Benzil
Encyclopedia
Benzil is the organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 with the formula (C6H5CO
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

)2, generally abbreviated (PhCO)2. This yellow solid is one of the most common diketone
Diketone
A diketone is a molecule containing two ketone groups. The simpliest diketone is diacetyl, also known as 2,3-butanedione. Diacetyl, acetylacetone, and hexane-2,5-dione are examples of 1,2-, 1,3-, and 1,4-diketones, respectively...

s. Its main use is as a photoinitiator
Photoinitiator
A photoinitiator is any chemical compound that decomposes into free radicals when exposed to light. Photoinitiators are found both in nature and in industry ....

 in polymer chemistry
Polymer chemistry
Polymer chemistry or macromolecular chemistry is a multidisciplinary science that deals with the chemical synthesis and chemical properties of polymers or macromolecules. According to IUPAC recommendations, macromolecules refer to the individual molecular chains and are the domain of chemistry...

.

Structure

The compound's most noteworthy structural feature is the long carbon-carbon bond
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...

 of 1.54 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter 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
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...

 of 117°. In less hindered analogues (glyoxal
Glyoxal
Glyoxal is an organic compound with the formula OCHCHO. This yellow colored liquid is the smallest dialdehyde . Its tautomer acetylenediol is unstable.-Production:...

, biacetyl, oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

 derivatives), the (RCO)2 groups adopts a planar, anti-conformation.

Applications

Most benzil is consumed for use in the free-radical curing
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...

 of polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 networks. Ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation decomposes benzil, generating free-radical species within the material, promoting the formation of cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

s.

Reactions

Benzil is a standard building block in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

. It condenses with amines to give diketimine
Diketimine
Diketimines or diimines are a family of ligands and ligand precursors derived from 1,2- and 1,3-diketones by replacement of the carbonyl oxygen atoms with NR groups, where R = aryl, alkyl...

s ligands. A classic organic reaction
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

 of benzil is the benzilic acid rearrangement
Benzilic acid rearrangement
The benzilic acid rearrangement is the rearrangement reaction of benzil with potassium hydroxide to benzilic acid. First performed by Justus Liebig in 1838 this reaction type is displayed by 1,2-diketones in general...

, in which base catalyses the conversion of benzil to benzilic acid. This reactivity is exploited in the preparation of the drug phenytoin
Phenytoin
Phenytoin sodium is a commonly used antiepileptic. Phenytoin acts to suppress the abnormal brain activity seen in seizure by reducing electrical conductance among brain cells by stabilizing the inactive state of voltage-gated sodium channels...

. Benzil also reacts with 1,3-diphenylacetone
Dibenzyl ketone
Dibenzyl ketone, or 1,3-diphenylacetone, is an organic compound composed of two benzyl groups attached to a central carbonyl group. This results in the central carbonyl carbon atom being electrophilic and the two adjacent carbon atoms slightly nucleophilic...

 in an aldol condensation
Aldol condensation
An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone....

 to give tetraphenylcyclopentadienone
Tetraphenylcyclopentadienone
Tetraphenylcyclopentadienone is a dark purple to black crystalline solid at standard temperature and pressure. Because of its highly conjugated nature, this organic compound can be analyzed in the UV spectrum. Tetraphenylcyclopentadienone has a "propeller" shape in its 3D conformation...

.

Preparation

Benzil is prepared from benzoin
Benzoin
Benzoin is an organic compound with the formula PhCHCPh. It is a hydroxy ketone attached to two phenyl groups. It appears as off-white crystals, with a light camphor-like odor. Benzoin is synthesized from benzaldehyde in the benzoin condensation...

, which in turn is easily obtained via the benzoin condensation
Benzoin condensation
The benzoin condensation is a reaction between two aromatic aldehydes, particularly benzaldehyde. The reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic carbene. The reaction product is an aromatic acyloin with benzoin as the parent compound...

 from benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...

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