Benzopinacol: Properties, Uses & Handling FAQs

Benzopinacol is a symmetric vicinal diol (1,1,2,2-tetraphenyl-1,2-ethanediol) widely used in organic synthesis as a precursor for the pinacol rearrangement and as a model substrate in photochemical studies.

Chemical Name

1,1,2,2-Tetraphenyl-1,2-ethanediol

Molecular Formula

C₂₆H₂₂O₂

Molecular Weight

366.45 g/mol

CAS Number

464-72-2

Appearance

White to off-white crystalline solid

Key Reaction

Undergoes acid-catalyzed pinacol rearrangement to yield benzopinacol (benzpinacolone)

What Is Benzopinacol and What Are Its Key Chemical Properties?

Benzopinacol (CAS 464-72-2), systematically named 1,1,2,2-tetraphenyl-1,2-ethanediol, is a symmetric vicinal diol with the molecular formula C₂₆H₂₂O₂ and a molecular weight of 366.45 g/mol. It appears as a white to off-white crystalline solid at room temperature. The compound features two adjacent carbon atoms each bearing two phenyl groups and one hydroxyl group, giving it a highly hindered, symmetric structure.

Benzopinacol is sparingly soluble in common protic solvents such as water and ethanol, but dissolves more readily in chlorinated solvents (e.g., dichloromethane, chloroform) and aromatic solvents such as benzene or toluene. Its melting point and exact solubility data should be confirmed against the certificate of analysis supplied with each lot (see review flags).

How Is Benzopinacol Synthesized?

The classical laboratory synthesis of benzopinacol involves the photochemical reductive coupling of benzophenone. When benzophenone is irradiated with UV light in the presence of a hydrogen-donor solvent (commonly isopropanol), it undergoes n→π* excitation to its triplet state. The excited benzophenone abstracts a hydrogen atom from the solvent to generate a ketyl radical (Ph₂C•–OH). Two ketyl radicals then couple to form benzopinacol.

This reaction is one of the most cited examples of a photochemical radical coupling process and is frequently used as a teaching experiment in undergraduate organic photochemistry courses. The yield depends critically on the light source intensity, solvent purity, and exclusion of oxygen, which quenches the triplet state and the radical intermediates.

What Are the Main Applications of Benzopinacol in Research?

Pinacol Rearrangement Precursor: Benzopinacol is the archetypal substrate for the pinacol rearrangement, an acid-catalyzed 1,2-shift reaction that converts vicinal diols into carbonyl compounds. Treatment with a protic or Lewis acid causes dehydration and migration of a phenyl group to yield the rearranged ketone product (benzpinacolone, 2,2,3-triphenylpropiophenone). This transformation is a cornerstone example in mechanistic organic chemistry.

Photochemistry Model Compound: Because its synthesis directly involves a photochemical radical step, benzopinacol is used as a standard compound to probe photochemical reaction efficiency, quantum yields, and triplet-state reactivity of carbonyl compounds.

Synthetic Intermediate: Researchers employ benzopinacol as a building block for constructing highly substituted carbon frameworks, leveraging its diol functionality for further derivatization such as cyclic carbonate formation, esterification, or elimination reactions.

Storage, Handling, and Safety Considerations

Storage: Store benzopinacol in a tightly sealed container in a cool, dry, well-ventilated location away from direct light and incompatible materials such as strong acids and strong oxidizers. Recommended storage temperature is typically room temperature or 2–8 °C for long-term preservation; confirm the specific recommendation on the product label or SDS.

Handling: Use standard laboratory personal protective equipment (PPE): lab coat, chemical-resistant gloves, and safety goggles. Avoid generating dust and work in a fume hood when handling larger quantities or performing reactions involving this compound.

Disposal: Dispose of in accordance with local, national, and institutional regulations for organic chemical waste. Do not pour down the drain without confirming local permissibility.

Note: Always consult the Safety Data Sheet (SDS) provided by Aladdin Scientific before use for authoritative hazard, first-aid, and regulatory information.

Purity, Grade, and Quality Specifications

Aladdin Scientific offers benzopinacol (SKU: B153219) as a research-grade reagent. Purity is verified by standard analytical techniques; the exact purity specification and analytical data (e.g., HPLC, GC, or titration) are provided on the Certificate of Analysis (CoA) accompanying each shipment.

When selecting a lot for sensitive applications such as quantum yield measurements or mechanistic studies, researchers should request the CoA to confirm purity, water content, and any relevant spectroscopic identity tests. For bulk or custom purity requirements, contact Aladdin Scientific's technical sales team.

Frequently asked questions

What is the difference between benzopinacol and benzpinacolone?

Benzopinacol (1,1,2,2-tetraphenyl-1,2-ethanediol) is the vicinal diol starting material, while benzpinacolone (2,2,3-triphenylpropiophenone, also called desyl phenyl ketone) is the rearranged ketone product formed when benzopinacol undergoes the acid-catalyzed pinacol rearrangement. The reaction involves dehydration followed by a 1,2-phenyl shift and produces one equivalent of water.

Why must oxygen be excluded during the photochemical synthesis of benzopinacol?

Molecular oxygen is an efficient quencher of the excited triplet state of benzophenone and also reacts rapidly with the ketyl radical intermediates to form peroxy species rather than allowing radical coupling. This drastically reduces the yield of benzopinacol. The reaction mixture should therefore be degassed (e.g., by freeze-pump-thaw cycles or sparging with inert gas such as argon or nitrogen) before irradiation.

In which solvents does benzopinacol dissolve well?

Due to its large, nonpolar tetraphenyl framework, benzopinacol dissolves more readily in nonpolar to moderately polar organic solvents such as dichloromethane, chloroform, toluene, and ethyl acetate. It has limited solubility in highly polar protic solvents like water and cold ethanol. Gentle warming or sonication may assist dissolution in borderline solvents.

How should benzopinacol be stored to maintain its purity over time?

Benzopinacol should be stored in a tightly closed container under cool, dry, and dark conditions. Protecting it from strong acids (which could trigger premature rearrangement) and strong oxidants is particularly important. Check the product label and SDS for the specific recommended storage temperature; refrigerated storage (2–8 °C) is advisable for long-term stock.

Can benzopinacol be used as a reference standard in photochemistry experiments?

Yes, benzopinacol is widely used as a reference or model compound in photochemical studies, particularly for benchmarking the efficiency of triplet-state hydrogen abstraction reactions and ketyl radical coupling. Its well-characterized synthesis from benzophenone makes it a convenient standard. Researchers requiring high-purity material for quantitative experiments should verify purity by requesting the Certificate of Analysis.

References

1. Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules. University Science Books, 2010. (Photochemical synthesis of benzopinacol via benzophenone ketyl radical coupling.)
2. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed. Wiley-Interscience, 1992. (Pinacol rearrangement mechanism and examples including benzopinacol.)
3. Aladdin Scientific Certificate of Analysis, Benzopinacol, SKU B153219. (Purity, melting point, and analytical data for each lot.)

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