Benzil

Benzil (systematically known as 1,2-diphenylethane-1,2-dione) is the organic compound with the formula (C6H5CO)2, generally abbreviated (PhCO)2. This yellow solid is one of the most common diketones. Its main use is as a photoinitiator in polymer chemistry.[3]

Benzil
Names
Preferred IUPAC name
Diphenylethanedione
Systematic IUPAC name
1,2-Diphenylethane-1,2-dione
Other names
Diphenylethane-1,2-dione
Benzil
Dibenzoyl
Bibenzoyl
Diphenylglyoxal
Identifiers
3D model (JSmol)
608047
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.689
EC Number
  • 205-157-0
RTECS number
  • DD1925000
UNII
Properties
C14H10O2
Molar mass 210.232 g·mol−1
Appearance yellow crystalline powder
Density 1.23 g/cm3, solid (1.255 g/cm3, x-ray)
Melting point 94.0 to 96.0 °C; 201.2 to 204.8 °F; 367.1 to 369.2 K
Boiling point 346.0 to 348.0 °C; 654.8 to 658.4 °F; 619.1 to 621.1 K
insoluble
Solubility in ethanol soluble
Solubility in diethyl ether soluble
Solubility in benzene soluble
-118.6·10−6 cm3/mol
Structure
P31,221[1]
3.8 D[2]
Hazards
Main hazards Irritant
GHS pictograms
GHS Signal word Warning
GHS hazard statements
 H315, H319, H335
P261, P264, P271, P280, P302+352, P304+340, P305+351+338, P312, P321, P332+313, P337+313, P362, P403+233, P405, P501
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
2
0
Related compounds
Related diketones
 biacetyl
Related compounds
 benzophenone
glyoxal
bibenzil
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure

The compound's most noteworthy structural feature is the long carbon-carbon bond of 1.54 Å, which indicates the absence of pi-bonding between the two carbonyl centers. The PhCO centers are planar, but the pair of benzoyl groups are twisted with respect to the other with a dihedral angle of 117°.[4] In less hindered analogues (glyoxal, biacetyl, oxalic acid derivatives), the (RCO)2 group adopts a planar, anti-conformation.

Applications

Most benzil is used in the free-radical curing of polymer networks. Ultraviolet radiation decomposes benzil, generating free-radical species within the material, promoting the formation of cross-links. Benzil is a potent inhibitor of human carboxylesterases, enzymes involved in the hydrolysis of carboxylesters and many clinically used drugs.[5]

Benzil is a relatively poor photoinitiator, and is seldom used. It absorbs at the 260 nm wavelength. It undergoes photobleaching, which allows the curing light to reach deeper layers of the material on longer exposure.[6]

Reactions

Benzil is a standard building block in organic synthesis. It condenses with amines to give diketimine ligands. A classic organic reaction of benzil is the benzilic acid rearrangement, in which base catalyses the conversion of benzil to benzilic acid. This reactivity is exploited in the preparation of the drug phenytoin. Benzil also reacts with 1,3-diphenylacetone in an aldol condensation to give tetraphenylcyclopentadienone.

Preparation

Benzil is prepared from benzoin, for example with copper(II) acetate:[7]

PhC(O)CH(OH)Ph + 2 Cu2+ → PhC(O)C(O)Ph + 2 H+ + 2 Cu+

Other suitable oxidizing agents such as nitric acid (HNO3) are used routinely.

Iron(III) chloride (FeCl3) can be used as an inexpensive catalyst for this chemical conversion.[8]

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References
  1. Acta Crystallogr. B43 398 (1987)
  2. Spectrochim. Acta A60 (8-9) 1805 (2004)
  3. Hardo Siegel, Manfred Eggersdorfer "Ketones" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, 2002 by Wiley-VCH, Weinheim. doi:10.1002/14356007.a15_077
  4. Quang. Shen, Kolbjoern. Hagen "Gas-phase molecular structure and conformation of benzil as determined by electron diffraction" J. Phys. Chem., 1987, 91 (6), pp 1357–1360. doi:10.1021/j100290a017.
  5. Wadkins. R. M. et al "Identification and characterization of novel benzil (diphenylethane-1,2-dione) analogues as inhibitors of mammalian carboxylesterases. J. Med. Chem., 2005 48 pp 2906–15.
  6. Arthur Green, W. (2010-04-22). Industrial Photoinitiators: A Technical Guide. ISBN 9781439827468.
  7. Depreux, P.; Bethegnies, G.; Marcincal-Lefebvre, A. (1988). "Synthesis of benzil from benzoin with copper(II) acetate". Journal of Chemical Education. 65 (6): 553. Bibcode:1988JChEd..65..553D. doi:10.1021/ed065p553.
  8. Bi, Xiaoxin; Wu, Lintao; Yan, Chaoguo; Jing, Xiaobi; Zhu, Hongxiang (2011). "One-Pot Synthesis Benzils from Aldehydes Via Nhc-Catalyzed Benzoin Dimerization Under Metal-Free Conditions in Water". Journal of the Chilean Chemical Society. 56 (2): 663. doi:10.4067/S0717-97072011000200008.
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