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Functional Group Detection: Carboxylic Acids, Phenols, and Alcohols, Study notes of Chemistry

Edith Cowan University (ECU)Chemistry

Various tests to identify functional groups in organic compounds. It covers the detection of carboxylic acids using litmus test, sodium bicarbonate test, and fluorescein test, phenolic compounds through fecl3 test and phthalic anhydride test, and alcohols via ceric ammonium nitrate test and iodoform test.

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2021/2022

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Functional group detection
1. Test for carboxylic acid group
(a) Litmus test- Add blue litmus solution (1 drop) to an aqueous solution of acid,
appearance of a red colour indicates the presence of a carboxylic acid (blue litmus paper
may be used in place of a blue litmus solution)
(b) Sodium bicarbonate test (Functional group test): To a saturated solution of
sodium bicarbonate add small amount of given organic compound. Brisk effervescence
indicates the presence of carboxylic acid group.
(c) Fluorescein test: Heat a small amount of organic compound, resorcinol and
conc. sulphuric acid (1-2 drops) in a clean and dry test tube till a dark brown coloured
liquid is formed. Then add few drops of this solution into a beaker containing dilute
NaOH solution (10 mL NaOH diluted to 100 mL). Appearance of red colour solution with
green fluorescence indicates the presence of dicarboxylic acid.
(d) Test for Oxalic acid (Blue ring test): Heat a small amount of given organic
compound, resorcinol (2-3 flakes) and water (1 mL) in a test tube. Cool the contents and
add few mL of conc. H2SO4 along the sides of the test tube. Appearance of blue ring at
the junction of two layers confirms the presence of oxalic acid.
2. Test for phenolic functional group:
(a) Neutral FeCl3 test (Functional group test): Dissolve the given organic compound
in water or alcohol and to this a drop or two of neutral FeCl3 solution. Appearance of
red/green/pink/blue-violet colours confirms the presence of phenolic functional group.
*****Preparation of neutral FeCl3 solution: Take the solution of FeCl3 in a test tube
and add NaOH solution till a small amount of precipitate is observed. Then add a drop or
two of FeCl3 solution to dissolve the precipitate.
(b) Phthalein test- The phenols having a free para position respond to this test. In a
dry test tube, gently heat a small amount of given organic compound with an equal
amount of phthalic anhydride (or phthalic acid) and conc. sulphuric acid (2-3 drops), for
1-2 minutes. Cool and pour the mixture into a beaker containing dilute sodium hydroxide
pf3
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Functional group detection

1. Test for carboxylic acid group

(a) Litmus test - Add blue litmus solution (1 drop) to an aqueous solution of acid, appearance of a red colour indicates the presence of a carboxylic acid (blue litmus paper may be used in place of a blue litmus solution)

(b) Sodium bicarbonate test ( Functional group test ): To a saturated solution of sodium bicarbonate add small amount of given organic compound. Brisk effervescence indicates the presence of carboxylic acid group.

(c) Fluorescein test: Heat a small amount of organic compound, resorcinol and conc. sulphuric acid (1-2 drops) in a clean and dry test tube till a dark brown coloured liquid is formed. Then add few drops of this solution into a beaker containing dilute NaOH solution (10 mL NaOH diluted to 100 mL). Appearance of red colour solution with green fluorescence indicates the presence of dicarboxylic acid.

(d) Test for Oxalic acid (Blue ring test): Heat a small amount of given organic compound, resorcinol (2-3 flakes) and water (1 mL) in a test tube. Cool the contents and add few mL of conc. H 2 SO 4 along the sides of the test tube. Appearance of blue ring at the junction of two layers confirms the presence of oxalic acid.

2. Test for phenolic functional group:

(a) Neutral FeCl 3 test ( Functional group test ): Dissolve the given organic compound in water or alcohol and to this a drop or two of neutral FeCl 3 solution. Appearance of red/green/pink/blue-violet colours confirms the presence of phenolic functional group.

*******Preparation of neutral FeCl 3 solution:** Take the solution of FeCl 3 in a test tube and add NaOH solution till a small amount of precipitate is observed. Then add a drop or two of FeCl 3 solution to dissolve the precipitate.

(b) Phthalein test - The phenols having a free para position respond to this test. In a dry test tube, gently heat a small amount of given organic compound with an equal amount of phthalic anhydride (or phthalic acid) and conc. sulphuric acid (2-3 drops), for 1-2 minutes. Cool and pour the mixture into a beaker containing dilute sodium hydroxide

solution. Appearance of pink, blue, green, red colouration indicates the presence of a phenol with free para position.

Compound Colour Phenol, o-Cresol Red m-Cresol Bluish purple Catechol Blue Resorcinol Red solution with green fluorescence 1-Naphthol Green 2-Naphthol Very faint green with slight fluorescence

3. Test for alcoholic functional group: (a) Ceric ammonium nitrate test ( Functional group test ): Dissolve a small amount of given organic compound in minimum amount of water or dioxane ( for water insoluble compounds ) and add freshly prepared ceric ammonium nitrate solution (few drops. Appearance of red colour shows the presence of alcoholic group. (b) Iodoform test: Iodoform test is given by alcohols which contain CH 3 CHOHR group and oxidize to CH 3 COR group during the reaction to give a positive iodoform test (same as described for carbonyl compounds). When a compound containing CH 3 CO- group or the group CH 3 CH(OH)- (which can be easily oxidized to CH 3 CO group), is treated with a solution of iodine in basic medium, iodoform, a yellow solid separates out. Procedure -Dissolve a small amount of given organic compound in minimum amount of water and add sodium hydroxide (~1 mL, 10%). Add to this a saturated solution of iodine-potassium iodide in water with stirring until a dark colour of iodine persists. Heat the solution in water bath and maintain the temperature at 80ºC for few minutes. Remove the colour of excess iodine by adding a few drops of sodium hydroxide solution. A yellow precipitate of iodoform indicates the presence of CH 3 CO group in the compound.

Chemistry of tests:

1. Test for carboxylic acid group

Sodium bicarbonate test:

a)

b)