4-Chlorobenzyl Bromide: Properties, Uses & Handling FAQ

4-Chlorobenzyl Bromide is a reactive benzyl halide building block widely used in organic synthesis as an alkylating and benzylating agent for the preparation of pharmaceuticals, agrochemicals, and fine chemicals.

CAS Number

939-55-9

Molecular Formula

C7H6BrCl

Molecular Weight

205.48 g/mol

Appearance

Colorless to pale yellow liquid or low-melting solid

Primary Reactivity

Electrophilic benzylation via SN2 mechanism

Aladdin SKU

C135867

What Is 4-Chlorobenzyl Bromide and What Are Its Key Properties?

4-Chlorobenzyl Bromide (CAS 939-55-9, SKU C135867) is an aryl-substituted benzyl bromide carrying a chlorine substituent at the para position of the aromatic ring. Its molecular formula is C₇H₆BrCl with a molecular weight of approximately 205.48 g/mol. The compound typically appears as a colorless to pale yellow liquid or low-melting solid at room temperature.

The molecule possesses two distinct halogen functionalities: the benzylic C–Br bond, which is highly activated toward nucleophilic substitution due to resonance stabilization of the incipient benzylic carbocation or transition state, and the aryl C–Cl bond, which is comparatively inert under mild conditions. This orthogonal reactivity makes the compound a versatile bifunctional building block. The para-chloro substituent also provides a handle for further cross-coupling chemistry (e.g., Suzuki or Negishi coupling) under palladium catalysis once the benzylic bromide has been reacted.

What Are the Main Synthetic Applications of 4-Chlorobenzyl Bromide?

4-Chlorobenzyl Bromide serves primarily as an electrophilic alkylating agent in organic synthesis. Its most common applications include:

• O-Alkylation (Benzylation of Alcohols and Phenols): In the presence of a base (e.g., K₂CO₃, NaH, or Cs₂CO₃), the reagent readily alkylates hydroxyl groups to form 4-chlorobenzyl ethers, which serve as protecting groups in multi-step synthesis.
• N-Alkylation: Amines and nitrogen heterocycles can be benzylated to introduce the 4-chlorobenzyl group, a pharmacophore found in several drug candidates and bioactive molecules.
• S-Alkylation: Thiols and thiolates react rapidly with 4-Chlorobenzyl Bromide to produce sulfide derivatives useful in medicinal chemistry.
• C-Alkylation: Active methylene compounds and stabilized carbanions (malonates, beta-ketoesters) undergo C-benzylation under basic conditions.
• Pharmaceutical and Agrochemical Intermediates: The 4-chlorobenzyl motif is incorporated into APIs and crop-protection compounds, making this reagent an important intermediate in industrial synthesis.

How Should 4-Chlorobenzyl Bromide Be Handled and Stored Safely?

4-Chlorobenzyl Bromide is a lachrymatory, corrosive, and potentially toxic alkyl halide. The following safety and storage guidelines are essential:

• Personal Protective Equipment (PPE): Always handle in a well-ventilated fume hood. Wear chemical-resistant gloves (nitrile or neoprene), a lab coat, and safety goggles. A face shield is recommended when handling larger quantities.
• Inhalation and Skin Contact: The compound has lachrymatory properties and is an irritant to the eyes, skin, and respiratory tract. Avoid inhalation of vapors. In case of skin contact, wash immediately with copious water for at least 15 minutes.
• Storage Conditions: Store in a tightly sealed container under cool (2–8 °C recommended), dry, and dark conditions, away from moisture, bases, and oxidizers. Protect from light to prevent decomposition and color development.
• Moisture Sensitivity: The benzylic bromide is susceptible to hydrolysis upon prolonged exposure to moisture; desiccant storage is advisable for long-term use.
• Waste Disposal: Dispose of as halogenated organic waste in accordance with local institutional and governmental regulations.

Always consult the current Safety Data Sheet (SDS) provided by Aladdin Scientific before first use.

What Reactions and Conditions Are Compatible with 4-Chlorobenzyl Bromide?

Understanding compatibility helps researchers design efficient reaction protocols:

• Compatible Bases: Mild inorganic bases such as K₂CO₃ and Cs₂CO₃ in polar aprotic solvents (DMF, acetone, MeCN) are commonly used for O- and N-alkylation. Stronger bases (NaH, LDA) are used for C-alkylation but require anhydrous conditions.
• Solvent Selection: DMF, DMSO, acetone, THF, and acetonitrile are suitable solvents. Protic solvents should be avoided unless the reaction specifically requires them, as they can promote competing hydrolysis.
• Temperature: Many reactions proceed efficiently at room temperature to 80 °C. Elevated temperatures accelerate the reaction but may promote elimination or polyalkylation side reactions.
• Incompatibilities: Avoid contact with strong oxidizers, strong bases (risk of elimination), metals (risk of coupling), and water (hydrolysis to 4-chlorobenzyl alcohol).
• Phase-Transfer Catalysis (PTC): The reagent is compatible with PTC conditions (e.g., TBAB or Aliquat 336) for biphasic alkylation reactions, offering practical advantages for scale-up.

Quality, Purity, and Ordering Information for Aladdin C135867

Aladdin Scientific offers 4-Chlorobenzyl Bromide under SKU C135867. Researchers should note the following when ordering:

• Purity: Check the current Certificate of Analysis (CoA) available on the product page for lot-specific purity data. Typical reagent-grade material is ≥95% pure by GC or titration; consult the CoA for the specific lot.
• Available Pack Sizes: Multiple pack sizes are typically available to suit discovery-scale and process-scale requirements. Contact Aladdin Scientific or visit the product page for current stock and pricing.
• Certificate of Analysis (CoA): A lot-specific CoA, including purity, melting/boiling point data, and analytical results, can be downloaded directly from the Aladdin product page using the lot number.
• Shipping: As a regulated halogenated organic compound, this product may be subject to special packaging and shipping conditions. Please review Aladdin's shipping and hazardous materials policies prior to ordering.
• Shelf Life and Re-testing: If the material has been stored for an extended period or shows visible discoloration (yellowing/browning), re-test purity before use in critical applications.

Frequently asked questions

What is the difference between the benzylic bromide and the aryl chloride functionalities in 4-Chlorobenzyl Bromide?

The benzylic C–Br bond is highly reactive toward nucleophilic substitution (SN2) because the benzylic position is activated by resonance with the aromatic ring. The aryl C–Cl bond is much less reactive under standard nucleophilic substitution conditions because the sp2 carbon is less electrophilic and the chloride is a poorer leaving group in that context. This orthogonal reactivity allows selective functionalization of each position under different conditions: the C–Br is addressed first under mild base conditions, while the C–Cl can later participate in palladium-catalyzed cross-coupling reactions.

How should I handle 4-Chlorobenzyl Bromide to minimize lachrymatory exposure in the laboratory?

Always transfer and handle 4-Chlorobenzyl Bromide inside a well-functioning chemical fume hood with the sash lowered as far as practical. Use a syringe or pre-weighed sealed vials to minimize open-vessel time. Wear nitrile or neoprene gloves, a lab coat, and safety goggles at all times. Promptly seal the container after use, and wash gloves before removal to prevent secondary contamination.

Can 4-Chlorobenzyl Bromide be used as a protecting group reagent, and how is the group removed?

Yes, 4-Chlorobenzyl Bromide is used to install the 4-chlorobenzyl (Cl-Bn) ether or ester protecting group on alcohols, phenols, and carboxylic acids. The 4-chlorobenzyl group is more electron-poor than a standard benzyl group, making it slightly more stable to acidic conditions. It is typically removed by catalytic hydrogenolysis (H2, Pd/C or Pd(OH)2) under standard conditions, similar to ordinary benzyl protecting groups. The presence of the aryl chloride does not significantly impede hydrogenolysis under typical conditions, though catalyst loading and reaction time may need optimization.

What solvents are recommended for reactions using 4-Chlorobenzyl Bromide?

Polar aprotic solvents are generally preferred: DMF and DMSO provide high polarity to facilitate SN2 reactions, while acetone and acetonitrile offer a good balance of polarity and ease of removal. THF is suitable for reactions requiring lower polarity or cryogenic conditions. Protic solvents such as water, methanol, or ethanol should be avoided or used with caution, as the benzylic bromide can undergo hydrolysis or solvolysis, reducing yield and purity.

How do I verify the purity of 4-Chlorobenzyl Bromide upon receipt?

Upon receipt, visually inspect the material for excessive discoloration (dark yellow, brown, or black color may indicate degradation). Download the lot-specific Certificate of Analysis (CoA) from Aladdin's product page using your lot number to confirm reported purity. For critical applications, in-house verification by GC (using an internal standard) or NMR is recommended. Key NMR indicators include the benzylic CH2 singlet near δ 4.4 ppm and the characteristic para-substituted aromatic pattern in the 7.2–7.4 ppm region.

References

1. March, J.; Smith, M. B. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th ed.; Wiley: Hoboken, NJ, 2013. (General SN2 alkylation mechanisms and benzyl halide reactivity.)
2. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 4th ed.; Wiley: Hoboken, NJ, 2006. (Benzyl and substituted benzyl protecting groups.)
3. Aladdin Scientific. Product Page and Safety Data Sheet: 4-Chlorobenzyl Bromide, SKU C135867. (Retrieved from Aladdin Scientific official website.)

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