Experiment: tests for the carboxyl group
Beyond NCERT In the JEE Main and NEET syllabus; the Class 12 book has no section for it.
Quick answer
Carboxylic acids usually give carbon dioxide effervescence with sodium hydrogencarbonate, unlike ordinary phenol; other sufficiently strong acids also respond. Litmus shows acidity, and ester formation with an alcohol and acid catalyst gives supporting evidence. Use the combined results: neither litmus nor an odour alone proves a carboxyl group.
How do you test for the carboxyl group?
Tests for the carboxyl group
A carboxylic acid turns moist blue litmus red, gives brisk effervescence of carbon dioxide with sodium hydrogencarbonate solution, and forms an ester, which the Lab Manual recognises by its sweet smell, when warmed with an alcohol and a little concentrated sulphuric acid (NCERT Laboratory Manual, Unit 8, pages 95–96). The sodium hydrogencarbonate test rests on these NCERT lines:
However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.
NCERT Chemistry Part II (Class 12), Section 8.9.1, page 249
The NCERT Class 12 textbook explains the chemistry behind the sodium hydrogencarbonate test (NCERT Section 8.9.1, page 249) and the ester test (Section 8.9.2, page 251), but it does not describe the method. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 8, Experiment 8.1, part V (pages 95–96).
In plain words: the Lab Manual reads red litmus as a carboxyl or a phenolic group, fizzing with sodium hydrogencarbonate as a carboxyl group (ordinary phenols are too weak to react, NCERT page 249), and a sweet smell after warming with an alcohol as a carboxyl group too. Strictly, litmus shows only acidity, other sufficiently strong acids also fizz, and ester formation is supporting evidence: smell alone does not confirm identity (reasoned).
Theory
- The name carboxyl comes from carbonyl and hydroxyl, since the group, –COOH, contains both (Lab Manual, page 95).
- Carboxylic acids turn blue litmus red and react with sodium hydrogencarbonate solution with effervescence of carbon dioxide; the Lab Manual calls this “a test that distinguishes carboxylic acids from phenols” (page 95), which holds for ordinary phenols (see the caution below). Phenols are much weaker acids: pKa 10 against 4.76 for acetic acid (NCERT page 250).
- With alcohols, in the presence of concentrated sulphuric acid, they give esters (Lab Manual, page 96; NCERT page 251).
Carboxyl group tests
RCOOH + NaHCO3 → RCOONa + H2O + CO2
RCOOH + R′OH ⇌ RCOOR′ + H2O (conc. H2SO4, heat)
Litmus test
- Procedure. With a glass rod, put a drop of the liquid compound, or of a solution of the compound, on moist blue litmus paper. If the paper turns red, the Lab Manual takes it to indicate “either a carboxylic group or a phenolic group” (page 96), so litmus alone cannot tell the two apart; the next test does.
- Strictly, red litmus shows acidity: NCERT Class 11 says acids in general “turn blue litmus paper into red” (Unit 6, page 189), so mineral acids do too. Phenol is a weak acid (pKa 10, NCERT page 250) and may not give a distinct red at the tested concentration (reasoned).
Sodium hydrogencarbonate test
- Procedure. Take 2 mL of saturated aqueous sodium hydrogencarbonate in a clean test tube and add a few drops of the liquid compound or a few crystals of the solid. Brisk effervescence of carbon dioxide “indicates the presence of carboxyl group” (Lab Manual, page 96). Strictly, it shows an acid strong enough to react with hydrogencarbonate, which is why the exceptions below exist (reasoned).
- A caution from the phenol tests: 2,4-dinitrophenol and 2,4,6-trinitrophenol are strong enough acids to dissolve even in sodium hydrogencarbonate (Lab Manual, page 91), so phenols like these, with strong electron-withdrawing groups, are exceptions to the rule (reasoned).
Ester test
- Procedure. Take about 0.1 g of the compound, add 1 mL of ethanol or methanol and 2–3 drops of concentrated sulphuric acid, and heat for 10–15 minutes in a hot water bath at about 50 °C. The Lab Manual then pours the mixture into aqueous sodium carbonate, to neutralise the excess sulphuric acid and carboxylic acid, and reads a “sweet smell of the substance formed” as showing a carboxyl group (page 96). Our safety additions: allow the mixture to cool, then transfer it slowly, as directed by the instructor, since neutralisation can froth as CO2 evolves. Treat the odour as supporting evidence, not proof (reasoned).
- For example, ethanoic acid and ethanol give ethyl ethanoate: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O (reasoned from NCERT page 251).
Results at a glance
| Test | Positive result | Shows |
|---|---|---|
| blue litmus | turns red | carboxyl or phenolic group (Lab Manual); strictly, any acid |
| NaHCO3 | brisk effervescence of CO2 | carboxyl group (Lab Manual); strictly, an acid strong enough |
| ester test | sweet smell | carboxyl group (Lab Manual); supporting evidence |
Precautions
- Add the compound slowly, so that the effervescence is clearly seen (Lab Manual, page 96).
- Heat the ester mixture only in the hot water bath: the Lab Manual marks ethyl alcohol as flammable (page 96).
- Do not directly sniff unknown chemicals or hot mixtures. If odour observation is part of an approved teaching protocol, observe the cooled sample only by the instructor’s wafting method; some esters have little odour, or a smell that is not sweet (reasoned).
- Handle concentrated sulphuric acid with care: the Lab Manual marks it as corrosive (page 91).
Solved examples
Example 1: Benzoic acid or phenol?
Two white solids are benzoic acid and phenol. Which test distinguishes them without relying on an uncertain litmus response?
Answer: Add each to saturated sodium hydrogencarbonate solution. Benzoic acid gives brisk effervescence of CO2: C6H5COOH + NaHCO3 → C6H5COONa + H2O + CO2. Phenol does not react (Lab Manual, pages 95–96; NCERT page 249). NCERT Exercise 8.13 (iii) asks for a test to tell these two apart.
Example 2: Carbon dioxide from benzoic acid
At most, what mass of carbon dioxide can 1.22 g of benzoic acid give with excess sodium hydrogencarbonate? (C = 12, H = 1, O = 16.)
Answer: M(C6H5COOH) = 84 + 6 + 32 = 122 g mol−1, so 1.22 g is 0.0100 mol, which gives 0.0100 mol of CO2, M = 12 + 32 = 44 g mol−1: 0.0100 × 44 = 0.44 g (reasoned from the Lab Manual’s equation, page 95).
Common mistakes to avoid
Common mistake
Taking a red litmus as proof of a carboxyl group
The Lab Manual reads red litmus as a carboxylic or a phenolic group (page 96), so it cannot tell the two apart: confirm with the sodium hydrogencarbonate test. Litmus also responds to any acid and depends on concentration, and even the hydrogencarbonate test is not unique to –COOH (reasoned).
Common mistake
Adding the compound to NaHCO all at once
Add it slowly, so that the effervescence can be seen clearly (Lab Manual, page 96).
Quick revision summary
- Litmus: blue to red for a carboxyl or phenolic group (Lab Manual); strictly any acid, and weak phenol may give no clear red.
- Sodium hydrogencarbonate: brisk effervescence of CO2 with carboxylic acids, not with ordinary phenols (2,4-dinitrophenol and 2,4,6-trinitrophenol dissolve in it).
- Ester test: 0.1 g + 1 mL ethanol or methanol + 2–3 drops conc. H2SO4, 10–15 min at about 50 °C, cool, then slowly add to Na2CO3 solution: a sweet smell shows carboxyl (Lab Manual), supporting evidence rather than proof.
Formula sheet
- RCOOH + NaHCO → RCOONa + HO + CO
- RCOOH + R′OH ⇌ RCOOR′ + HO (conc. HSO, heat)
Questions students ask
Why do carboxylic acids give CO2 with sodium hydrogencarbonate when most phenols do not?
Acid transfer to hydrogencarbonate is coupled to formation and escape of CO2. The textbook pKa near 6.37 refers to the combined dissolved CO2/H2CO3 system, not isolated molecular H2CO3. Using that effective value gives a net equilibrium constant near 106.37−4.76 ≈ 40 for acetic acid, but only 106.37−10 ≈ 2 × 10−4 for phenol, before accounting for gas escape. Strongly electron-withdrawing groups can make a phenol acidic enough to respond, as the NCERT manual notes for dinitrophenol and trinitrophenol.
Can sodium carbonate or sodium hydroxide be used instead of sodium hydrogencarbonate in the test for carboxylic acids?
Sodium hydroxide cannot replace sodium hydrogencarbonate in the test, because it gives no gas and forms salts with phenols as well as carboxylic acids (NCERT pages 205 and 249). Sodium carbonate does give CO2 with carboxylic acids (NCERT page 249), but in excess carbonate a few drops of acid are used up turning CO32− into HCO3−, so little gas may be seen (reasoned). Beyond the NCERT text, carbonate can take a proton from phenol, since HCO3−, pKa 10.25 (NCERT Class 11, Table 6.8, page 200), is a weaker acid than phenol, 10 (NCERT page 250), so phenol partly dissolves without effervescence (reasoned).
How can formic acid be told apart from acetic acid in the lab?
Formic acid can be told apart from acetic acid by Tollens’ or Fehling’s test: both acids turn blue litmus red and give brisk effervescence with sodium hydrogencarbonate (Lab Manual, page 96; NCERT page 249), but only formic acid, HCOOH, gives a silver mirror or red copper(I) oxide, a point beyond the NCERT text (Wikipedia, “Fehling’s solution”). Acetic acid is the kind of product these reagents give from aldehydes, so it is not oxidised further (reasoned from NCERT page 239).
Can the ester test be used to detect alcohols as well as carboxylic acids?
The ester test can detect an alcohol in principle, with the roles swapped: warming the alcohol with ethanoic acid and a few drops of concentrated sulphuric acid gives an ester with its sweet smell, since carboxylic acids are esterified by alcohols (NCERT page 251; reasoned). The NCERT Laboratory Manual does not use it that way: its tests for the alcoholic group are the ceric ammonium nitrate, iodoform and Lucas tests (Unit 8, pages 88–90). Odour alone cannot establish a functional group. Phenols generally esterify poorly in the ordinary acid/alcohol Fischer procedure and are usually acylated with an acyl chloride or anhydride instead.
Practice questions
Try each question first, then open the answer to check your working.
-
Q1. Why is the sodium hydrogencarbonate test better than the litmus test for a carboxyl group?
Show answerHide answer
Litmus turns red with both carboxylic acids and phenols (Lab Manual, page 96), but only carboxylic acids react with sodium hydrogencarbonate to give CO2, so that test distinguishes them (Lab Manual, page 95). The exceptions are strongly acidic phenols such as 2,4,6-trinitrophenol (Lab Manual, page 91); a mineral acid would fizz too (reasoned).
-
Q2. In the ester test, why is the mixture poured into sodium carbonate solution before an instructor-approved observation of the cooled product?
Show answerHide answer
It neutralises the excess sulphuric acid and the excess carboxylic acid (Lab Manual, page 96), removing residual acid and reducing interference from a volatile carboxylic acid’s odour. Sulphuric acid itself is nonvolatile; do not describe it as a sharp-smelling vapour (reasoned).