Trifluoroacetic Acid (TFA)

Trifluoroacetic Acid

Other Names: TFA, 2,2,2-Trifluoroacetic Acid, Trifluoroethanoic Acid, Perfluoroacetic Acid
Chemical Formula: C₂HF₃O₂
Structural Formula: CF₃COOH
Molar Mass: 114.02 g/mol
CAS Number: 76-05-1
SMILES: FC(F)(F)C(=O)O
Appearance: Colorless, volatile liquid
Odor: Sharp, pungent acidic odor

Trifluoroacetic acid (TFA) is a fluorinated carboxylic acid derived structurally from acetic acid by replacement of all three hydrogen atoms of the methyl group with fluorine atoms. The strongly electron-withdrawing CF₃ group markedly increases the acidity of the neighboring carboxyl group by stabilizing the trifluoroacetate anion. As a result, TFA is substantially stronger than non-fluorinated carboxylic acids and is widely used as a volatile strong organic acid in synthetic, analytical and peptide chemistry. NIST reports the molecular formula C₂HF₃O₂, molecular mass 114.0233 g/mol and CAS Registry Number 76-05-1.

Chemical and physical properties

Trifluoroacetic acid is a colorless liquid under ambient conditions. Reference data report a melting point of approximately −15.4 °C, a boiling point of approximately 72 °C and a density of about 1.48 g/cm³. Its relatively low boiling point allows TFA to be removed comparatively easily from many reaction mixtures by evaporation or distillation.

TFA is miscible with water and with many polar and moderately polar organic solvents. It is highly polar, hygroscopic and capable of forming strong hydrogen-bonding interactions. Mixtures of TFA and water display non-ideal phase behavior, including formation of an azeotropic composition.

The acidity of trifluoroacetic acid results primarily from the strong negative inductive effect of the three fluorine atoms. Literature values for its aqueous pK_a vary depending on experimental method and conditions. Classical reference sources commonly report values around 0.2–0.3, while a recent ¹⁹F NMR determination reported pK_a = 0.03 ± 0.08 at 300 K. In all cases, TFA is considerably more acidic than acetic acid.

Applications

Trifluoroacetic acid is extensively used as an acid reagent in organic synthesis. Its combination of strong acidity, relatively low nucleophilicity and volatility makes it useful for acid-catalyzed rearrangements, condensations, solvolysis reactions and transformations involving acid-sensitive protecting groups. It may also be used as a reaction medium or co-solvent in transformations that require a strongly acidic environment.

One of the most important applications of TFA is the cleavage of tert-butoxycarbonyl (Boc) protecting groups from amines. Protonation and fragmentation of the Boc group under acidic conditions releases the protected amine, making TFA a standard reagent in both conventional organic synthesis and peptide chemistry. Experimental studies of solid-phase peptide synthesis have specifically examined the dependence of Boc removal on TFA concentration.

In Fmoc-based solid-phase peptide synthesis, concentrated TFA mixtures are widely used at the end of synthesis to cleave peptides from acid-labile resin linkers and simultaneously remove many acid-sensitive side-chain protecting groups. Scavengers are commonly incorporated into cleavage mixtures to trap reactive carbocationic species generated during deprotection and thereby reduce undesired peptide modification.

TFA is also widely employed as a mobile-phase modifier in reversed-phase high-performance liquid chromatography, particularly for peptides and other basic compounds. Addition of small amounts of TFA lowers mobile-phase pH and promotes ion-pairing interactions, which can improve chromatographic peak shape and retention behavior. However, TFA can substantially suppress electrospray ionization signals in LC-MS because of gas-phase ion pairing and changes in spray properties, so alternative volatile modifiers are often preferred when maximum mass-spectrometric sensitivity is required.

Trifluoroacetic acid is additionally used to prepare trifluoroacetate salts of amines and other basic compounds. Such salts are frequently encountered during isolation and purification of peptides, amino-containing intermediates and other basic organic molecules after TFA-mediated deprotection or chromatography.

Handling and safety

Trifluoroacetic acid is strongly corrosive and can cause severe chemical burns to skin, eyes and mucous membranes. Because it is volatile, inhalation of concentrated vapors is also a significant hazard. Work with neat TFA or concentrated solutions should therefore be performed with effective ventilation, appropriate chemically resistant gloves, protective clothing and suitable eye or face protection.

The compound should be stored in tightly closed, chemically compatible containers and protected from incompatible bases and reactive materials. Its volatility and hygroscopicity should also be considered when preparing accurately defined solutions or storing partially used material.

Scientific references

NIST Chemistry WebBook, SRD 69. Trifluoroacetic acid. CAS Registry Number 76-05-1.

Eidman, K. F. Trifluoroacetic Acid. Encyclopedia of Reagents for Organic Synthesis. Wiley. DOI: 10.1002/047084289X.rt236.pub2.

Reid, R. E. Solid phase peptide synthesis. Effect of trifluoroacetic acid concentration on the removal of the tert-butyloxycarbonyl protecting group. Journal of Organic Chemistry, 1976, 41, 1027–1031. DOI: 10.1021/jo00868a024.

King, D. S.; Fields, C. G.; Fields, G. B. Trifluoroacetic acid cleavage and deprotection of resin-bound peptides following synthesis by Fmoc chemistry. Methods in Enzymology, 1997, 289, 67–83. DOI: 10.1016/S0076-6879(97)89044-1.

Apffel, A.; Fischer, S.; Goldberg, G.; Goodley, P. C.; Kuhlmann, F. E. Enhanced sensitivity for peptide mapping with electrospray liquid chromatography-mass spectrometry in the presence of signal suppression due to trifluoroacetic acid-containing mobile phases. Journal of Chromatography A, 1995, 712, 177–190. DOI: 10.1016/0021-9673(95)00175-M.

Experimental Determination of pK_a for 10 PFAS, Mono-, Di-, and Trifluoroacetic Acid by ¹⁹F-NMR. Environmental Science & Technology Letters, 2025.

Safety

Signal Word: Danger

First Aid Measures
Eye Contact: Rinse cautiously with water for at least 15 minutes. Remove contact lenses if present and easy to do. Seek immediate medical attention.
Skin Contact: Remove contaminated clothing. Rinse skin immediately with plenty of water. Seek medical attention.
Inhalation: Move the person to fresh air. If breathing is difficult, administer oxygen and seek immediate medical attention.
Ingestion: Do not induce vomiting. Rinse mouth thoroughly with water. Seek immediate medical assistance.

Handling and Storage
Storage Conditions: Store in tightly closed containers in a cool, dry, and well-ventilated area away from heat and incompatible materials such as bases and oxidizing agents.
Handling Precautions: Use only with adequate ventilation. Avoid inhalation and contact with skin and eyes. Wear appropriate protective equipment.