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TFA is an organic compound highly acidic in nature. It is an organofluorine compound with carboxylic acid as the functional group. It has three fluorine atoms substituted in the place of the three alpha hydrogens in acetic acid. The three fluorine atoms are the reason why trifluoroacetic acid has such a strong acidic nature. TFA stands for Trifluoroacetic Acid, also known as 2,2,2-trifluoroacetic acid. It is a colourless and odourless liquid which finds its applications in both organic and inorganic chemistry. It has a molecular formula of CF3CO2H and a molecular weight of 114.023 g/mol.
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Key Terms: Alpha Carbon, Atom, Fluorine, Functional Group, Carboxylic Acid, Acidity, Acetic Acid, Hydrogen
Formula and Structure of TFA
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One molecule of TFA has two carbon atoms, three fluorine atoms, two oxygen atoms and one hydrogen atom. Hence, the molecular formula of TFA is CF3CO2H. TFA is an analogue of acetic acid or ethanoic acid and has three substituted fluorine atoms in place of the three alpha hydrogens attached to the alpha carbon in acetic acid. The alpha carbon is the carbon that is attached to the primary functional group which is -COOH, i.e, carboxylic acid and the hydrogens attached to the alpha carbon are the corresponding alpha hydrogens.
Structure of Trifluoroacetic Acid
Synthesis of TFA
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There are two methods to prepare trifluoroacetic acid as mentioned below:
- Trifluoroacetic acid can be prepared via Swarts fluorination method by preparing trichlorotrifluoropropene. On treating hexachloropropene with AgF, we obtain tri chloro trifluoropropene or 1,1,1-trifluoro-2,3,3-trichloropropene, which on further oxidation with potassium permanganate gives TFA or trifluoroacetic acid. This is a more conventional way of preparation of TFA which is not used to an industrial extent now.
- Trifluoroacetic acid is also prepared by the electrochemical fluorination of acetic acid or acetyl chloride or acetic anhydride using Simons method which is followed by the hydrolysis of the trifluoroacetyl fluoride formed in the previous method. The reactions is
CH3COCl + 4 HF → CF3COF + 3 H2 + H+ + Cl-
Physical Properties of TFA
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Mentioned below are the physical properties of TFA:
- TFA is a carboxylic acid with three fluorine atoms and is a colourless fuming liquid with pungent odour, more like that of vinegar.
- It has a density of 1.489 g.m.L-1 and pKa value of 0.52.
- The Enthalpy of Vaporisation is 33 KJ.mol.
- TFA has melting and boiling points of -15.4? and 72.4? respectively.
Chemical Properties of TFA
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Mentioned below are the chemical properties of TFA:
- Trifluoroacetic acid is a highly acidic substance which is corrosive to skin, eyes and corrodes the skin on contact with this acid.
- It is also soluble in water and is much denser than water.
- The acidity of TFA or trifluoroacetic acid is much higher than that of acetic acid, this is because of the three highly electronegative fluorine atoms attached to the alpha carbon.
- Moreover, fluorine atoms are highly electron withdrawing groups and carry high negative inductive effects. Therefore, the carbon is highly electron deficient and can easily lose the proton or hydrogen atom in the primary functional group.
Applications of TFA
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TFA or trifluoroacetic acid has many varied applications such as:
- It is used as a calibrant in mass spectroscopy.
- It is a suitable and versatile solvent for NMR spectroscopy (for substances that are stable in acid).
- TFA is used as an ion pairing agent for organic compounds in liquid chromatography, the HPLC technique, at low concentrations. Example, for peptides and small proteins.
- TFA is widely used for the production of many fluorinated compounds including trifluoroacetic anhydride, trifluoroperacetic acid, trifluoroacetate salts, and 2,2,2- trifluoroethanol.
Things to Remember
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- Trifluoroacetic acid is an organofluorine compound and is an analogue of acetic acid or ethanoic acid.
- It has three substituted fluorine atoms in place of the three alpha hydrogens attached to the alpha carbon in acetic acid.
- The IUPAC name of trifluoroacetic acid is 2,2,2-trifluoroethanoic acid.
- TFA or Trifluoroacetic acid is a very useful reagent in both organic and inorganic chemistry due to its higher pKa value and thus, its higher strength as an acid.
- It is a well-known and suitable reagent in many reactions based on organic chemistry.
- The conjugate base of trifluoroacetic acid is stable which makes it an even better acid.
- Trifluoroacetic acid is also available in its anhydrous form than HCl. It is much less oxidising as compared to sulfuric acid.
Solved Questions
Ques. The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order acetic acid < trichloroacetic acid < water. Support your answer in this regard. (2 marks)
Ans. Fluorine is more electronegative than Chlorine and thus, trifluoroacetic acid is a much stronger acid than trichloroacetic acid which is even stronger than acetic acid because of the difference in electron deficiency over the alpha carbon atom. The depression in freezing point depends upon its degree of dissociation or the number of particles or ions in its aqueous solution, thus the increasing order for the depression in freezing point for the same amount of substances are in this manner.
Ques. Why is trifluoroacetic acid stronger than acetic acid which is considered a weak acid? (2 marks)
Ans. TFA or trifluoroacetic acid is a stronger acid than acetic acid. This is due to the higher acid ionisation constant i.e. Ka value of trifluoroacetic acid which is approximately 34,000 times higher than acetic acid. This is solely due to the highly electronegative fluorine atoms which have strong electron-withdrawing nature or negative inductive effect. This property of the trifluoro derivative reduces the electron density over the alpha carbon which in turn attracts electron from the carbon of the carboxylic acid and hence, weakens the oxygen-hydrogen bond which leads to the easy release of the hydrogen atom and thus, greater acidity in trifluoroacetic acid.
Ques. The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C) and formic acid (D) is: (5 marks)
a) A>B>C>D
b) A>C>B>D
c) B>A>D>C
d) B>D>C>A
Give your explanation for the same.
Ans. The correct option is c).
Explanation: This is due to the various factors that affect acidity electronegativity of the substituent atoms, size of the anions and resonance. TFA has the maximum acidity due to the highly electronegative fluorine atoms which have strong electron-withdrawing nature or negative inductive effect. Due to the relatively lesser electronegativity of a chlorine atom the acidity of trichloroacetic acid is lesser than TFA but higher than acetic acid and formic acid. Acetic acid has a comparatively lesser acidic nature than formic acid due to the positive inductive effect of the -CH group, also known as the methyl group. The +I effect of the methyl group increases the electron density over alpha carbon atoms and the electron withdrawal from the -COOH group is lesser thus, hydrogen-oxygen bond is not weakened. Therefore, proton release which is vital for determining acidic strength is not favourable and acetic acid is less acidic than formic acid. Formic acid or HCOOH has no substituent atoms or additional groups along with the primary functional group -COOH therefore, acidity is not influenced by other factors and therefore, is less acidic than trifluoroacetic acid and trichloroacetic acid but more acidic than acetic acid.
Ques. What is trifluoroacetic acid used for? (3 marks)
Ans. The uses of Trifluoroacetic acid are-
- Trifluoroacetic acid has been used as the most suitable reagent for the removal of nitrogen and oxygen protecting groups by solvolysis under aqueous or anhydrous conditions.
- TFA is used as a strong acid to remove t-butyl derived side-chain protecting groups in Fmoc peptide synthesis, and in other organic syntheses to remove the t-butoxycarbonyl protecting group.
- It is a suitable and versatile solvent for NMR spectroscopy, for substances that are stable in acid.
- TFA is used as an ion pairing agent for organic compounds in liquid chromatography, the HPLC technique, at low concentrations. Example for- peptides and small proteins.
Ques. Compound 'A' undergoes formation of cyanohydrins which on hydrolysis gives lactic acid. Therefore, compound 'A' is: (2 marks)
a) Acetaldehyde
b) Benzaldehyde
c) Acetone
d) Formaldehyde
Ans. The correct option is option (c) Acetone
The HCN addition reaction occurs via the following reaction:
Ques. Write the reagents required in the following reactions: (All India 2015) (3 marks)
Ans. The reagents required in the following reactions are:
- Pyridinium chlorochromate
- Ammonia
The reaction is
Ques. Identify B in the following reaction (3 marks)
Ans. The B in the given reaction is CH2 = CHCOOH. The reaction is:
Ques. In the following chemical reactions, the compounds ‘A’ and ‘B’ are respectively:
a) Nitrobenzene and chlorobenzene
b) Nitrobenzene and fluorobenzene
c) Phenol and benzene
d) Benzene diazonium chloride and fluorobenzene
Ans. The compounds A and B are Benzene diazonium chloride and fluorobenzene.
Explanation: The reaction is:
Ques. Give the significance of TSA in organic and inorganic chemistry? (3 marks)
Ans. TFA or Trifluoroacetic acid is a very useful reagent in both organic and inorganic chemistry due to its higher pKa value and thus, its higher strength as an acid. TFA is a well-known and suitable reagent in many reactions based on organic chemistry. This is because of its convenient properties such as:
- More volatile liquid
- More soluble in organic solvents
- More dense than water
- Higher pKa value and hence, higher acidity
- More available in its anhydrous form than several other inorganic acids
- Less oxidising as compared to sulfuric acid
This is why organic reactions are feasible with trifluoroacetic acid or TFA.
Ques. Why does the carboxylic group remain intact in electrochemical fluorination reaction? Is there any other method of synthesis? (2 marks)
Ans. The carboxylic group remains intact due to the anodic reaction of the electrolysis of the mixture of hydrogen fluoride (HF) and acetic acid occurring below the voltage at which elemental fluorine is developed. This reaction occurs mildly and as a result, the carboxylic acid remains intact.
Apart from electrochemical fluorination reactions, there are some other preparation reactions also but they are not well-known due to higher number of complications and less yield of the product.
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