Benzil

Not to be confused with benzyl.

Benzil (i.e. Bz₂, 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.^[4]

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
CAS Number	134-81-6 Y
3D model (JSmol)	Interactive image Interactive image
Beilstein Reference	608047
ChEBI	CHEBI:51507 Y
ChEMBL	ChEMBL189886 Y
ChemSpider	8329 Y
ECHA InfoCard	100.004.689
EC Number	205-157-0
PubChem CID	8651
RTECS number	DD1925000
UNII	S85X61172J Y
CompTox Dashboard (EPA)	DTXSID3044380
InChI InChI=1S/C14H10O2/c15-13(11-7-3-1-4-8-11)14(16)12-9-5-2-6-10-12/h1-10H Y Key: WURBFLDFSFBTLW-UHFFFAOYSA-N Y InChI=1/C14H10O2/c15-13(11-7-3-1-4-8-11)14(16)12-9-5-2-6-10-12/h1-10H Key: WURBFLDFSFBTLW-UHFFFAOYAZ
SMILES O=C(C(=O)c1ccccc1)c2ccccc2 c1ccccc1C(=O)C(=O)c2ccccc2
Properties
Chemical formula	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
Solubility in water	insoluble
Solubility in ethanol	soluble
Solubility in diethyl ether	soluble
Solubility in benzene	soluble
Magnetic susceptibility (χ)	-118.6·10−6 cm3/mol
Structure
Crystal structure	P31,221[1]
Dipole moment	3.8 D[2]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Irritant
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501
NFPA 704 (fire diamond)	2 1 0
Lethal dose or concentration (LD, LC):
LD50 (median dose)	>3 g/kg (mouse, oral)[3]
Related compounds
Related diketones	diacetyl
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). Y verify (what is YN ?) 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°.^[5] In less hindered analogues (glyoxal, biacetyl, oxalic acid derivatives), the (RCO)₂ group adopts a planar, anti-conformation.

Applications

Most benzil can be used as a photoinitiator in the free-radical curing of polymer networks. It absorbs ultraviolet radiation at a wavelength of 260 nm, leading to decomposition with formation of free-radical species and formation of cross-links within the material. However, it is a relatively poor photoinitiator, and is seldom used. It undergoes photobleaching, which allows the curing light to reach deeper layers of the material on longer exposure.^[6] Acetal derivatives, such as 2,2-dimethoxy-2-phenylacetophenone, have better properties for this application.^[6]

Benzil is a potent inhibitor of human carboxylesterases, enzymes involved in the hydrolysis of carboxylesters and many clinically used drugs.^[7]

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:^[8]

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.^[9]

References

1. Acta Crystallogr. B43 398 (1987)
2. Spectrochim. Acta A60 (8-9) 1805 (2004)
3. "Benzil".
4. 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
5. 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.
6. Green, W. Arthur (2010-04-22). Industrial Photoinitiators: A Technical Guide. CRC Press. p. 31. ISBN 9781439827468. Retrieved 2022-05-21.
7. 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.
8. 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.
9. 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.