Tests for Functional Groups in Organic Chemistry (JEE/NEET Focus)

 Over 100 Q&A on Tests for Functional Groups in Organic Chemistry (JEE/NEET Focus)

Part 1: Alcohols and Phenols

Q. No.	Functional Group	Test Name/Reagent	Observation/Key Result
1	-OH (Alcohol)	Sodium Metal Test (Na)	Effervescence of H2​ gas (2ROH+2Na→2RO−Na++H2​).
2	-OH (Alcohol)	Esterification Test (RCOOH/H+)	Fruity smell of the ester.
3	1∘,2∘,3∘ Alcohols	Lucas Test (ZnCl2​/HCl)	3∘ shows immediate turbidity; 2∘ after 5 min; 1∘ no turbidity at RT.
4	CH3​CH(OH)−	Iodoform Test (I2​/NaOH)	Yellow precipitate of CHI3​.
5	Phenol (Ar−OH)	Neutral FeCl3​ Test	Violet, green, or blue color (complex formation).
6	Phenol (Ar−OH)	Bromine Water Test (Br2​(aq))	White precipitate of 2,4,6-Tribromophenol.
7	Phenol (Ar−OH)	Litmus Test	Turns blue litmus red (weakly acidic).
8	1∘ vs 2∘ Alcohol	Cu at 573 K	1∘→Aldehyde; 2∘→Ketone; 3∘→Alkene (Dehydration).

Part 2: Aldehydes and Ketones (C=O) Group

Q. No.	Functional Group	Test Name/Reagent	Observation/Key Result
9	C=O (Aldehyde/Ketone)	2,4-DNP Test	Yellow, orange, or red precipitate (Schiff's base).
10	Aldehyde (RCHO)	Tollen's Test (Ag(NH3​)2+​)	Silver Mirror on the inner wall of the test tube.
11	Aldehyde (RCHO)	Fehling's Test (Cu2+ complex)	Red-brown precipitate of Cu2​O.
12	Aldehyde (RCHO)	Schiff's Test	Restores the pink/magenta color of Schiff's reagent.
13	Ketone (RCOR′)	Tollen's/Fehling's	No reaction (except α-hydroxy ketones).
14	CH3​CO− (Methyl Ketone)	Iodoform Test (I2​/NaOH)	Yellow precipitate of CHI3​.
15	Aromatic Aldehyde (ArCHO)	Tollen's/Fehling's	Tollen's OK (Silver mirror); Fehling's Fail (Aromatic aldehyde usually resists Fehling’s).
16	Ketones that don't respond to Iodoform	Tollen's/Fehling's	Both tests are negative.

Part 3: Carboxylic Acids and Esters

Q. No.	Functional Group	Test Name/Reagent	Observation/Key Result
17	-COOH (Carboxylic Acid)	Sodium Bicarbonate Test (NaHCO3​)	Brisk effervescence of CO2​ gas.
18	-COOH (Carboxylic Acid)	Blue Litmus Test	Turns blue litmus red (stronger acid than phenol).
19	-COOH (Carboxylic Acid)	Esterification Test (R′OH/H+)	Fruity smell of the ester formed.
20	-COOH (Carboxylic Acid)	Neutral FeCl3​ Test (for Aromatic COOH)	Buff-colored precipitate of ferric carboxylate.
21	Ester (RCOOR′)	Hydroxamic Acid Test	Deep red color with FeCl3​ after hydrolysis (formation of RCONHOH).
22	Formic Acid (HCOOH)	Tollen's Test	Silver Mirror (HCOOH is easily oxidized like an aldehyde).

Part 4: Amines and Nitrogen Compounds

Q. No.	Functional Group	Test Name/Reagent	Observation/Key Result
23	NH2​ (1∘ Amine)	Carbylamine Test (CHCl3​/KOH)	Foul/offensive smell of Isocyanide (R−NC).
24	1∘,2∘,3∘ Amines	Hinsberg's Test (C6​H5​SO2​Cl)	1∘→Soluble in KOH; 2∘→Insoluble; 3∘→No reaction.
25	NH2​ (Amine)	Litmus Test	Turns red litmus blue (basic nature).
26	Aromatic 1∘ Amine (Aniline)	Azo Dye Test (Diazotization + Coupling with Phenol)	Bright orange/yellow dye (p-hydroxyazobenzene).
27	Nitro Compound (RNO2​)	Reduction followed by FeCl3​	RNO2​→RNH2​→Azo Dye or Carbylamine.
28	Amide (RCONH2​)	Alkaline Hydrolysis (NaOH/heat)	NH3​ gas evolved (detected by Nessler's reagent or red litmus).

Part 5: Unsaturated Hydrocarbons and Halides

Q. No.	Functional Group	Test Name/Reagent	Observation/Key Result
29	Alkene (C=C)	Bromine Water Test (Br2​/CCl4​)	Decolorizes the reddish-brown color (Addition).
30	Alkene (C=C)	Baeyer's Test (Cold Dil. KMnO4​)	Decolorizes the pink/purple color and forms a brown MnO2​ ppt.
31	Terminal Alkyne (R−C≡CH)	Ammoniacal AgNO3​ (Tollen’s)	White precipitate of silver acetylide (R−C≡C−Ag).
32	Terminal Alkyne (R−C≡CH)	Ammoniacal Cu2​Cl2​	Red precipitate of copper acetylide (R−C≡C−Cu).
33	Alkyl Halide (R−X)	AgNO3​ Test (after hydrolysis/boiling with KOH)	Precipitate of AgX (AgCl white, AgBr pale yellow, AgI yellow).
34	Aryl Halide (Ar−X)	AgNO3​ Test (direct)	No precipitate (C−X bond is strong/no ionization).

Part 6: Lassiagne's Test (Detection of Elements)

Q. No.	Element	Test Principle/Reagent	Observation/Key Result
35	Nitrogen (N)	Prussian Blue Test (FeSO4​+FeCl3​)	Prussian blue/deep green color or ppt (Fe4​[Fe(CN)6​]3​).
36	Sulphur (S)	Sodium Nitroprusside Test (Na2​[Fe(CN)5​NO])	Deep violet/purple color.
37	Halogens (X)	AgNO3​ Test (in Lassaigne's filtrate)	White/yellow/pale yellow precipitate (AgX).
38	Phosphorus (P)	Ammonium Molybdate Test	Canary yellow precipitate (Phosphomolybdate).
39	What is the purpose of preparing Lassaigne's Extract?	To convert elements (N, S, X) from covalent to ionic form for aqueous testing.
40	What is used to make Lassaigne's Extract?	Sodium metal (fused with the organic compound).

Part 7: Distinguishing Tests (JEE/NEET Specific)

Q. No.	Compounds to Distinguish	Recommended Test	Key Difference
41	Methanol vs Ethanol	Iodoform Test (I2​/NaOH)	Ethanol gives yellow ppt; Methanol gives none.
42	Propanal (Aldehyde) vs Propanone (Ketone)	Tollen's Test	Propanal gives Silver Mirror; Propanone gives none.
43	Phenol vs Ethanol	Neutral FeCl3​ Test	Phenol gives violet color; Ethanol gives none.
44	But-1-yne (Terminal) vs But-2-yne (Non-terminal)	Ammoniacal AgNO3​ Test	But-1-yne gives white ppt; But-2-yne gives none.
45	Carboxylic Acid vs Phenol	NaHCO3​ Test	Carboxylic Acid gives CO2​ effervescence; Phenol gives none.
46	Aniline (1∘) vs N,N-Dimethylaniline (3∘)	Hinsberg's Test	Aniline gives a KOH-soluble product; N,N-dimethylaniline is unreactive.
47	Chloroform (CHCl3​) vs Carbon Tetrachloride (CCl4​)	Carbylamine Test	Chloroform gives foul smell (when 1∘ amine is added); CCl4​ gives none.
48	Alkyl Chloride (RCl) vs Aryl Chloride (ArCl)	Aqueous AgNO3​ Test	RCl gives immediate AgCl ppt; ArCl gives none.
49	Glucose (Aldehyde) vs Fructose (Ketone)	Seliwanoff's Test	Fructose (ketose) reacts faster (Resorcinol/HCl).
50	Sucrose vs Glucose	Fehling's Test	Glucose (reducing) gives red ppt; Sucrose (non-reducing) gives none.

Part 8: Further Concepts and Applications

Q. No.	Concept/Application	Detail
51	Why does the Iodoform Test require the CH3​CO− group?	This group is easily halogenated and cleaved by the base (NaOH) to yield CHI3​.
52	Why must Tollen’s Reagent be freshly prepared?	The complex (Ag(NH3​)2​OH) is unstable and decomposes upon standing, forming explosive Ag2​O.
53	What is the active ingredient in Tollen's Reagent?	Diamminesilver(I) hydroxide (Ag(NH3​)2​OH).
54	What is the active species in Fehling's Solution?	Alkaline Cu2+ ion complexed with tartrate ions.
55	Which amines (1∘,2∘,3∘) form a sulphonamide precipitate in Hinsberg's Test?	1∘ and 2∘ amines.
56	What is the role of NaOH in the Carbylamine Test?	It acts as a base to generate the reactive intermediate, Dichlorocarbene (CCl2​).
57	Why does Formic Acid give Tollen's Test?	It contains the H−C=O group, which is easily oxidized to CO2​.
58	What is the final product of the reaction between Acetone and 2,4-DNP?	Acetone 2,4-dinitrophenylhydrazone.
59	How do you prepare Bromine Water in the lab?	By dissolving Br2​ in water.
60	What is the structural feature confirmed by a positive Baeyer's Test?	Presence of an unsaturation (C=C or C≡C).
61	What is the common name for the gas evolved in the NaHCO3​ test?	Carbon dioxide (CO2​).
62	What is the color of 2,4-DNP reagent itself?	Orange-yellow.
63	Why is Ethanol added to the AgNO3​ solution when testing R−X?	To increase the solubility of the alkyl halide.
64	What must be done before testing a salt for a halogen by AgNO3​ in the lab?	Boil with HNO3​ to decompose CN− and S2− ions, which interfere.
65	What is the name of the test used to distinguish Aldose (Glucose) from Ketose (Fructose) rapidly?	Seliwanoff's Test.
66	What is the characteristic observation of a positive Molisch’s Test (for Carbohydrates)?	Purple or violet ring at the junction of the two liquids.
67	What is the role of concentrated H2​SO4​ in the Molisch’s Test?	It dehydrates the sugar to form furfural derivatives.
68	What is the general rule for using d-Glucose in the lab?	Always use D-Glucose (naturally occurring D-form).
69	What is the final product in the Ring Test (Nitrate) responsible for the color?	Nitroso ferrous sulphate ([Fe(H2​O)5​NO]SO4​).
70	What is the observation when S2− is present in Lassaigne’s Extract?	Violet color with Sodium Nitroprusside.
71	What test confirms the presence of Alcohol (R−OH) over Phenol (Ar−OH)?	Lucas Test (reactivity difference).
72	Why is Acetone used as the solvent for the Finkelstein Reaction (Halide exchange)?	To precipitate the less soluble NaCl or NaBr, driving the equilibrium forward.
73	What is the color change in the Chromyl Chloride Test (Cl− detection)?	Red-brown vapors (CrO2​Cl2​) turning to yellow solution (Na2​CrO4​).
74	What is the term for the yellow precipitate formed in the Iodoform Test?	Triiodomethane (CHI3​).
75	Why do Ketones generally fail the Tollen’s Test?	The C−C bond is strong and requires harsh oxidation conditions.
76	What is the key functional group necessary for a compound to be a Carbohydrate (chemically)?	Polyhydroxy (many OH groups).
77	What is the observation when Ethane-1,2-diol is added to NaHCO3​?	No reaction (It's not acidic enough).
78	How do you confirm Fe3+ in qualitative analysis?	K4​[Fe(CN)6​] giving a Prussian Blue ppt.
79	How do you confirm S2− in qualitative analysis?	Lead Acetate Paper turning Black (PbS).
80	What is the confirmatory test for Nitrite (NO2−​)?	Starch-Iodide Test (Blue color formation).
81	What is the difference in the Bromination of Phenol in CS2​ vs H2​O?	CS2​→Monosubstitution; H2​O→Trisubstitution (2,4,6-Tribromophenol).
82	What is the reagent used in the Haloform reaction?	X2​ (halogen) and NaOH (or NaOX).
83	What is the precipitate in the DNPH test?	Hydrazone (Schiff’s base).
84	Which amines (1∘,2∘,3∘) react with HNO2​ to give an alcohol and N2​ gas?	Primary Aliphatic Amines (RNH2​).
85	What is the product of the reaction between a primary amine and Nitrous Acid (HNO2​)?	Alcohol (R−OH).
86	What is the test used to distinguish between a primary and secondary halide using AgNO3​?	No, AgNO3​ tests only for the presence of X− ion (reactivity difference is better seen in SN​1/SN​2 conditions).
87	Why is the Sulphur test usually negative in Lassaigne’s if Nitrogen is also present?	They combine to form NaCNS (Sodium thiocyanate), which needs a separate test.
88	What color does Fehling’s Solution A and B form when mixed?	Blue color.
89	What is the Acid-Base indicator used in the Carbylamine reaction?	None, it is a functional group test based on smell.
90	What is the term for the characteristic smell of the product of the Esterification reaction?	Fruity smell.
91	What is the difference between Methyl Ketone and Ethyl Ketone?	Methyl Ketones have the CH3​CO− group and give the Iodoform Test.
92	What is the simplest alcohol that gives the Iodoform Test?	Ethanol (CH3​CH2​OH).
93	What is the test used to confirm Phenol over Carboxylic Acid after the NaHCO3​ test?	Neutral FeCl3​ Test.
94	Which reagent is used to confirm the presence of reducing sugar in a solution?	Fehling's Solution or Benedict's Solution.
95	How is the H2​ gas evolved in the Sodium Metal Test confirmed?	It burns with a pop sound.
96	What is the formula of the Nessler’s Reagent?	Alkaline solution of K2​[HgI4​] (Potassium Tetraiodomercurate(II)).
97	What is the observation in the Nessler’s Test for Ammonia?	Brown/reddish-brown precipitate or coloration.
98	What is the role of concentrated HNO3​ in the Halogen test in Lassaigne’s?	It removes the unreacted CN− and S2− before adding AgNO3​.
99	How can you distinguish between Acetic Acid and Formic Acid?	Tollen’s Test (Formic acid gives silver mirror, Acetic acid does not).
100	Why is the Bromine Water Test not sufficient to prove unsaturation in all cases?	Phenol also decolorizes Br2​ water (Substitution vs Addition).
101	What color does Acidified KMnO4​ change to when an alkene is present?	Pink to colorless (Baeyer’s Test).
102	What is the reagent used in the Diazotization reaction?	NaNO2​/HCl at 0−5∘C.
103	What is the test that produces a precipitate that is soluble in excess NaOH?	Aluminium Hydroxide (Al3+ confirmatory test).
104	Which amines (1∘,2∘,3∘) react with Benzoyl Chloride?	1∘ and 2∘ amines.
105	What is the term for the precipitate formed when DNPH reacts with Ketones?	Ketone 2,4-dinitrophenylhydrazone.