Experiment: tests for alcoholic and phenolic –OH groups

How do you test for alcoholic and phenolic –OH groups?

The NCERT Class 12 textbook explains the chemistry behind the Lucas test (NCERT Section 7.4.4, page 208), the solubility of phenols in NaOH (pages 206–207) and, in Unit 8, the iodoform reaction (page 240), but it does not mention the ceric ammonium nitrate, ferric chloride or phthalein dye tests and does not describe the method. The JEE Main 2027 and NEET (UG) 2026 syllabuses list “hydroxyl (alcoholic and phenolic)” among the functional groups to detect in Unit 20. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 8, Experiment 8.1, parts II and III (pages 88–92).

In plain words: a red colour with ceric ammonium nitrate says “alcohol”; how fast a cloudy layer forms with Lucas reagent says which class; and a violet, blue, green or red colour with neutral ferric chloride says “phenol”.

Tests for the alcoholic –OH group

Ceric ammonium nitrate test

• Theory. Alcohols give a red colour with ceric ammonium nitrate, because a complex forms (Lab Manual, page 88):

• Procedure. Take 1 mL of the compound dissolved in a suitable solvent and add a few drops of ceric ammonium nitrate solution. A red colour shows an alcoholic –OH group (Lab Manual, page 89).
• Note. The red colour fades on standing, and also disappears with excess reagent, so do not add too much (Lab Manual, page 89).

Iodoform test

• Theory. Ethanol and secondary alcohols with a CH₃–CH(OH)R group give iodoform. Probably, sodium hypochlorite first turns potassium iodide into hypoiodite, which oxidises CH₃–CH(OH)R to CH₃COR and then replaces its α-hydrogens by iodine; the C–C bond then breaks, giving iodoform, CHI₃ (Lab Manual, page 88). The NCERT textbook uses the same reaction to detect a CH₃CO or CH₃CH(OH) group (NCERT Unit 8, page 240). With iodine and sodium hydroxide, as in the second method below, the overall balanced equation for ethanol is (reasoned):

• Procedure, first method. To 0.2 mL of the compound add 10 mL of 10% aqueous KI and 10 mL of freshly prepared NaOCl solution, and warm gently: yellow crystals of iodoform separate (Lab Manual, page 89).
• Second method. Dissolve 0.1 g (4–5 drops) of the compound in 2 mL of water, adding dioxane drop by drop if it does not dissolve. Add 2 mL of 5% NaOH, then potassium iodide–iodine reagent drop by drop with shaking until a dark iodine colour persists. Leave for 2–3 minutes; if no iodoform separates, warm at 60 °C in a water bath, adding more reagent until the iodine colour lasts through two minutes of heating. Remove the excess iodine with a few drops of NaOH, dilute with an equal volume of water and leave for 10–15 minutes. A yellow precipitate of iodoform means a positive test (Lab Manual, page 89).
• Methanol and tertiary alcohols have no CH₃–CH(OH)– group, so they give no iodoform (reasoned from NCERT Unit 8, page 240).
• NCERT slip (Laboratory Manual, page 88): it says that primary, secondary and tertiary alcohols can be told apart by the iodoform test and the Lucas test. Only the Lucas test sorts them by class; the iodoform test detects a CH₃–CH(OH)– group, so ethanol (1°) and propan-2-ol (2°) give it while propan-1-ol (1°) and 2-methylpropan-2-ol (3°) do not. In an exam, use the Lucas test for the class.

Lucas test

• Theory. Lucas reagent is zinc chloride in concentrated hydrochloric acid. Alcohols dissolve in it, but the alkyl chlorides formed do not, so a second layer shows that the reaction has happened (Lab Manual, page 88). Tertiary alcohols form the halides easily (NCERT page 208), because they give the most stable carbocations (reasoned). The textbook side of the test is in the Lucas test section of the alcohols and phenols page.

• Procedure. Take 1 mL of the compound, add 10 mL of Lucas reagent, shake well and time how long two distinct layers take to separate (Lab Manual, page 90).

• The Lab Manual adds that primary alcohols react very slowly, taking 10 minutes to several days (page 88), so no layer appears in the few minutes of the test; NCERT says primary alcohols give no turbidity at room temperature (page 208). The test works only for alcohols that dissolve in the reagent, since it relies on the halide separating out (page 90).

Tests for the phenolic –OH group

• Solubility. An –OH group attached directly to a ring carbon is a phenolic –OH group. Phenols are weakly acidic: they dissolve in NaOH solution but are not acidic enough to dissolve in sodium hydrogencarbonate solution. Phenols with electron-withdrawing groups, such as 2,4,6-trinitrophenol and 2,4-dinitrophenol, are strong enough acids to dissolve even in sodium hydrogencarbonate (Lab Manual, pages 90–91; see acid strength).

Ferric chloride (iron(III) chloride) test

• Theory. Phenols give a coloured complex with neutral ferric chloride, in which the phenoxide ions act as ligands (reasoned): phenol gives a violet complex. Resorcinol and the cresols give violet or blue colours; catechol gives a green colour that quickly darkens; 1- and 2-naphthol give no characteristic colour (Lab Manual, page 90).

• Procedure. To 2 mL of an aqueous or alcoholic solution of the compound, add neutral ferric chloride solution drop by drop. A blue, green, violet or red colour shows a phenolic –OH group (Lab Manual, page 91).
• Neutral ferric chloride is made by adding dilute NaOH to ferric chloride solution drop by drop until a small but permanent brown precipitate appears, then filtering (Lab Manual, page 91).

Phthalein dye test

• Theory. Phenols condense with phthalic anhydride in the presence of concentrated H₂SO₄; phenol gives phenolphthalein, which turns dark pink with NaOH solution (Lab Manual, page 90).
• Procedure. Heat 0.1 g of the compound and 0.1 g of phthalic anhydride with 1–2 drops of concentrated H₂SO₄ in an oil bath for about 1 minute; cool, and pour into 15 mL of dilute NaOH. A pink, blue, green or red colour shows a phenolic –OH group; a large excess of NaOH makes the colour disappear (Lab Manual, page 91).
• Other phenols give other colours (NCERT Laboratory Manual, Table 8.1, page 91): o-cresol red, m-cresol bluish-purple and p-cresol none; catechol usually blue, which takes longer to appear; resorcinol the green fluorescence of fluorescein.

Results at a glance

Precautions

• Use freshly prepared, neutral and very dilute ferric chloride solution (Lab Manual, page 92).
• Avoid excess ceric ammonium nitrate, which bleaches the red colour (Lab Manual, page 89).
• Handle phenol with care: it is toxic and corrosive (Lab Manual, page 92).

Solved examples

Common mistakes to avoid

Quick revision summary

• Alcoholic –OH: red colour with ceric ammonium nitrate (avoid excess).
• Lucas reagent (ZnCl₂ + conc. HCl): 3° layers at once, 2° in 1–5 minutes, 1° none; the alkyl chloride is the insoluble layer.
• Iodoform (KI + NaOCl, or I₂/KI + NaOH): yellow CHI₃ from ethanol and CH₃CH(OH)R alcohols.
• Phenolic –OH: soluble in NaOH, not in NaHCO₃ (2,4-dinitrophenol and 2,4,6-trinitrophenol excepted); violet, blue, green or red colour with neutral FeCl₃; coloured dye in the phthalein test (phenol gives phenolphthalein, pink in NaOH).

Questions students ask

Do phenols give the ceric ammonium nitrate test?

Phenols do react with ceric ammonium nitrate, a point beyond the NCERT text: IGNOU’s practical course lists it as a test for phenols as well (IGNOU eGyanKosh), and they turn the reagent a deeper red or brown rather than the clear red that alcohols give (Labster theory notes). The NCERT Laboratory Manual uses the test only for the alcoholic –OH group (page 88), so a colour change alone does not prove an alcohol when a phenol may be present; check with neutral ferric chloride as well (reasoned).

Why does the red colour in the ceric ammonium nitrate test fade?

The red colour fades because cerium(IV) is a strong oxidising agent: the red complex an alcohol forms (Lab Manual, page 88) goes on to oxidise the alcohol as the cerium returns to its +3 state (reasoned from NCERT Unit 4, page 109: E^⊖(Ce⁴⁺/Ce³⁺) = +1.74 V; lanthanoids). Beyond the NCERT text, the change from Ce(IV) to Ce(III) shows as the orange reagent fading to pale yellow (Wikipedia, “Ceric ammonium nitrate”), and hot reagent oxidises many organic compounds, so the test is done at room temperature (IGNOU eGyanKosh). So read the colour at once (reasoned from Lab Manual, page 89).

Why does ethanol give the iodoform test when it is a primary alcohol?

Ethanol gives the iodoform test because it has the CH₃–CH(OH)– group, as CH₃–CH(OH)R with R = H: hypoiodite oxidises it to ethanal, CH₃CHO, then replaces that CH₃’s three hydrogens by iodine, giving CI₃CHO (Lab Manual, page 88). Ethanal is the one aldehyde with a CH₃CO– group, and alkali breaks the C–C bond of CI₃CHO into iodoform and methanoate, hence HCOONa in the ethanol equation (reasoned). The test looks for this unit, not for the alcohol’s class; the reaction is the haloform reaction (NCERT Unit 8, page 240).

Does every phenol give a colour with neutral ferric chloride?

Not every phenol gives a colour with neutral ferric chloride: the NCERT Laboratory Manual notes that 1- and 2-naphthol give no characteristic colour (page 90), and, beyond the NCERT text, IGNOU’s practical course adds that picric acid (2,4,6-trinitrophenol) gives none, that some phenols colour only in other solvents, and that a negative result means little without other evidence (IGNOU eGyanKosh). A positive result is not proof either, since enols, which also carry an –OH group on a C=C carbon, give colours with FeCl₃ (IGNOU eGyanKosh). So the Lab Manual’s other phenol tests, solubility and the phthalein dye test, are run as well (pages 90–91; reasoned).

Why does the pink colour of the phthalein dye test vanish in excess NaOH?

The pink colour vanishes because, beyond the NCERT text, phenolphthalein is pink only as its doubly ionised form: in dilute NaOH its two phenolic –OH groups each lose a proton, but above about pH 13 hydroxide adds to the ion and breaks its conjugated system, slowly giving a colourless form (Wikipedia, “Phenolphthalein”). So the NCERT Laboratory Manual uses dilute NaOH and warns against a large excess (page 91). The dye forms when two phenol molecules join phthalic anhydride through the ring carbon para to their –OH, so p-cresol, its para carbon taken by a methyl group, forms no such dye (reasoned) and gives no colour (NCERT Laboratory Manual, Table 8.1, page 91).

Are the tests for alcohols and phenols in NCERT and in the JEE Main 2027 and NEET (UG) 2026 syllabuses?

The tests for alcohols and phenols are described in the NCERT Class 12 Laboratory Manual, Unit 8, Experiment 8.1, parts II and III (pages 88–92); of these tests, NCERT Class 12 Chemistry Part II names only the Lucas test (Unit 7, page 208) and the iodoform reaction (Unit 8, page 240), never ceric ammonium nitrate, ferric chloride or the phthalein dye test. Unit 20, Principles Related to Practical Chemistry, of both JEE Main 2027 and NEET (UG) 2026 lists the detection of “hydroxyl (alcoholic and phenolic)” groups, and Unit 17 of both lists the “Identification of primary, secondary and tertiary alcohols” (JEE Main 2027 wording), the Lucas test’s job; neither names any of them (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why does a second layer form when a tertiary alcohol is shaken with Lucas reagent?

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   The alcohol dissolves in the reagent, but it is quickly converted to the tertiary alkyl chloride, which is insoluble and separates as a layer (Lab Manual, page 88; NCERT page 208).

2. Q2. A compound dissolves in NaOH solution but not in sodium hydrogencarbonate solution, and gives a violet colour with neutral ferric chloride. What group does it contain?

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   A phenolic –OH group: phenols are weakly acidic, soluble in NaOH but not in NaHCO₃, and give coloured complexes with neutral FeCl₃ (Lab Manual, pages 90–91).