Benzophenone

Benzophenone is the organic compound with the formula (C₆H₅)₂CO, generally abbreviated Ph₂CO. Substituted benzophenones such as oxybenzone and dioxybenzone are used in sunscreen.

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Uses

Benzophenone can be used as a photo initiator in UV-curing applications such as inks, imaging and clear coatings in the printing industry. Benzophenone prevents ultraviolet (UV) light from damaging scents and colors in products such as perfumes and soaps. It can also be added to the plastic packaging as a UV blocker. This allows manufacturers to package the product in clear glass or plastic. Without it, opaque or dark packaging would be used.

In laboratories, solvents are often distilled with sodium and benzophenone as desicants. The product of these two chemicals in the absence of air and water is a dark blue ketyl; a solution of this ketyl can be used to qualitatively test for the absence of air and water.

Synthesis

Benzophenone can be prepared by the reaction of benzene with carbon tetrachloride followed by hydrolysis of the resulting diphenyldichloromethane,^[1], or by Friedel-Crafts acylation of benzene with benzoyl chloride in the presence of a Lewis acid (e.g. aluminum chloride) catalyst.

Chemistry

Benzophenone is a common photosensitizer in photochemistry. It crosses from the S₁ state into the triplet state with nearly 100 % yield. The resulting diradical will abstract a hydrogen atom from a suitable hydrogen donor to form a ketyl radical.

Benzophenone radical anion

Sodium is commonly used as a desiccant to purify solvents by reacting with water to give the hydroxide, which is nonvolatile:

Na + H₂O → NaOH + ¹/₂ H₂

The main problem with the use of sodium as a desiccant (below its melting point) is associated with the slow rate of reaction between a solid and a solution. When however, the desiccant is soluble, the speed of drying is much higher. Benzophenone is often used to generate such a soluble drying agent. An advantage to this application is the intense blue color of the ketyl radical anion. Thus, sodium/benzophenone can be used as an indicator of air-free and moisture-free conditions in the purification of solvents by distillation.^[2][3]

Derivatives

• p,p'-bis(N,N-dimethylamino)benzophenone or Michler's ketone has dimethylamino substituents at each para position.

See also

• Desiccant

References

• Merck Index, 11th Edition, 1108.

1. ^ Marvel, C. S.; Sperry, W. M. (1941). "Benzophenone". Org. Synth.; Coll. Vol. 1: 95. 
2. ^ Nathan L. Bauld (2001). Unit 6: Anion Radicals. University of Texas.
3. ^ W. L. F. Armarego and C. Chai (2003). Purification of laboratory chemicals. Butterworth-Heinemann. ISBN 0750675713. 

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