# Benzil
**Benzil** (1,2-diphenylethane-1,2-dione; also called dibenzoyl or diphenylglyoxal) is an aromatic alpha-diketone, ethane-1,2-dione with a phenyl group on each carbonyl carbon, formula C₁₄H₁₀O₂. It is a yellow crystalline solid best known as the oxidation product of benzoin and as the namesake substrate of the [benzilic acid rearrangement](https://www.edgepedia.org/benzilic-acid-rearrangement), the first reported example of a molecular rearrangement reaction.[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) • [2](https://www.youtube.com/watch?v=JYCPV6vbfJE)
| Key fact | Detail |
|---|---|
| Identity | Alpha-diketone, C₁₄H₁₀O₂, molecular weight 210.23 g/mol, CAS 134-81-6, EC 205-157-0[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) • [3](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch) |
| Appearance | Yellow crystalline solid; melting point 94.90 °C[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) |
| Physical properties | XLogP3 3.4 (experimental logP 3.38), water solubility 0.5 g/L at 20 °C, density 1.230[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) • [3](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch) |
| Crystal structure | Trigonal, noncentrosymmetric space group P3₁21; a = 8.4095 Å, c = 13.6695 Å; O=C–C=O torsion about 110°[4](https://onlinelibrary.wiley.com/doi/10.1155/2022/5968496) • [5](https://www.sciencedirect.com/org/science/article/pii/S2041652023025518) |
| Classic synthesis | Oxidation of benzoin with concentrated nitric acid; one undergraduate procedure reports 84% yield[6](https://www.youtube.com/watch?v=BNS4vpkhqAw) |
| Signature reaction | Benzilic acid rearrangement with potassium hydroxide to benzilic acid, first performed by Justus von Liebig in 1838[2](https://www.youtube.com/watch?v=JYCPV6vbfJE) |
| Hazard profile | GHS warning: H315 (skin irritation), H319 (eye irritation), H335 (respiratory irritation); rat oral LD50 > 3000 mg/kg[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) • [3](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch) |
## Structure and physical properties
Benzil consists of two benzoyl groups joined carbonyl-to-carbonyl, PhCOCOPh. It has no hydrogen bond donors and two hydrogen bond acceptors, with a topological polar surface area of 34.1 Ų and three rotatable bonds.[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) The molecule is not planar: in the crystal and in solution the O=C–C=O unit is twisted into a helical, skewed cisoid conformation with a torsion angle of about 110°, and the barrier to rotation about this bond is low enough that benzil is optically inactive in solution and in the melt.[5](https://www.sciencedirect.com/org/science/article/pii/S2041652023025518)
**Chiral crystals from an achiral molecule.** Benzil is a prominent example of an achiral molecule that crystallizes in a chiral space group, P3₁21 (space group 154); single-crystal [X-ray diffraction](https://www.edgepedia.org/x-ray-diffraction) gives lattice parameters a = b = 8.4095 Å, c = 13.6695 Å, with α = β = 90° and γ = 120°.[4](https://onlinelibrary.wiley.com/doi/10.1155/2022/5968496) Its spontaneous mirror symmetry breaking on crystallization has been studied more closely than for any other organic crystal, and the phenomenon extends to benzil-based soft crystalline, cubic liquid crystalline, and isotropic liquid phases.[5](https://www.sciencedirect.com/org/science/article/pii/S2041652023025518) Crystal packing is dominated by weak C–H···O interactions that link molecules into inversion dimers.[4](https://onlinelibrary.wiley.com/doi/10.1155/2022/5968496)
## Synthesis
The standard route is oxidation of the secondary alcohol in benzoin to a second carbonyl. Heating benzoin in excess concentrated nitric acid is the common undergraduate laboratory method; the reaction is monitored by thin-layer chromatography or by the evolution of brown nitrogen dioxide vapor, and the crude product is precipitated with water and recrystallized from ethanol.[6](https://www.youtube.com/watch?v=BNS4vpkhqAw) • [7](https://home.miracosta.edu/dlr/211exp10.htm) One published procedure reports 84% yield with a clean product by TLC, melting point, and ¹H NMR.[6](https://www.youtube.com/watch?v=BNS4vpkhqAw)
**Alternative oxidants.** Mild oxidants including [Fehling's solution](https://www.edgepedia.org/fehlings-solution), copper(II) sulfate in pyridine, and sodium dichromate also work, though dichromate gives lower yield because some benzoin cleaves back to benzoic acid.[7](https://home.miracosta.edu/dlr/211exp10.htm) Homogeneous systems reported for benzoin-to-benzil oxidation include nitric acid, thallium nitrate, ammonium nitrate–copper acetate, bismuth nitrate–copper acetate, and ferric nitrate.[8](https://www.sciencedirect.com/science/article/abs/pii/S1566736708001222) A green-chemistry teaching comparison contrasts three methods: concentrated nitric acid as oxidant and solvent, catalytic copper(II) acetate with ammonium nitrate in 80% acetic acid, and a solvent-free melt using iron(III) chloride hexahydrate at 110 °C.[9](https://pubs.acs.org/doi/10.1021/acs.jchemed.5c01819) A patented Chinese process introduces an oxygen-containing gas into the nitric acid–benzoin system at 2–5 MPa, reducing the nitric acid required and avoiding nitrogen oxide generation.[10](https://patents.google.com/patent/CN115745762A/en) Diarylalkynes can also be converted directly to alpha-diketones by visible-light aerobic oxidation catalyzed by copper-superoxo at room temperature.[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil)
## The benzilic acid rearrangement
Treating benzil with alcoholic potassium hydroxide converts it to the potassium salt of benzilic acid, (Ph)₂C(OH)CO₂K, which on acidification gives benzilic acid, an alpha-hydroxy acid.[2](https://www.youtube.com/watch?v=JYCPV6vbfJE) • [11](https://library.sciencemadness.org/library/books/Experiments_in_Organic_Chemistry_by_Fieser_2nd_ed_1941.pdf) The reaction is formally the 1,2-rearrangement of 1,2-diketones to alpha-hydroxycarboxylic acids under base, and it works best when the ketone groups have no adjacent enolizable protons, which would otherwise allow aldol condensation to compete.[2](https://www.youtube.com/watch?v=JYCPV6vbfJE) More generally, 1,2-diketones lacking alpha-hydrogens react with hydroxide ion to give alpha-hydroxy acids, with the driving force lying in ionization of the product's carbonyl group.[12](https://archive.nptel.ac.in/content/storage2/courses/104103022/module8/lec17/4.html)
The accepted mechanism proceeds by hydroxide addition to one carbonyl, C–C rotation, intramolecular 1,2-migration of a phenyl group with its electron pair to the adjacent carbonyl carbon, and proton relay to the product anion.[13](https://pubmed.ncbi.nlm.nih.gov/16496961/) • [14](https://mu.menofia.edu.eg/PrtlFiles/Faculties/pharm/Portal/Files/pharmaceutical%20organic%20chemistry.pdf) The rearrangement can be viewed as an intramolecular disproportionation, one carbon center oxidized while the other is reduced, and it was first performed by [Justus von Liebig](https://www.edgepedia.org/justus-von-liebig) in 1838, making it the first reported example of a rearrangement reaction.[2](https://www.youtube.com/watch?v=JYCPV6vbfJE) Studies with caustic alkalis indicate the transformation is triggered by coordination of the metal cation M⁺ to benzil and propagated by nucleophilic attack of OH⁻ on the complexed substrate.[15](https://onlinelibrary.wiley.com/doi/abs/10.1002/bscb.19810900307) Benzil itself is also a good oxidizing agent for alcohols in the presence of their alkoxides; Doering and Urban showed that the rearrangement fails with primary and secondary alkoxide ions because of competitive hydride transfers of the Meerwein–Ponndorf–Verley–Oppenauer type.[16](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/benzilic-acid)
## Photochemistry and applications
Benzil absorbs ultraviolet and near-visible light, undergoes intersystem crossing from the excited singlet S₁ to the triplet state T₁, and in the triplet state abstracts hydrogen atoms from suitable donors to form a ketyl radical and a donor-derived radical.[17](https://www.sciencedirect.com/science/article/abs/pii/S0032386100001725) • [18](https://pmc.ncbi.nlm.nih.gov/articles/PMC8434379/) This hydrogen-abstraction behavior places benzil among type II (H-abstraction) photoinitiators, alongside benzophenone, camphorquinone, and thioxanthone, used with proton donors for radical photopolymerization.[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC9145830/) Benzil–amine photoinitiator systems polymerize vinyl monomers by a mechanism that depends on the irradiation wavelength, and benzil has been examined as a photoinitiator for acrylic monomers.[20](https://www.sciencedirect.com/science/article/abs/pii/S1010603003001758) • [17](https://www.sciencedirect.com/science/article/abs/pii/S0032386100001725) Radical-scavenger studies and reisolation of benzil after irradiation in oxygen-free benzene provide strong evidence against benzil undergoing photocleavage in solution.[21](https://www.sciencedirect.com/topics/chemistry/exciplex-formation)
In glassy polymer matrices, photon-induced reaction of benzil with molecular oxygen converts it to benzoyl peroxide, whose subsequent thermal or photochemical decomposition produces crosslinks; this underlies photoperoxidation studies in polystyrene and related polymers.[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC8434379/) Beyond photochemistry, benzil serves as a reagent for synthesizing various heterocycles, and benzil derivatives find uses in pharmaceutical, polymer, and materials chemistry.[22](https://aces.onlinelibrary.wiley.com/doi/10.1002/ajoc.202100650) It is also listed as a metabolite found in or produced by the yeast *Saccharomyces cerevisiae*.[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil)
## Safety
Under GHS classification benzil carries the signal word Warning with hazard statements H315 (causes skin irritation), H319 (causes serious eye irritation), and H335 (may cause respiratory irritation).[1](https://pubchem.ncbi.nlm.nih.gov/compound/benzil) The rat oral LD50 exceeds 3000 mg/kg, and the substance is essentially insoluble in water (0.5 g/L at 20 °C) with a logP of 3.38, suggesting some potential to bioaccumulate.[3](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch) It is listed as active on the US TSCA inventory and on the Canadian DSL and several other national inventories, with no REACH authorization or restriction entries.[3](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch)
## References
1. [Benzil, CID 8651, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/benzil)
2. [Benzil – Benzilic Acid Rearrangement (video description), IITian explains by Unacademy](https://www.youtube.com/watch?v=JYCPV6vbfJE)
3. [Benzil Safety Data Sheet, Thermo Fisher Scientific (Alfa Aesar), revision 23 January 2024](https://assets.thermofisher.com/directwebviewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAAA12579&SUBFORMAT=d__CLP1&page=NewSearch)
4. [Structural and Vibrational Investigation of Benzil, Kanagathara et al., Journal of Chemistry (2022)](https://onlinelibrary.wiley.com/doi/10.1155/2022/5968496)
5. [Spontaneous mirror symmetry breaking in benzil-based phases, ScienceDirect](https://www.sciencedirect.com/org/science/article/pii/S2041652023025518)
6. [Oxidation of benzoin to benzil – laboratory experiment (video description), Chem Help ASAP](https://www.youtube.com/watch?v=BNS4vpkhqAw)
7. [Chemistry 211 Experiment 10, MiraCosta College](https://home.miracosta.edu/dlr/211exp10.htm)
8. [Selective liquid phase oxidation of benzoin to benzil over transition metals doped MCM-41, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S1566736708001222)
9. [What Makes a Good Reaction? An Introduction to Green Chemistry in an Undergraduate Practical, Journal of Chemical Education](https://pubs.acs.org/doi/10.1021/acs.jchemed.5c01819)
10. [CN115745762A – Oxidation method for preparing benzil from benzoin, Google Patents](https://patents.google.com/patent/CN115745762A/en)
11. [Experiments in Organic Chemistry, Fieser, 2nd ed. (1941)](https://library.sciencemadness.org/library/books/Experiments_in_Organic_Chemistry_by_Fieser_2nd_ed_1941.pdf)
12. [Molecular Rearrangements, NPTEL Module 8, Lecture 17](https://archive.nptel.ac.in/content/storage2/courses/104103022/module8/lec17/4.html)
13. [A FMO-controlled reaction path in the benzil-benzilic acid rearrangement, PubMed](https://pubmed.ncbi.nlm.nih.gov/16496961/)
14. [Chemistry of aromatic compounds, Menoufia University course notes](https://mu.menofia.edu.eg/PrtlFiles/Faculties/pharm/Portal/Files/pharmaceutical%20organic%20chemistry.pdf)
15. [Coordinative Role of Alkali Cations in Organic Reactions VI, Poonia, Bulletin des Sociétés Chimiques Belges (1981)](https://onlinelibrary.wiley.com/doi/abs/10.1002/bscb.19810900307)
16. [Benzilic Acid – an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/benzilic-acid)
17. [Photoreactions in polymers containing benzil units, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S0032386100001725)
18. [Benzil Photoperoxidations in Polymer Films, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8434379/)
19. [A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9145830/)
20. [Free radical macrophotoinitiators: an overview on recent advances, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S1010603003001758)
21. [Exciplex Formation – an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/chemistry/exciplex-formation)
22. [Benzils: A Review on their Synthesis, Shaikh, Kamble, and Bayannavar, Asian Journal of Organic Chemistry (2022)](https://aces.onlinelibrary.wiley.com/doi/10.1002/ajoc.202100650)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Aldehydes and ketones › Dicarbonyls and poly-carbonyl compounds › Aryl dicarbonyls and diaryl ketones*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 11, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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