2-Bromomesitylene: Properties, Handling & Lab Use Guide

2-Bromomesitylene is a sterically hindered aryl bromide reagent widely employed in organic synthesis as a versatile electrophilic partner and precursor to organometallic intermediates.

IUPAC Name

2-bromo-1,3,5-trimethylbenzene

Molecular Formula

C₉H₁₁Br

Molecular Weight

199.09 g/mol

CAS Number

576-83-0

Appearance

Colorless to pale yellow liquid

Key Hazard

Irritant; harmful if inhaled or absorbed through skin

Chemical Properties and Structure

2-Bromomesitylene (CAS 576-83-0), also known as 2-bromo-1,3,5-trimethylbenzene, is an aryl bromide with the molecular formula C₉H₁₁Br and a molecular weight of approximately 199.09 g/mol. The compound features a benzene ring symmetrically substituted with three methyl groups at the 1, 3, and 5 positions, and a bromine atom at the 2 position. This arrangement creates significant steric hindrance around the C–Br bond, which profoundly influences its reactivity compared to less hindered aryl bromides.

The steric bulk imparted by the flanking methyl groups at positions 1 and 3 renders direct nucleophilic attack at the ipso carbon difficult, making 2-bromomesitylene a useful model substrate for studying steric effects in cross-coupling and metal-insertion reactions. Despite this hindrance, the compound undergoes lithium-halogen exchange and magnesium insertion under appropriate conditions to generate the corresponding mesityl organolithium and Grignard reagents, respectively.

Synthetic Applications

2-Bromomesitylene serves as a valuable building block in several classes of organic transformations:

• Organolithium generation: Treatment with n-butyllithium or tert-butyllithium at low temperature (typically –78 °C in THF or diethyl ether) affords mesityllithium, a hindered aryllithium species useful for additions to carbonyl compounds and as a base.
• Grignard reagent synthesis: Reaction with magnesium turnings in anhydrous THF or diethyl ether produces mesitylmagnesium bromide, employable in standard Grignard additions and transmetalation protocols.
• Transition-metal-catalyzed cross-coupling: 2-Bromomesitylene participates in Suzuki–Miyaura, Negishi, and Kumada cross-coupling reactions, though its steric profile often necessitates the use of bulky, electron-rich phosphine ligands or specialized catalyst systems to achieve satisfactory yields.
• Steric probe in mechanistic studies: Its well-defined steric environment makes it an excellent substrate for benchmarking new catalytic systems and for probing the steric tolerance of transition-metal complexes.

Safe Handling and Storage

2-Bromomesitylene should be handled in accordance with standard laboratory safety practices for reactive organic halides:

• Personal protective equipment (PPE): Wear chemical-splash goggles, nitrile gloves, and a laboratory coat. Work in a well-ventilated fume hood to avoid inhalation of vapors.
• Skin and eye contact: The compound is an irritant. In case of skin contact, wash thoroughly with soap and water; for eye contact, rinse with copious water and seek medical attention.
• Storage: Store in a tightly sealed container in a cool, dry, well-ventilated area away from heat sources and incompatible materials such as strong oxidizers and strong bases. Keep protected from light to prevent degradation.
• Disposal: Dispose of waste material in accordance with local, regional, and national regulations governing halogenated organic waste. Do not pour down the drain.

Always consult the current Safety Data Sheet (SDS) supplied with the product before use.

Quality, Purity and Specification Considerations

For reliable synthetic outcomes, the purity of 2-bromomesitylene is a critical consideration. Key specification parameters to evaluate when sourcing this reagent include:

• Purity (GC assay): Research-grade material is typically ≥97–98% pure by GC analysis. Lower purity grades may introduce side reactions, particularly in sensitive organometallic transformations.
• Water content: Reactions involving organolithium or Grignard generation require rigorously anhydrous conditions. Verify that the reagent and all solvents are dry before use; use molecular sieves or freshly distilled solvents as appropriate.
• Color and appearance: High-quality material is colorless to pale yellow. Progressive yellowing or browning may indicate decomposition or the presence of HBr formed by light-induced elimination; such material should be redistilled or replaced.
• Packaging integrity: Confirm that ampule or bottle seals are intact on receipt. Store under inert atmosphere (nitrogen or argon) after opening to minimize moisture uptake.

Frequently asked questions

What is 2-Bromomesitylene used for in organic synthesis?

2-Bromomesitylene is primarily used as a precursor to sterically hindered organometallic reagents, including mesityllithium (via lithium–halogen exchange) and mesitylmagnesium bromide (Grignard reagent). It also participates in transition-metal-catalyzed cross-coupling reactions and serves as a steric probe in mechanistic studies.

Why does 2-Bromomesitylene require specialized catalyst systems in cross-coupling reactions?

The three methyl substituents on the mesitylene ring create significant steric bulk around the C–Br bond. This steric hindrance slows oxidative addition of the C–Br bond to palladium or other transition-metal centers, requiring bulky, electron-rich phosphine ligands or specially designed catalysts to facilitate the reaction at practical rates and yields.

How should 2-Bromomesitylene be stored to maintain its quality?

Store 2-Bromomesitylene in a tightly sealed container in a cool, dry, well-ventilated area away from light, heat, and strong oxidizers. After opening, keep the reagent under an inert atmosphere (nitrogen or argon) to prevent moisture uptake and light-induced degradation. Discard or redistill material that shows significant discoloration.

Is 2-Bromomesitylene compatible with aqueous reaction conditions?

2-Bromomesitylene is not compatible with reactions requiring anhydrous organometallic intermediates such as Grignard or organolithium reagents, which decompose rapidly in the presence of water. For cross-coupling reactions in which aqueous bases are used (e.g., Suzuki–Miyaura), limited water tolerance is acceptable, but thorough drying of the aryl bromide itself is still recommended for reproducible results.

References

1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley: New York, 1992. General reference for aryl halide reactivity and organometallic transformations.
2. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012. Textbook coverage of Grignard reagents, organolithium chemistry, and cross-coupling.
3. Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; University Science Books: Sausalito, CA, 2010. Reference for transition-metal-catalyzed cross-coupling with hindered aryl halides.

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