaking process. If the must is fermented at too high a temperature, acetobacter will overpower the yeast naturally present in the grapes.

As demand for vinegar for culinary, medical, and sanitary purposes increased, winemakers quickly learned to use other organic materials to produce vinegar in the warm summer months, before the grapes ripen and are ready for processing in came. However, this method was slow and not always successful, and winemakers did not understand the process.

One of the first modern business processes was the "quick method" or 'German method', first practiced in Germany in 1823. In this process, fermentation takes place in a tower packed with wood chips or charcoal. The alcoholic input is pumped into the top of the tower and fresh air is supplied from the bottom, by natural or forced convection. The improved air supply in this process reduces the time to prepare vinegar from months to weeks.

Most vinegar today is made in submerged tank culture, first described in 1949 by Otto Hromatka and Heinrich Ebner. In this method, alcohol is fermented to vinegar in a continuously stirred tank, and oxygen is supplied by bubbling air through the solution. Using modern applications of this method, 15% acetic acid vinegar can be made in just 24 hours in a batch process, even 20% in 60 hours.

Anaerobic fermentation

Some species of anaerobic bacteria, including members of the genus Clostridium, can convert sugars to acetic acid directly, without using ethanol as an intermediate. The total chemical reaction carried out by these bacteria can be represented by:

C6H12O6Δ Δ 3CH3COOH{displaystyle C_{6}H_{12}O_{6}longrightarrow 3CH_{3}COOH}

More interesting from an industrial chemist's point of view is the fact that these acetogens can produce acetic acid from one-carbon compounds, including methanol, carbon monoxide, or a mixture of carbon dioxide and hydrogen:

2CO2+4H2Δ Δ CH3COOH+2H2O{displaystyle 2CO_{2}+4H_{2}longrightarrow CH_{3}COOH+2H_{2}O}

This ability of Clostridium to use sugars directly, or to produce acetic acid from less expensive inputs, means that these bacteria could produce acetic acid more efficiently than ethanol oxidizers such as Acetobacter . However, Clostridium bacteria are less acid tolerant than Acetobacter. Even the most acid-tolerant Clostridium strains can only produce vinegar of a very low percentage concentration of acetic acid, compared to Acetobacter strains that can produce vinegar of up to 20% acid acetic. At present, it is still more cost effective to produce vinegar using Acetobacter than to produce it using Clostridium and then concentrate it. As a result, although acetogenic bacteria have been known since the 1940s, their industrial use remains confined to a few applications.

Applications and uses

• In apculture, it is used for the control of the larvae and eggs of the wax moths, disease called galleriosis, which destroy the wax panes that the honey bees work to raise or accumulate the honey.
• Its applications in the chemical industry are very linked to its esters, such as vinyl acetate or cellulose acetate (base for the manufacture of nylon, rayon, zealot, etc.).
• Its properties are widely known as a bite in fixative solutions, for the preservation of tissues (histology), where it acts empirically as a nucleoprotein fixer, and not as a plasmatic protein, whether globular or fibrous. (Results endorsed by J. Baker).
• In the revealing of black and white photographs, it was and is used in a very weak solution like "bath of unemployment": when the revealed material was immersed in it, the alkalinity of the revealing bathroom was neutralized and the process was stopped; then the fixing bathroom eliminated the rest of unrevealed material. The fixers in turn, which mainly use sodium thyosulphate as a main component, incorporate acetic acid as an acidulant (acid agent) to keep the pH of the solution low enough compared to that of the revealing bathroom.
• The cellulose acetate for its qualities of transparency and flexibility is also used as one of the basic materials or support for the manufacture of film and photographic films on which the photosensitive emulsion is applied. It was also used for a time as a support or basis for the manufacture of tape magnetophonic, although for this application it was replaced advantageously by the polyester.
• Other uses in medicine are like dye in colpocopies to detect infection by human papillomavirus, when the cérvix tissue is stained white with acetic acid is positive for human papilloma virus infection, this stain is known as positive white acetate.

Also, mixed with alcohol, it is useful for the prevention of external otitis.

• It also serves in cleaning stains in general.
• It is also used for kitchen uses such as vinegar and also cleaning.
• In Radiology, radio plaque revealing processors use an acetic acid bathroom to stop the process of development, where the radiographic path comes from.

Security

Concentrated acetic acid is corrosive and should therefore be handled with appropriate care, as it can cause skin burns, permanent eye damage, and irritation to mucous membranes. These burns may not appear until hours after exposure. Latex gloves offer no protection, so especially heavy-duty gloves, such as those made of nitrile rubber, should be worn when handling this compound.

Concentrated acetic acid is difficult to ignite in the laboratory. There is a risk of flammability if the ambient temperature exceeds 39 °C (102 °F), and it can form explosive mixtures with air above this temperature (explosive limit: 5.4%-16%).

The dangers of acetic acid solutions depend on their concentration. The following table lists the EU classification of acetic acid solutions:

Security Symbol

Solutions of more than 25% acetic acid are handled in a fume hood, due to the corrosive and pungent vapor. Dilute acetic acid, in the form of vinegar, is harmless. However, the ingestion of strong solutions is dangerous to human and animal life in general. It can cause severe damage to the digestive system, and cause a potentially lethal change in the acidity of the blood.

Due to incompatibilities, it is recommended that acetic acid be stored away from chromic acid, ethylene glycol, nitric acid, and perchloric acid.