2-Bromohexanoic Acid: Synthesis, Handling & Application Protocol

2-Bromohexanoic Acid is an alpha-bromocarboxylic acid widely employed as a versatile electrophilic building block in organic synthesis, enabling SN2 substitution, enolate chemistry, and the preparation of chiral intermediates for pharmaceuticals and specialty chemicals.

CAS Number

616-05-7

Molecular Formula

C₆H₁₁BrO₂

Molecular Weight

195.06 g/mol

Appearance

Colorless to pale yellow liquid or low-melting solid

Functional Groups

Alpha-bromo, carboxylic acid

SKU

B152316

Typical Application

Electrophilic alkylating agent, chiral amino acid synthesis, polymer modification

Chemical Properties and Structure

2-Bromohexanoic Acid (CAS 616-05-7, MW 195.06 g/mol, molecular formula C₆H₁₁BrO₂) is an alpha-bromocarboxylic acid featuring a bromine substituent at the carbon alpha to the carboxyl group. This structural arrangement imparts high electrophilicity at C-2, making it significantly more reactive toward nucleophilic substitution than unactivated alkyl bromides. The molecule possesses a chiral center at C-2, and the commercially available racemic mixture (±) can be resolved or used directly depending on the synthetic objective. The compound is typically a colorless to pale yellow liquid or low-melting solid at room temperature with a pungent, irritating odor characteristic of alpha-halo acids.

The carboxylic acid moiety enables direct manipulation via esterification, amide coupling, or decarboxylative strategies, while the alpha-bromo group serves as the primary site of reactivity toward nucleophiles including amines, thiols, alkoxides, and soft carbon nucleophiles.

Synthesis and Preparation Methods

The most established route to 2-Bromohexanoic Acid is the Hell–Volhard–Zelinsky (HVZ) reaction, in which hexanoic acid is treated with bromine (Br₂) in the presence of a catalytic amount of phosphorus tribromide (PBr₃) or red phosphorus. The reaction proceeds via in situ formation of the acyl bromide intermediate, which undergoes enolization and selective alpha-bromination, followed by hydrolysis to furnish the alpha-bromo acid.

1. Reagents: Hexanoic acid, Br₂ (1.0–1.05 equiv), PBr₃ (cat., ~0.1 equiv).
2. Procedure: Add PBr₃ to neat hexanoic acid at 0 °C with stirring. Add Br₂ dropwise, allowing temperature to rise gradually to room temperature. Stir until bromine color is consumed. Quench carefully with cold water.
3. Workup: Extract the product into ethyl acetate or diethyl ether, wash with cold water, dry over MgSO₄, and concentrate under reduced pressure. Purify by vacuum distillation if high purity is required.
4. Yield and Purity: Typical yields under optimized conditions are reported in the literature to be moderate-to-good; exact figures should be verified against primary literature.

Alternatively, 2-Bromohexanoic Acid can be prepared by hydrolysis of the corresponding alpha-bromo ester (e.g., ethyl 2-bromohexanoate) under basic or acidic aqueous conditions.

Key Applications in Organic Synthesis

2-Bromohexanoic Acid is a broadly applicable electrophilic building block exploited in multiple synthetic contexts:

• SN2 Alkylation: The activated alpha-carbon undergoes efficient SN2 reactions with nitrogen, oxygen, sulfur, and carbon nucleophiles to install a wide range of functional groups while retaining a carboxylic acid handle for further elaboration.
• Synthesis of Non-Natural Amino Acids: Reaction of 2-Bromohexanoic Acid with ammonia equivalents (e.g., phthalimide/Gabriel synthesis, azide displacement followed by reduction) provides access to 2-aminohexanoic acid (norleucine) derivatives and other alpha-amino acid scaffolds.
• Thioether and Thioester Formation: Thiol nucleophiles react selectively at C-2 to yield alpha-thio carboxylic acids, useful in bioconjugation and materials science.
• Polymer and Surface Functionalization: Used to introduce pendant carboxylic acid groups onto polymeric backbones or to modify surfaces via esterification or amide-bond chemistry.
• Pharmaceutical Intermediates: Serves as a precursor in multi-step synthesis of bioactive molecules requiring a hexyl-substituted chiral alpha-carbon framework.

Safety, Handling, and Storage

2-Bromohexanoic Acid is a corrosive, irritating alpha-halo acid and must be handled with appropriate precautions:

• Personal Protective Equipment (PPE): Wear chemical-resistant gloves (nitrile or neoprene), lab coat, and safety goggles. Use in a well-ventilated fume hood at all times.
• Skin and Eye Contact: The compound is corrosive to skin and eyes. In case of contact, flush immediately with copious water for at least 15 minutes and seek medical attention.
• Inhalation: Vapors and aerosols are irritating to the respiratory tract. Avoid inhalation; use respiratory protection if ventilation is insufficient.
• Storage: Store in a tightly sealed container in a cool, dry, well-ventilated area away from heat sources, oxidizers, and bases. Refrigerated storage (2–8 °C) is recommended to minimize hydrolysis and decomposition. Keep away from moisture.
• Waste Disposal: Dispose of as halogenated organic waste in accordance with local institutional and governmental regulations.
• Stability: Alpha-bromo acids can undergo slow hydrolysis or dimerization upon prolonged storage or exposure to moisture; check purity before use in sensitive reactions.

Always consult the Safety Data Sheet (SDS) provided by Aladdin Scientific for comprehensive hazard information before use.

Protocol: SN2 Amination to Synthesize 2-Aminohexanoic Acid Derivatives

This representative protocol illustrates the use of 2-Bromohexanoic Acid (B152316) as an electrophile in the Gabriel synthesis of norleucine (2-aminohexanoic acid):

1. Materials: 2-Bromohexanoic Acid (B152316, 1.0 equiv), potassium phthalimide (1.05 equiv), anhydrous DMF, hydrazine hydrate or aqueous HBr/acetic acid for deprotection.
2. Step 1 – N-Alkylation: Dissolve potassium phthalimide (1.05 equiv) in anhydrous DMF (0.5 M) under nitrogen atmosphere. Add 2-Bromohexanoic Acid (1.0 equiv). Heat at 80–100 °C for 6–12 hours with stirring. Monitor by TLC (EtOAc/hexane).
3. Step 2 – Workup of N-Alkylation: Cool to room temperature, pour onto ice water, extract with ethyl acetate, wash with brine, dry over Na₂SO₄, filter, and concentrate. The crude N-phthaloyl-norleucine product can be recrystallized if needed.
4. Step 3 – Phthalimide Deprotection: Treat the N-phthaloyl intermediate with hydrazine hydrate (2–3 equiv) in ethanol at reflux for 2–4 hours, or hydrolyze with 6 M HCl at reflux. Filter off the precipitated phthalhydrazide; concentrate the filtrate and purify the free amino acid by ion-exchange chromatography or recrystallization.
5. Step 4 – Analysis: Confirm product identity by ¹H NMR, ¹³C NMR, HRMS, and optical rotation if chiral resolution was performed.

Expected Outcome: 2-Aminohexanoic acid (norleucine) or its derivatives, depending on subsequent protecting-group strategy. Yield and ee (if applicable) should be determined experimentally.

Note: Specific reaction times, temperatures and yields may require optimization and should be validated in your laboratory. Consult primary literature for validated procedures.

Frequently asked questions

What is the difference between 2-Bromohexanoic Acid and hexanoic acid in terms of reactivity?

2-Bromohexanoic Acid has a bromine atom at the alpha-carbon adjacent to the carboxyl group, which dramatically activates that position toward nucleophilic substitution (SN2). Hexanoic acid lacks this leaving group and is far less reactive toward substitution chemistry at C-2. The alpha-bromo functionality also enables reactions not available to simple carboxylic acids, such as elimination to form alpha,beta-unsaturated acids.

How should 2-Bromohexanoic Acid be stored to maintain stability?

2-Bromohexanoic Acid should be stored in a tightly sealed container under cool (ideally 2–8 °C), dry, and dark conditions away from moisture, bases, and oxidizing agents. Moisture can promote hydrolysis of the C–Br bond over time. Check purity by NMR or GC before use in sensitive synthetic steps.

Is 2-Bromohexanoic Acid (B152316) available in racemic or enantiopure form?

The standard commercial offering of 2-Bromohexanoic Acid (SKU B152316) from Aladdin Scientific is the racemic (±) mixture, as is typical for HVZ-derived alpha-bromo acids. If an enantiopure form is required, enzymatic resolution or chiral synthesis strategies must be applied. Please contact Aladdin Scientific directly for information on availability of specific enantiomers or custom specifications.

What nucleophiles react efficiently with 2-Bromohexanoic Acid?

The alpha-bromo carbon of 2-Bromohexanoic Acid reacts efficiently with a variety of hard and soft nucleophiles including primary and secondary amines, alkali metal azides (for azide displacement), thiolates (for thioether formation), alkoxides (for ether formation), and stabilized carbon nucleophiles (e.g., malonates, enolates). Reaction conditions (solvent, base, temperature) should be selected based on the specific nucleophile to maximize yield and minimize side reactions such as elimination.

References

1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992. (Hell–Volhard–Zelinsky reaction, alpha-halo acid chemistry)
2. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012. (Alpha-bromination of carboxylic acids, Gabriel synthesis)
3. Aladdin Scientific. 2-Bromohexanoic Acid, SKU B152316, Product Specification Sheet and Safety Data Sheet.

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