Acrylic acid

Description

Acrylic acid, scientifically known as prop-2-enoic acid (CH2=CHCO2H), is the fundamental unsaturated carboxylic acid featuring a vinyl group directly attached to a carboxylic acid group. This colorless liquid, recognized by its pungent, tart odor, is highly soluble in water, alcohols, ethers, and chloroform. With an annual production exceeding one billion kilograms, acrylic acid's high reactivity allows it to polymerize into polyacrylic acid, utilized in absorbent hygiene products. Additionally, it can esterify with alcohols to form acrylates, which are integral in numerous polymer formulations. Despite the common misconception, polymers known as acrylics, such as Plexiglas and textiles, often derive from sources other than acrylic acid, even though they contain acrylate monomers.

Chemical properties

Acrylic acid is a colorless, flammable, and corrosive liquid or solid (below 13 C) with an irritating, rancid, odor. Sinks and mixes with water; irritating vapor is produced.

The Uses of Acrylic acid

Acrylic acid undergoes the typical reactions of a carboxylic acid and, when reacted with an alcohol, it will form the corresponding ester. The most common alkyl esters of acrylic acid are methyl-, butyl-, ethyl-, and 2-ethylhexyl-acrylate.
Acrylic acid and its esters readily combine with themselves (to form polyacrylic acid) or other monomers (e.g. acrylamides, acrylonitrile, vinyl, styrene, and butadiene) by reacting at their double bond, forming homopolymers or copolymers.

What are the applications of Application

Acrylic acid is synthesised by oxidation of acrolein or hydrolysis of acrylonitrile and can be used:
(1) The production of various plastics;
(2) As a component of paints, polishes and adhesives to enhance their adhesive and protective properties;
(3) As a coating for leather to improve its durability and appearance;
(4) Used as an antimicrobial agent；
(5) used as an intermediate in the synthesis of acrylates, which are essential for the manufacture of various polymers.

Definition

Acrylic acid is an unsaturated liquid carboxylic acid with a pungent odor. The acid and its esters are used to make acrylic resins.

Production Methods

Acrylic acid is produced from propene which is a by product of ethylene and gasoline production. CH₂=CHCH₃ + 1.5 O₂→ CH₂=CHCO₂H + H₂O Because acrylic acid and its esters have long been valued commercially, many other methods have been developed but most have been abandoned for economic or environmental reasons. An early method was the hydrocarboxylation of acetylene ("Reppe chemistry") : HCCH + CO + H₂O → CH₂=CHCO₂H This method requires nickel carbonyl and high pressures of carbon monoxide. It was once manufactured by the hydrolysis of acrylonitrile which is derived from propene by ammoxidation, but was abandoned because the method cogenerates ammonium derivatives. Other now abandoned precursors to acrylic acid include ethenone and ethylene cyanohydrin.

Preparation

There are three routes to acrylic acid which have commercial significance; they are based on propylene, acetylene and ethylene respectively. At the present time, most acrylic acid is produced via the propylene route.
(i) Propylene route. This route involves the two-stage oxidation of propylene:

A mixture of propylene, air and steam is fed into a reactor containing a catalyst at about 320??C to give acrolein. This intermediate is not isolated but is passed directly to a second reactor, also containing a catalyst, at about 280??C. The effluent is cooled by contact with cold aqueous acrylic acid.
Acrylic acid is extracted from the solution with a solvent and then separated by distillation. Because of the ready availability of low cost propylene, this route has become the preferred route for the production of acrylic acid. (ii) Acetylene route. This route involves the reaction of acetylene, carbon monoxide and water:

In one process, the reaction is conducted in solution in tetrahydrofuran at about 200??C and 6-20 MPa (60--200 atmospheres). Nickel bromide is used as catalyst. The solution of acrylic acid in tetrahydrofuran, after separation of the unconverted acetylene and carbon monoxide in a degassing column, passes to a distillation tower where tetrahydrofuran is taken overhead and acrylic acid is the bottom product. The reaction between acetylene, carbon monoxide and water may also be carried out by using nickel carbonyl as the source of carbon monoxide. In this case, milder reaction conditions are possible. Owing to the high cost of acetylene, this route is now of little commercial importance.
(iii) Ethylene route. This route consists of the following sequence:

The addition of hydrogen cyanide to ethylene oxide takes place at 55-60??C in the presence of a basic catalyst such as diethylamine. The reaction is exothermic and is carried out in solution to facilitate control; the solvent is conveniently ethylene cyanohydrin. The reaction mixture is neutralized and ethylene cyanohydrin is separated by distillation. The second stage of the synthesis involves the dehydration and hydrolysis of ethylene cyanohydrin; these reactions are carried out in one step by heating the cyanohydrin with aqueous sulphuric acid at about 175??C. (It is possible, of course, that the intermediate in this conversion may be acrylonitrile, as shown, or P-hydroxypropionic acid or both.) At one time this was the standard route for the preparation of acrylic acid but it has been largely displaced by the more economical propylene route.

Reactivity Profile

Acrylic acid may polymerize violently especially when the frozen acid is partially thawed (freezing point 12°C or 53°F). Frozen acid should be melted at room temperature and the process should be well stirred. Do not use heat during the melting process. Corrodes iron and steel and polymerization may occur on contact with iron salts. The uninhibited acid polymerizes exothermically at ambient temperature and explodes if confined. The inhibitor (usually hydroquinone) greatly reduces the tendency to polymerize. Explosive polymerization can also occur with strong bases, amines, ammonia, oleum, chlorosulfonic acid, and peroxides. Mixing with 2-aminoethanol, 28% ammonium hydroxide, ethylenediamine or ethyleneimine in a closed container causes an increase in temperature and pressure. Can react violently with oxidizing reagents and strong bases.

Health Hazard

Acrylic acid is a corrosive liquid that cancause skin burns. Spill into the eyes candamage vision. The vapors are an irritantto the eyes. The inhalation hazard is oflow order. An exposure to 4000 ppm for 4 hours was lethal to rats.

Safety

Acrylic acid is severely irritating and corrosive to the skin and the respiratory tract. Eye contact can result in severe and irreversible injury. Low exposure will cause minimal or no health effects, while high exposure could result in pulmonary edema. The LD₅₀ is 340 mg/kg (rat, oral).

Environmental Fate

Acrylic acid is corrosive, and its toxicity occurs at the site of contact.

Purification Methods

It can be purified by steam distillation, or vacuum distillation through a column packed with copper gauze to inhibit polymerisation. (This treatment also removes inhibitors such as methylene blue that may be present.) Azeotropic distillation of the water with *benzene converts aqueous acrylic acid to the anhydrous material.

Substituents

As a substituent acrylic acid can be found as an acyl group or a carboxyalkyl group depending on the removal of the group from the molecule. More specifically these are :
The acryloyl group, with the removal of the -OH from carbon-1.
The 2-carboxy ethenyl group, with the removal of a -H from carbon-3. This substituent group is found in chlorophyll..

Toxicity evaluation

Acrylic acid’s large-scale use and production results in its release into the environment. The most likely route of exposure is inhalation because acrylic acid has a low vapor pressure. The miscibility of acrylic acid in water combined with its low vapor pressure prevent it from accumulating in the soil. Acrylic acid that is emitted into the atmosphere is degraded photochemically by reaction with hydroxyl radicals. There is no potential for long-range atmospheric transport of acrylic acid because it has an atmospheric lifetime of 1 month.