Acetone

Acetone is produced directly or indirectly from propylene. Approximately 83% of acetone is produced via the cumene process;[20] as a result, acetone production is tied to phenol production. In the cumene process, benzene is alkylated with propylene to produce cumene, which is oxidized by air to produce phenol and acetone:

Other processes involve the direct oxidation of propylene (Wacker-Hoechst process), or the hydration of propylene to give 2-propanol, which is oxidized (dehydrogenated) to acetone.[20]

Previously, acetone was produced by the dry distillation of acetates, for example calcium acetate in ketonic decarboxylation.

${\displaystyle {\ce {Ca(CH3COO)2 -> CaO_{(s)}{}+ CO2_{(g)}{}+ (CH3)2CO v}}}$

After that time, during World War I, acetone was produced using acetone-butanol-ethanol fermentation with Clostridium acetobutylicum bacteria, which was developed by Chaim Weizmann (later the first president of Israel) in order to help the British war effort,[20] in the preparation of Cordite.[37] This acetone-butanol-ethanol fermentation was eventually abandoned when newer methods with better yields were found.[20]

Like most ketones, acetone exhibits the keto–enol tautomerism in which the nominal keto structure (CH₃)₂C=O of acetone itself is in equilibrium with the enol isomer (CH₃)C(OH)=(CH₂) (prop-1-en-2-ol). In acetone vapor at ambient temperature, only 2.4×10⁻⁷% of the molecules are in the enol form.[38] Yet the enol form is chemically important in some chemical reactions.

In the presence of suitable catalysts, two acetone molecules also combine to form the compound diacetone alcohol (CH₃)C=O(CH₂)C(OH)(CH₃)₂, which on dehydration gives mesityl oxide (CH₃)C=O(CH)=C(CH₃)₂. This product can further combine with another acetone molecule, with loss of another molecule of water, yielding phorone and other compounds.

One might expect acetone to also form polymers and (possibly cyclic) oligomers of two types. In one type, units could be acetone molecules linked by ether bridges −O− derived by from the opening of the double bond, to give a polyketal-like (PKA) chain [−O−C(CH₃)₂−]_n. The other type could be obtained through repeated aldol condensation, with one molecule of water removed at each step, yielding a poly(methylacetylene) (PMA) chain [−CH=C(CH₃)−]_n.[39]

The conversion of acetone to a polyketal (PKA) would be analogous to the formation of paraformaldehyde from formol, and of trithioacetone from thioacetone. In 1960, Kargin, Kabanov and others observed that the thermodynamics of this process is unfavourable for liquid acetone, so that it (unlike thioacetone and formol) is not expected to polymerise spontaneously, even with catalysts. However, they observed that the thermodynamics became favourable for crystalline solid acetone at the melting point (−96 °C). They claimed to have obtained such a polymer (a white elastic solid, soluble in acetone, stable for several hours at room temperature) by depositing vapor of acetone, with some magnesium as a catalyst, onto a very cold surface.[40]

In 1962, Wasaburo Kawai reported the synthesis of a similar product, from liquid acetone cooled to −70 to −78 °C, using n-butyl lithium or triethylaluminium as catalysts. He claimed that the infrared absorption spectrum showed the presence of −O− linkages but no C=O groups.[41] However, conflicting results were obtained later by other investigators.[39]

Structure of possible acetone polymer

The PMA type polymers of acetone would be equivalent to the product of polymerisation of propyne, except for a keto end group.[39]

Small amounts of acetone are produced in the body by the decarboxylation of ketone bodies. Certain dietary patterns, including prolonged fasting and high-fat low-carbohydrate dieting, can produce ketosis, in which acetone is formed in body tissue. Certain health conditions, such as alcoholism and diabetes, can produce ketoacidosis, uncontrollable ketosis that leads to a sharp, and potentially fatal, increase in the acidity of the blood. Since it is a byproduct of fermentation, acetone is a byproduct of the distillery industry.

Acetone can be produced from the oxidation of ingested isopropanol, or from the spontaneous/enzymatic breakdown of acetoacetate (a ketone body) in ketotic individuals.

Although some biochemistry textbooks and current research publications[42] indicate that acetone cannot be metabolized, there is evidence to the contrary. It can then be metabolized either by CYP2E1 via methylglyoxal to D-lactate and pyruvate, and ultimately glucose/energy, or by a different pathway via propylene glycol to pyruvate, lactate, acetate (usable for energy) and propionaldehyde.[43][44][45]

About a third of the world's acetone is used as a solvent, and a quarter is consumed as acetone cyanohydrin, a precursor to methyl methacrylate.[19]

Acetone is a good solvent for many plastics and some synthetic fibers. It is used for thinning polyester resin, cleaning tools used with it, and dissolving two-part epoxies and superglue before they harden. It is used as one of the volatile components of some paints and varnishes. As a heavy-duty degreaser, it is useful in the preparation of metal prior to painting or soldering, and to remove rosin flux after soldering (to prevent adhesion of dirt and electrical leakage and perhaps corrosion or for cosmetic reasons), although it attacks many electronic components (for example polystyrene capacitors) so it is unsuitable for cleaning many circuit boards.

Although itself flammable, acetone is used extensively as a solvent for the safe transportation and storage of acetylene, which cannot be safely pressurized as a pure compound. Vessels containing a porous material are first filled with acetone followed by acetylene, which dissolves into the acetone. One litre of acetone can dissolve around 250 litres of acetylene at a pressure of 10 bars (1.0 MPa).[46][47]

Acetone is used to synthesize methyl methacrylate. It begins with the initial conversion of acetone to acetone cyanohydrin via reaction with hydrogen cyanide (HCN):

${\displaystyle {\ce {(CH3)2CO + HCN -> (CH3)2C(OH)CN}}}$

In a subsequent step, the nitrile is hydrolyzed to the unsaturated amide, which is esterified:

${\displaystyle {\ce {(CH3)2C(OH)CN + CH3OH -> CH2=C(CH3)CO2CH3 + NH3}}}$

The third major use of acetone (about 20%)[19] is synthesizing bisphenol A. Bisphenol A is a component of many polymers such as polycarbonates, polyurethanes, and epoxy resins. The synthesis involves the condensation of acetone with phenol:

${\displaystyle {\ce {(CH3)2CO + 2 C6H5OH -> (CH3)2C(C6H4OH)2 + H2O}}}$

Many millions of kilograms of acetone are consumed in the production of the solvents methyl isobutyl alcohol and methyl isobutyl ketone. These products arise via an initial aldol condensation to give diacetone alcohol.[20]

${\displaystyle {\ce {2 (CH3)2CO -> (CH3)2C(OH)CH2C(O)CH3}}}$

Condensation with acetylene gives 2-methylbut-3-yn-2-ol, precursor to synthetic terpenes and terpenoids.

HPLC readout of an Excedrin tablet. Peaks from left to right are acetaminophen, aspirin, and caffeine.

Spectroscopy techniques are useful when the sample being tested is pure, or a very common mixture. When an unknown mixture is being analyzed it must be broken down into its individual parts. Chromatography techniques can be used to break apart mixtures into their components allowing for each part to be analyzed separately.

Thin layer chromatography (TLC) is a quick alternative to more complex chromatography methods. TLC can be used to analyze inks and dyes by extracting the individual components.[48] This can be used to investigate notes or fibers left at the scene since each company's product is slightly different and those differences can be seen with TLC. The only limiting factor with TLC analysis is the necessity for the components to be soluble in whatever solution is used to carry the components up the analysis plate.[48] This solution is called the mobile phase.[48] The forensic chemist can compare unknowns with known standards by looking at the distance each component travelled.[48] This distance, when compared to the starting point, is known as the retention factor (R_f) for each extracted component.[48] If each R_f value matches a known sample, that is an indication of the unknown's identity.[48]

High-performance liquid chromatography can be used to extract individual components from a mixture dissolved in a solution. HPLC is used for nonvolatile mixtures that would not be suitable for gas chromatography.[49] This is useful in drug analysis where the pharmaceutical is a combination drug since the components would separate, or elute, at different times allowing for the verification of each component.[50] The eluates from the HPLC column are then fed into various detectors that produce a peak on a graph relative to its concentration as it elutes off the column. The most common type of detector is an ultraviolet-visible spectrometer as the most common item of interest tested with HPLC, pharmaceuticals, have UV absorbance.[51]

Gas chromatography (GC) performs the same function as liquid chromatography, but it is used for volatile mixtures. In forensic chemistry, the most common GC instruments use mass spectrometry as their detector.[52] GC-MS can be used in investigations of arson, poisoning, and explosions to determine exactly what was used. In theory, GC-MS instruments can detect substances whose concentrations are in the femtogram (10⁻¹⁵) range.[53] However, in practice, due to signal-to-noise ratios and other limiting factors, such as the age of the individual parts of the instrument, the practical detection limit for GC-MS is in the picogram (10⁻¹²) range.[54] GC-MS is also capable of quantifying the substances it detects; chemists can use this information to determine the effect the substance would have on an individual. GC-MS instruments need around 1,000 times more of the substance to quantify the amount than they need simply to detect it; the limit of quantification is typically in the nanogram (10⁻⁹) range.[54]

In the laboratory, acetone is used as a polar, aprotic solvent in a variety of organic reactions, such as S_N2 reactions. The use of acetone solvent is critical for the Jones oxidation. It does not form an azeotrope with water (see azeotrope tables).[55] It is a common solvent for rinsing laboratory glassware because of its low cost and volatility. Despite its common use as a supposed drying agent, it is not effective except by bulk displacement and dilution. Acetone can be cooled with dry ice to −78 °C without freezing; acetone/dry ice baths are commonly used to conduct reactions at low temperatures. Acetone is fluorescent under ultraviolet light, and its vapor can be used as a fluorescent tracer in fluid flow experiments.[56]

Acetone is used to precipitate proteins.[57] Alternatives for protein precipitation are trichloroacetic acid or ethanol.

Low-grade acetone is also commonly used in academic laboratory settings as a glassware rinsing agent for removing residue and solids before a final wash.[58] Acetone leaves a small amount of residue on a surface when dried that is harmful to surface samples.

A mixture of acetone and dry ice is a popular cooling bath that maintains a temperature of −78 °C as long as there is some dry ice left.[59]

Acetone is used in the field of pathology to find lymph nodes in fatty tissues for tumor staging (such as looking for lymph nodes in the fat surrounding the intestines).[60] This helps dissolve the fat, and hardens the nodes, making finding them easier.[61]

Acetone also used for destaining microscope slides of certain stains.[62]

Acetone is a weak Lewis base that forms adducts with soft acids like I₂ and hard acids like phenol. Acetone also forms complexes with divalent metals.[63][64]

Acetone is used as a solvent by the pharmaceutical industry and as a denaturant in denatured alcohol.[65] Acetone is also present as an excipient in some pharmaceutical drugs.[66]

Dermatologists use acetone with alcohol for acne treatments to chemically peel dry skin. Common agents used today for chemical peeling are salicylic acid, glycolic acid, azelaic acid, 30% salicylic acid in ethanol, and trichloroacetic acid (TCA). Prior to chemexfoliation, the skin is cleaned and excess fat removed in a process called defatting. Acetone, Septisol, or a combination of these agents is commonly used in this process.

Acetone has been shown to have anticonvulsant effects in animal models of epilepsy, in the absence of toxicity, when administered in millimolar concentrations.[67] It has been hypothesized that the high-fat low-carbohydrate ketogenic diet used clinically to control drug-resistant epilepsy in children works by elevating acetone in the brain.[67] Because of their higher energy requirements, children have higher acetone production than most adults – and the younger the child, the higher the expected production. This indicates that children are not uniquely susceptible to acetone exposure. External exposures are small compared to the exposures associated with the ketogenic diet.[68]

Acetone is often the primary component in cleaning agents such as nail polish and superglue removers. It will attack some plastics, however.

Make-up artists use acetone to remove skin adhesive from the netting of wigs and mustaches by immersing the item in an acetone bath, then removing the softened glue residue with a stiff brush.

Acetone is often used for vapor polishing of printing artifacts on 3D-printed models printed with ABS plastic. The technique, called acetone vapor bath smoothing, involves placing the printed part in a sealed chamber containing a small amount of acetone, and heating to around 80 degrees Celsius for 10 minutes. This creates a vapor of acetone in the container. The acetone condenses evenly all over the part, causing the surface to soften and liquefy. Surface tension then smooths the semi-liquid plastic. When the part is removed from the chamber, the acetone component evaporates leaving a glassy-smooth part free of striation, patterning, and visible layer edges, common features in untreated 3D printed parts.[69]

Acetone efficiently removes felt-tipped pen marks from glass and metals.

The most hazardous property of acetone is its extreme flammability. In small amounts Acetone burns with a dull blue flame, in larger amounts the evaporation of fuel causes incomplete combustion and a bright yellow flame. At temperatures greater than acetone's flash point of −20 °C (−4 °F), air mixtures of between 2.5% and 12.8% acetone, by volume, may explode or cause a flash fire. Vapors can flow along surfaces to distant ignition sources and flash back. Static discharge may also ignite acetone vapors, though acetone has a very high ignition initiation energy point and therefore accidental ignition is rare. Even pouring or spraying acetone over red-glowing coal will not ignite it, due to the high concentration of vapour and the cooling effect of evaporation of the liquid.[70] It auto-ignites at 465 °C (869 °F). Auto-ignition temperature is also dependent upon the exposure time, thus at some tests it is quoted as 525 °C. Also, industrial acetone is likely to contain a small amount of water which also inhibits ignition.

When oxidized, acetone forms acetone peroxide as a byproduct, which is a highly unstable, primary high explosive compound. It may be formed accidentally, e.g. when waste hydrogen peroxide is poured into waste solvent containing acetone. Due to its instability, it is rarely used, despite its simple chemical synthesis.

Acetone has been studied extensively and is believed to exhibit only slight toxicity in normal use. There is no strong evidence of chronic health effects if basic precautions are followed.[71] It is generally recognized to have low acute and chronic toxicity if ingested and/or inhaled.[72] Acetone is not currently regarded as a carcinogen, a mutagenic chemical nor a concern for chronic neurotoxicity effects.[70]

Acetone can be found as an ingredient in a variety of consumer products ranging from cosmetics to processed and unprocessed foods. Acetone has been rated as a generally recognized as safe (GRAS) substance when present in beverages, baked foods, desserts, and preserves at concentrations ranging from 5 to 8 mg/L.[72]

Acetone is however an irritant, causing mild skin irritation and moderate to severe eye irritation. At high vapor concentrations, it may depress the central nervous system like many other solvents.[73] Acute toxicity for mice by ingestion (LD₅₀) is 3 g/kg, and by inhalation (LC₅₀) is 44 g/m³ over 4 hours.[74]

In 1995, the United States Environmental Protection Agency (EPA) removed acetone from the list of "toxic chemicals" maintained under Section 313 of the Emergency Planning and Community Right to Know Act (EPCRA). In making that decision, EPA conducted an extensive review of the available toxicity data on acetone and found that acetone "exhibits acute toxicity only at levels that greatly exceed releases and resultant exposures", and further that acetone "exhibits low toxicity in chronic studies".

• Genotoxicity. Acetone has been tested in more than two dozen in vitro and in vivo assays. These studies indicate that acetone is not genotoxic.
• Carcinogenicity. EPA in 1995 concluded, "There is currently no evidence to suggest a concern for carcinogenicity". (EPCRA Review, described in Section 3.3). NTP scientists have recommended against chronic toxicity/carcinogenicity testing of acetone because "the prechronic studies only demonstrated a very mild toxic response at very high doses in rodents".
• Neurotoxicity and Developmental Neurotoxicity. The neurotoxic potential of both acetone and isopropanol, the metabolic precursor of acetone, have been extensively studied. These studies demonstrate that although exposure to high doses of acetone may cause transient central nervous system effects, acetone is not a neurotoxicant. A guideline developmental neurotoxicity study has been conducted with isopropanol, and no developmental neurotoxic effects were identified, even at the highest dose tested. (SIAR, pp. 1, 25, 31).
• Environmental. When the EPA exempted acetone from regulation as a volatile organic compound (VOC) in 1995, EPA stated that this exemption would "contribute to the achievement of several important environmental goals and would support EPA's pollution prevention efforts". 60 Fed. Reg. 31,634 (June 16, 1995). 60 Fed. Reg. 31,634 (June 16, 1995). EPA noted that acetone could be used as a substitute for several compounds that are listed as hazardous air pollutants (HAP) under section 112 of the Clean Air Act.