L-Phenylalanine ethyl ester hydrochloride, ethyl (2S)-2-amino-3-phenylpropanoate hydrochloride, is released as a white to off-white crystalline powder under CAS Registry Number 3182-93-2. The molecular formula C₁₁H₁₅NO₂·HCl gives a molar mass of 229.70 g·mol⁻¹ and a theoretical chloride content of 15.43 %. Commercial product models are normally differentiated as peptide-synthesis grade or pharmaceutical-intermediate grade; the former is typically controlled at ≥98.0 % assay by HPLC, while the latter may be controlled at ≥99.0 % on the anhydrous basis with individual unspecified impurities ≤0.5 %. Release documents commonly list appearance, identity by infrared absorption spectrophotometry, assay by HPLC, enantiomeric purity by chiral HPLC, loss on drying, residue on ignition, chloride content, and residual solvents. Because no harmonised pharmacopoeial monograph exists for this specific ester salt, suppliers align test procedures with USP <621>, USP <731>, USP <281>, USP <467>, USP <781>, and USP <232>/<233> or equivalent Ph. Eur. methods. The dry crystalline salt is less sensitive to atmospheric carbon dioxide than the corresponding free base, but it remains hygroscopic and should be stored in a desiccated environment at +2 °C to +8 °C for multi-year retention. The established shorthand designation L-Phe-OEt·HCl is used in synthetic route descriptions and supplier documentation.

How Is the Hydrochloride Salt Produced and Dried Without Ester Hydrolysis?

The hydrochloride salt is commonly prepared by Fischer esterification of L-phenylalanine with anhydrous ethanol in the presence of hydrogen chloride gas. In a glass-lined reactor fitted with an external scrubber and brine-cooled condenser, hydrogen chloride is sparged below the liquid surface at 0–10 °C. The acid-catalysed esterification is equilibrium-limited; water removal or a large molar excess of ethanol is used to drive the reaction toward the ester. Residual water above approximately 0.5 % w/w in the reaction mass promotes acid-catalysed hydrolysis of the newly formed ethyl ester and reduces yield. After conversion, the product is crystallised by controlled addition of an antisolvent such as methyl tert-butyl ether or diethyl ether at ≤15 °C. The slurry is transferred to a pressure filter or agitated nutsche filter-dryer, washed with cold solvent, and dried under vacuum at 35–40 °C. Drying above 45 °C or over-drying to less than 0.5 % residual solvent can generate electrostatic caking and crystal fracture; residual ethanol is therefore monitored by headspace gas chromatography using USP <467> or Ph. Eur. 2.4.24. On pilot scale, rotary vacuum drying with a jacket temperature of 35 °C and an absolute pressure of 20–50 mbar typically reduces residual ethanol below 0.1 % within 6–10 h, although filter cake thickness and wash composition produce observable batch-to-batch variation.

Chiral integrity is monitored during manufacture because prolonged exposure to strong acid at elevated temperature can erode enantiomeric purity. Pilot records for amino acid ester hydrochloride production generally limit esterification and crystallisation temperatures to ≤25 °C and minimise hold time between conversion and isolation. Published data for the exact activation energy of racemisation in this salt is limited. The optical rotation and chiral HPLC results on release provide a direct measure of retained stereochemistry. Crystallisation control also influences downstream filtration, because rapid antisolvent addition above 15 °C produces finely divided crystals with a broad particle-size distribution, slow filtration, and higher retained solvent.

In solution-phase peptide synthesis, the hydrochloride is not introduced as a direct nucleophile. The amino group is first liberated with a tertiary amine such as N,N-diisopropylethylamine or N-methylmorpholine in an anhydrous aprotic solvent, usually dichloromethane, tetrahydrofuran, or dimethylformamide. A stoichiometric excess of tertiary amine above 1.0 equivalent is avoided when the ethyl ester must survive long reaction times, because residual base accelerates saponification of the carboxyl-protecting group. Coupling to an N-protected amino acid is performed with carbodiimide reagents such as N,N′-dicyclohexylcarbodiimide or with uranium/phosphonium salts at a reaction temperature of -5 °C to +5 °C to limit racemization via oxazolone formation on the carboxyl component. After coupling, the product is normally isolated by aqueous work-up at pH 6–7; the ethyl ester has sufficient base resistance to survive a short bicarbonate wash, but prolonged exposure to pH above 8.0 leads to hydrolysis and the appearance of free L-phenylalanine. Reaction completion is monitored by thin-layer chromatography with ninhydrin staining and by HPLC at 210 nm.

Because the ethyl ester is an ester-protected carboxyl group and the hydrochloride is an amino-protecting salt form, the compound is useful in convergent routes that require a terminal phenylalanine residue to be installed without exposing an unprotected carboxyl group to the coupling medium. The ethyl ester is removed later by alkaline saponification with lithium hydroxide or sodium hydroxide in aqueous tetrahydrofuran, whereas the hydrochloride salt is simply neutralised before coupling. This protection strategy is not suitable for routes that require acid-mediated global deprotection, because the ethyl ester is stable to trifluoroacetic acid and requires a separate alkaline step.

Comparative Physicochemical Profile Across Methyl, Ethyl, and tert-Butyl Ester Salts

The hydrochloride salt differs from the free base and from the methyl and tert-butyl ester salts in handling, solubility, and orthogonal protection behaviour. The free base of L-phenylalanine ethyl ester is a low-melting material or liquid with greater sensitivity to atmospheric carbon dioxide, whereas the hydrochloride is a crystalline solid that can be weighed with lower risk of carbonate salt formation. The L-configuration is essential in pharmaceutical intermediate use; the D-enantiomer is generally limited to reference standards or impurity markers. Compared with methyl ester hydrochloride, the ethyl ester hydrochloride carries two additional methylene units, giving a higher calculated log P and a slower rate of alkaline ester hydrolysis. Compared with tert-butyl ester hydrochloride, the ethyl ester is stable in trifluoroacetic acid, whereas the tert-butyl ester is acid-labile and is removed under acidic conditions. These properties make the ethyl ester a base-labile, acid-stable carboxyl-protected building block, orthogonal to the tert-butyl ester in multistep synthesis.

The hydrochloride salt also differs from the free base in automated solid dispensing. The free base can be difficult to dispense in sub-gram quantities because of its physical form and tendency to creep, whereas the crystalline hydrochloride has a stable particle shape and higher bulk density. The salt form increases water solubility relative to the free base, which is relevant when a downstream step requires aqueous charging of the amino ester as a concentrated solution. The hydrochloride also stabilises the primary amino group against atmospheric carbon dioxide and oxidation; however, it introduces chloride ion, which must be considered in corrosion-resistant reactor selection and in waste-stream management.

When Tertiary Amine Neutralisation Generates an Aqueous Phase in DMF Couplings

A recurring production issue occurs when the hydrochloride salt is neutralised with aqueous sodium bicarbonate or sodium carbonate before coupling in dimethylformamide. The aqueous work-up can carry residual water into the coupling medium; if the water content exceeds approximately 0.3 % w/w, carbodiimide reagents are hydrolysed to ureas, the activated ester intermediate is quenched, and the resulting batch shows reduced conversion and increased N-acylurea by-products. Aqueous base also promotes saponification of the ethyl ester during the subsequent wash, producing free L-phenylalanine that is difficult to remove from the final peptide ester. To avoid this, scale-up records generally specify neutralisation with a nonaqueous tertiary amine in dichloromethane or tetrahydrofuran, followed by filtration of the precipitated tertiary ammonium chloride and immediate use of the filtrate. Where in situ neutralisation is used, the tertiary ammonium chloride by-product increases slurry viscosity and may settle if agitation is insufficient. Published data for exact stirrer speed thresholds in this specific salt are limited; process development reports for amino acid ester hydrochlorides describe similar salt settling and reduced conversion at low agitation rates, and pilot-scale batches are typically monitored by attenuated total reflectance infrared spectroscopy or off-line HPLC to confirm neutralisation before addition of the coupling reagent.

The base-catalysed hydrolysis of amino acid ethyl esters is generally second order, first order in hydroxide ion and first order in ester. For this specific hydrochloride, published rate constants under peptide coupling conditions are limited; process development therefore relies on end-of-reaction HPLC rather than a predictive kinetic model. Maintaining pH below 8.0 and temperature below 5 °C reduces the hydrolysis background while allowing coupling to proceed. The main stereochemical risk in carbodiimide-mediated coupling is oxazolone-mediated racemisation of the carboxyl component; the ethyl ester amino component contributes less risk when neutralised and used immediately at low temperature.

Release documentation for pharmaceutical intermediate shipments usually includes an HPLC chromatogram at 210 nm or 220 nm, a chiral purity result, residual solvent analysis, and chloride content by titration. The hydrochloride salt is hygroscopic and is best handled in an area maintained below 60 % relative humidity. Repacking of milled material into low-density polyethylene bags inside fibre drums is common, but storage above 25 °C can lead to gradual yellowing and ester hydrolysis. Stability studies reported in supplier documentation for amino acid ester hydrochloride salts generally show that material held at 25 °C/60 % RH for 24 months develops free L-phenylalanine levels from an initial <0.1 % to approximately 0.2–0.5 % depending on package integrity. At 40 °C/75 % RH, hydrolysis accelerates; long-term storage is therefore specified at +2 °C to +8 °C with desiccant. The compound should not be stored in contact with strong bases or in unbuffered aqueous solutions above pH 8.0 for extended periods, because saponification releases ethanol and L-phenylalanine.

Published ICH Q1A stability data for this exact ester salt is limited in the public domain; the above degradation behaviour is consistent with the general hydrolysis sensitivity of amino acid ethyl ester hydrochlorides and should be confirmed on the specific batch using ICH Q1A storage conditions.

Release Testing and Stability Controls in Bulk Handling

The above release framework is aligned with ICH Q3C for residual solvents and ICH Q3D for elemental impurities, but the scope of testing varies by grade and intended use. For pharmaceutical intermediate applications, a supplier-specific specification should be reviewed against the downstream synthesis step, particularly where residual ethanol or chloride can interfere with moisture-sensitive or halogen-sensitive reactions.