Tetrapropylammonium bromide (CAS#1941-30-6)

Tetrapropylammonium bromide (TPAB), also designated tetra-n-propylammonium bromide, is a quaternary ammonium salt consisting of a central nitrogen atom bonded to four n-propyl groups, with bromide as the counterion. It is supplied as a white to almost white crystalline powder or crystal with a molecular weight of 266.26–266.27 g/mol and a melting point of 266–272 °C (with decomposition reported near 270 °C). The compound is highly soluble in water (approximately 100 g/L at 20 °C), yielding a clear solution with a pH of 5–7, and is also soluble in polar organic solvents such as alcohols, but insoluble in diethyl ether.

The tetrapropylammonium cation is a compact, symmetrical quaternary ammonium species with four short alkyl chains. This structure produces a lipophilic–hydrophilic balance that is intermediate between the shorter tetraethyl and the longer tetrabutyl homologues, a feature that tunes both its phase-transfer activity and its ability to act as a template in zeolite synthesis. TPAB is hygroscopic and should be stored under an inert atmosphere in a cool, dark place to prevent moisture uptake and caking. The bromide counterion can undergo halide exchange—for example, with sodium iodide to form the corresponding iodide salt—and the cation can be converted to the hydroxide form by treatment with potassium hydroxide in ethanol, a transformation relevant to its use as a base or template precursor.

Applications

The two dominant applications of Tetrapropylammonium bromide are as a phase-transfer catalyst and as a structure-directing agent (template) in zeolite synthesis.

As a phase-transfer catalyst, TPAB facilitates the migration of charged species—typically anions—from an aqueous or solid phase into an organic phase where the desired reaction occurs. It is employed in a broad range of organic transformations, including nucleophilic substitutions, oxidations, reductions, and carbene-mediated reactions, where the quaternary ammonium cation pairs with the reactive anion and transports it across the phase boundary. TPAB is valued for its environmental compatibility, operational simplicity, non-corrosiveness, and ease of recovery and reuse compared with homogeneous catalysts, and its use aligns with green chemistry principles of catalysis and waste reduction.

The single most important application of TPAB is as the structure-directing agent for the synthesis of ZSM-5 zeolite. ZSM-5 is a high-silica, shape-selective molecular sieve that serves as a major catalyst in petroleum refining (fluid catalytic cracking, isomerization, alkylation) and in fine chemical production. The tetrapropylammonium cation occupies the intersections and channel segments of the MFI framework during crystallization, directing the formation of the characteristic 10-membered-ring pore system; the organic template is subsequently removed by calcination to generate the active microporous solid. TPAB is also used as a template for the synthesis of other microporous and mesoporous materials with potential applications in electronics (conducting polymers) and catalysis.

Additional applications include use as a surfactant, stabilizer, and antistatic agent, and as a reagent in organic synthesis where a soluble quaternary ammonium bromide is required.

Preparation Method

Tetrapropylammonium bromide is prepared by the S_N2 nucleophilic substitution of tripropylamine with propyl bromide. The tertiary amine attacks the primary alkyl bromide, displacing bromide and forming the quaternary ammonium cation; the displaced bromide becomes the counterion of the product.

In a representative laboratory procedure, tripropylamine (1 mol) and propyl bromide (1.5 mol) are combined in N,N-dimethylformamide (100 mL) and stirred at 110 °C for 15 hours under a nitrogen atmosphere. After completion, the reaction mixture is cooled in an ice bath, and a mixture of n-hexane and toluene (2:1) is added to precipitate the product. The solid is filtered and washed with n-hexane/toluene (3:1) to afford TPAB.

On an industrial scale, the reaction is conducted in a polar solvent—nitromethane, dimethylformamide, N,N-dimethylpyrrolidone, or hexamethylphosphoric acid triamide—at temperatures between 60 °C and 160 °C, with a preferred range of 100–130 °C and reaction times of 2–15 hours. TPAB is essentially insoluble in the reaction medium and precipitates as it forms; it is recovered by centrifugation or filtration, washed with an organic solvent, and vacuum-dried to remove residual starting materials. A reported optimized process using tri-n-propylamine and 1-bromopropane in ethyl acetate under 3 MPa N₂ at 120 °C for 14 hours achieved a yield of 84.12% and a purity of 98.92%. The horizontal reactor design described in patent literature allows the solid product to be discharged continuously, and the mother liquor can be recycled to improve overall efficiency. An alternative laboratory route starts from tripropylamine and propyl bromide in DMF at 110 °C, with precipitation and washing as described above.

Safety Information

Tetrapropylammonium bromide is classified under GHS with the signal word Warning. The harmonized hazard statements are H315 (Causes skin irritation), H319 (Causes serious eye irritation), and H335 (May cause respiratory irritation) . The substance is assigned to Skin Irritation Category 2, Serious Eye Irritation Category 2, and Specific Target Organ Toxicity (Single Exposure) Category 3 for respiratory irritation. Acute oral toxicity is low, with an LD₅₀ of approximately 3,500 mg/kg in rabbits. The compound is hygroscopic, and its dust may cause mechanical irritation; no carcinogenic, mutagenic, or reproductive toxicity classifications have been assigned. It is not regulated as dangerous goods for transportation under DOT or IATA criteria.

For safe handling, avoid breathing dust and use only in a well-ventilated area or outdoors (P261, P271). Wear protective gloves, protective clothing, and eye/face protection (P280), and wash face, hands, and any exposed skin thoroughly after handling (P264). In case of skin contact, wash with plenty of soap and water (P302+P352); if skin irritation occurs, seek medical attention (P332+P313). For eye contact, rinse cautiously with water for several minutes, remove contact lenses if present and easy to do so, and continue rinsing (P305+P351+P338); if eye irritation persists, seek medical advice (P337+P313). If inhaled and feeling unwell, move the person to fresh air and keep comfortable for breathing, and call a poison center or physician (P304+P340, P312). Store in a well-ventilated place with the container kept tightly closed (P403+P233), protected from moisture and compatible with the hygroscopic nature of the solid. Dispose of contents and containers to an approved waste disposal plant (P501); the substance is water-soluble and may be mobile in the environment, so release to drains and watercourses should be avoided.