Phenylacetic acid

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Phenylacetic acid
Names
IUPAC name
Phenylethanoic acid
Preferred IUPAC name
Phenylethanoic acid
Other names
2-Phenylacetic acid
Alpha tolylic acid
Benzeneacetic acid
PAA
α-Toluic acid
β-Phenylacetic acid
Properties
C8H8O2
Molar mass 136.15 g/mol
Appearance White solid
Odor Honey-like
Density 1.081 g/cm3 (25 °C)
Melting point 76.7 °C (170.1 °F; 349.8 K)
Boiling point 265.5 °C (509.9 °F; 538.6 K)
1.50 g/100 ml (20 °C)
1.73 g/100 ml (25 °C)
Solubility Very soluble in acetone, carbon disulfide, diethyl ether, ethanol
Slightly soluble in chloroform
Insoluble in ligroin
Vapor pressure 3.8·10-3 mmHg at 25 °C
Acidity (pKa) 4.31
Thermochemistry
Hazards
Safety data sheet Sigma-Aldrich
Flash point 132 °C (270 °F; 405 K)
Related compounds
Related compounds
Benzoic acid
Phenylpropanoic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phenylacetic acid (PAA), also called benzeneacetic acid, 2-phenylacetic acid and β-phenylacetic acid, is an organic chemical compound, an aromatic carboxylic acid with the chemical formula C8H8O2 or C6H5CH2COOH.

Properties

Chemical

Phenylacetic acid reacts with bases to form their respective salts.

Heating a mixture of phenylacetic acid and acetic anhydride yields phenylacetone. Other acetates, such as sodium, potassium and lead acetate can be used, giving different yields. A large excess of acetic anhydride must be used in this reaction, otherwise dibenzyl ketone will be formed[1]

Physical

Phenylacetic acid is a white solid, with a honey-like odor. It is slightly solible in water, but more soluble in organic solvents.

Availability

Phenylacetic acid is sold by chemical suppliers, but due to its legal status as drug precursor (used in the production of P2P and amphetamine-class drugs), it cannot be purchased by the home chemist without a hard to acquire permit. Since it's classified as List I drug precursor in most countries, it's almost impossible to be acquired by the hobby chemist. In the United States it is classified as a DEA List I chemical.

Preparation

There are several routes to PAA:

Hydrolysis of benzyl cyanide gives phenylacetic acid. Sulfuric acid is often used as catalyst, usually as 70% concentration, however concentrated hydrochloric acid can also be used.[2][3] This is the most common route to PAA.

Oxidation of phenethyl alcohol using acidified KMnO4 will yield phenylacetic acid.

Another route, involves reacting benzylmagnesium chloride (obtained by reacting perfectly dry benzyl chloride with Grignard reagent-grade magnesium flakes) in a dry and inert reaction flask with dry carbon dioxide. This yields very pure phenylacetic acid, though this route is expensive.[4]

Mandelic acid can be reduced with HI and red phoshporus to phenylacetic acid.[5]

Phenylacetic acid can also be obtained from acetophenone, by reacting the ketone with boron trifluoride etherate and lead(IV) acetate in dry benzene. This produces methyl phenylacetate, which is hydrolyzed with NaOH and acidified with HCl to yield the free acid.[6]

An interesting route involves the oxidation of ethylbenzene with sodium dichromate solution in an autoclave, at 275 °C for several hours. The yield of this reaction is given as 90%. While dangerous, this route is attractive since the reagents are cheap and easy to acquire.[7]

Projects

  • Preparation of diclofenac (Voltaren)
  • Preparation of sodium phenylacetate (used in Ammonul, as treatment of urea cycle disorders)
  • Preparation of methyl phenylacetate, which has an odor very similar to that of honey
  • Preparation of phenylacetone

Handling

Safety

Phenylacetic is only mildly corrosive and has low toxicity, though it's irritant.

Storage

In closed airtight bottles, away from bases and light.

Disposal

No special disposal is required. Can be diluted in water and slowly poured down the drain.

References

  1. https://erowid.org/archive/rhodium/chemistry/phenylacetone.html#phenylacetic
  2. Perfumery Essent. Oil Record 14, 336 (1924)
  3. https://erowid.org/archive/rhodium/chemistry/phenylacetic.html
  4. https://pubs.acs.org/doi/10.1021/ja01341a031
  5. https://onlinelibrary.wiley.com/doi/10.1002/hlca.19390220174
  6. https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-1981-29358
  7. https://pubs.acs.org/doi/10.1021/jo01048a006

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