Chemca
Top 30 Distinction Tests
Master the art of chemical identification. These 30 critical pairs require specific reagents producing distinct visual observations (precipitates, colors, effervescence). Click to reveal the chemistry and a visual representation of the test.
Part 1: Alcohols & Phenols
Q1. Distinguish: Propan-1-ol, Propan-2-ol, and 2-Methylpropan-2-ol
Lucas Reagent: Conc. HCl + Anhyd. ZnCl2 $S_N1$ Mechanism
Observation: The test relies on the difference in reactivity of $1^\circ, 2^\circ$, and $3^\circ$ alcohols towards $S_N1$ nucleophilic substitution. The formation of insoluble alkyl chlorides produces turbidity (cloudiness).
- 2-Methylpropan-2-ol ($3^\circ$): Forms a highly stable $3^\circ$ carbocation. Reacts instantly. Turbidity appears immediately.
- Propan-2-ol ($2^\circ$): Forms a less stable $2^\circ$ carbocation. Turbidity appears in about 5 minutes.
- Propan-1-ol ($1^\circ$): Forms an unstable $1^\circ$ carbocation. No turbidity at room temperature.
Q2. Distinguish: Ethanol and Methanol
Reagent: $\ce{I2}$ + Aqueous $\ce{NaOH}$ Haloform Reaction
Observation: This test identifies the presence of a $\ce{CH3-CH(OH)-}$ group (which oxidizes to a methyl ketone).
- Ethanol ($\ce{CH3CH2OH}$): Contains the requisite group. Upon heating with $\ce{I2/NaOH}$, it forms a pale yellow precipitate of Iodoform ($\ce{CHI3}$) with a characteristic antiseptic smell.
- Methanol ($\ce{CH3OH}$): Lacks the $\ce{CH3-CH(OH)-}$ group. No yellow precipitate is formed.
Ethanol (Yellow ppt)
Methanol (Clear)
Q3. Distinguish: Phenol and Ethanol
Reagent: Neutral aqueous $\ce{FeCl3}$ Enol Complex
Observation: This test detects the presence of a phenolic $-\ce{OH}$ group or an enolic group.
- Phenol ($\ce{C6H5OH}$): Reacts with neutral $\ce{FeCl3}$ to form an intensely colored water-soluble coordination complex. A deep violet (or blue/green) coloration is observed.
- Ethanol ($\ce{CH3CH2OH}$): An aliphatic alcohol; it does not form this complex. No violet coloration (remains yellowish/brown due to $\ce{FeCl3}$).
Phenol (Violet)
Ethanol (No Color)
Q4. Distinguish: Phenol and Benzoic Acid
Reagent: Aqueous $\ce{NaHCO3}$ Acidity Difference
Observation: This test distinguishes a stronger acid (carboxylic) from a weaker acid (phenol).
- Benzoic Acid ($\ce{C6H5COOH}$): Strong enough to decompose sodium bicarbonate. Brisk effervescence of colorless, odorless $\ce{CO2}$ gas is observed.
- Phenol ($\ce{C6H5OH}$): Weaker than carbonic acid ($\ce{H2CO3}$). No effervescence occurs.
Benzoic acid (Bubbles)
Phenol (No action)
Q5. Distinguish: Phenol and Cyclohexanol
Reagent: $\ce{Br2(aq)}$ Electrophilic Aromatic Substitution
Observation: Detects highly activated aromatic rings.
- Phenol: The $-\ce{OH}$ group strongly activates the ring. Reacts instantly with bromine water to decolorize it and form a white precipitate of 2,4,6-tribromophenol.
- Cyclohexanol: An aliphatic cyclic alcohol. It does not react. The red-brown color of bromine water is not discharged.
(Note: The neutral FeCl3 test can also be used, giving violet with phenol and no color with cyclohexanol).
Part 2: Aldehydes & Ketones
Q6. Distinguish: Propanal (Aldehyde) and Propanone (Ketone)
Reagent: Ammoniacal Silver Nitrate ($\ce{[Ag(NH3)2]+}$) Mild Oxidation
Observation: Tollens' reagent is a mild oxidizing agent that oxidizes aldehydes to carboxylates, while the silver ion is reduced to metallic silver.
- Propanal (Aldehyde): Reacts on warming. A brilliant silver mirror forms on the inner walls of the test tube.
- Propanone (Ketone): Resists mild oxidation. No silver mirror is formed.
Q7. Distinguish: Ethanal and Propanone
Reagent: Alkaline $\ce{Cu^2+}$ complexed with tartrate Aliphatic Aldehydes
Observation: Fehling's solution (deep blue) is a mild oxidizing agent specifically for aliphatic aldehydes.
- Ethanal (Aliphatic Aldehyde): Reduces the blue $\ce{Cu^2+}$ complex to insoluble Cuprous oxide ($\ce{Cu2O}$). A reddish-brown precipitate is formed.
- Propanone (Ketone): Does not react. The solution remains blue.
Ethanal (Red ppt)
Ketone (Remains Blue)
Q8. Distinguish: Acetaldehyde and Benzaldehyde
Fehling's: Aliphatic vs Aromatic Iodoform: Methyl ketone/aldehyde
Option 1 (Fehling's): Fehling's reagent is too weak to oxidize aromatic aldehydes.
- Acetaldehyde (Aliphatic): Gives a red-brown precipitate of $\ce{Cu2O}$.
- Benzaldehyde (Aromatic): Does not react. Remains deep blue.
Option 2 (Iodoform):
- Acetaldehyde ($\ce{CH3CHO}$): Contains the required $\ce{CH3-CO-}$ group. Gives a yellow precipitate of $\ce{CHI3}$.
- Benzaldehyde ($\ce{C6H5CHO}$): Lacks a methyl group attached to the carbonyl. No yellow precipitate.
Q9. Distinguish: Pentan-2-one and Pentan-3-one
Reagent: $\ce{I2 / NaOH}$ Methyl Ketone Detector
Observation: Since both are ketones, Tollens' and Fehling's will fail for both. We rely on the specific structural motif $\ce{CH3-CO-}$.
- Pentan-2-one ($\ce{CH3-CO-CH2CH2CH3}$): It is a methyl ketone. Heating with $\ce{I2/NaOH}$ produces a yellow precipitate of iodoform ($\ce{CHI3}$).
- Pentan-3-one ($\ce{CH3CH2-CO-CH2CH3}$): An ethyl ketone. It lacks the terminal methyl group attached to the carbonyl. No yellow precipitate forms.
Q10. Distinguish: Acetophenone and Benzophenone
Reagent: $\ce{I2 / NaOH}$ Methyl Ketone Detector
Observation: Both are aromatic ketones. We look for the methyl group.
- Acetophenone ($\ce{C6H5-CO-CH3}$): Possesses the required methyl ketone group. Yields a yellow precipitate of Iodoform ($\ce{CHI3}$).
- Benzophenone ($\ce{C6H5-CO-C6H5}$): Flanked by two phenyl rings, no methyl group. Yields no precipitate.
Part 3: Carboxylic Acids & Derivatives
Q11. Distinguish: Formic acid (Methanoic acid) and Acetic acid (Ethanoic acid)
Unique structural feature of Formic Acid
Observation: Both are acids, so $\ce{NaHCO3}$ will cause effervescence for both. We must use a redox test. Formic acid ($\ce{H-COOH}$) is unique; if you look at it from the left side, it contains an aldehyde-like moiety ($\ce{H-C=O}$).
- Formic acid: Easily oxidized to $\ce{CO2}$ and $\ce{H2O}$. Forms a silver mirror with Tollens' reagent (and a red ppt with Fehling's).
- Acetic acid ($\ce{CH3-COOH}$): Lacks the aldehydic hydrogen. Does not react with Tollens' or Fehling's.
Q12. Distinguish: Benzoic acid and Ethyl benzoate
Acid vs Ester
Observation:
- Benzoic acid ($\ce{C6H5COOH}$): A moderately strong organic acid. Reacts with $\ce{NaHCO3}$ to produce brisk effervescence of colorless $\ce{CO2}$ gas.
- Ethyl benzoate ($\ce{C6H5COOCH2CH3}$): An ester. Esters are neutral compounds and do not react with weak bases like sodium bicarbonate. No effervescence.
Part 4: Amines
Q13. Distinguish: Aniline and N-Methylaniline
Reagent: $\ce{CHCl3}$ + Ethanolic $\ce{KOH}$ Specific to $1^\circ$ Amines
Observation: This test is exclusively positive for primary ($1^\circ$) aliphatic and aromatic amines.
- Aniline ($1^\circ$, $\ce{C6H5NH2}$): Upon heating, forms phenyl isocyanide. An extremely foul, intolerable, offensive odor is produced.
- N-Methylaniline ($2^\circ$, $\ce{C6H5NHCH3}$): Secondary amines do not form isocyanides because they lack the required two protons on the nitrogen. No foul odor is observed.
Q14. Distinguish: Diethylamine and Triethylamine
Reagent: Benzene sulfonyl chloride ($\ce{C6H5SO2Cl}$)
Observation: Differentiates based on the presence of replaceable hydrogen atoms on the nitrogen.
- Diethylamine ($2^\circ$): Reacts with Hinsberg reagent to form N,N-diethylbenzene sulfonamide. This product has no acidic hydrogen on the nitrogen, so it does not dissolve in aqueous $\ce{KOH/NaOH}$ (remains an insoluble solid/oil).
- Triethylamine ($3^\circ$): Lacks a hydrogen atom on nitrogen. Does not react with Hinsberg's reagent under normal conditions.
Q15. Distinguish: Ethylamine and Aniline
Reagents: $\ce{NaNO2/HCl}$ (0-5°C), then $\beta$-Naphthol in $\ce{NaOH}$ Aliphatic vs Aromatic $1^\circ$
Observation: Both are primary amines (Carbylamine fails to distinguish them). We use the stability of diazonium salts.
- Aniline (Aromatic): Forms a stable benzene diazonium chloride at 0-5°C. When reacted with alkaline $\beta$-naphthol, a coupling reaction occurs, forming a brilliant orange-red dye.
- Ethylamine (Aliphatic): Forms a highly unstable aliphatic diazonium salt that instantly decomposes to yield ethanol and brisk effervescence of nitrogen gas ($\ce{N2}$). No dye is formed.
Part 5: Halides & Hydrocarbons
Q16. Distinguish: Chlorobenzene and Benzyl chloride
Reagent: Aqueous $\ce{KOH}$, boil, then dilute $\ce{HNO3}$ + $\ce{AgNO3(aq)}$ $S_N1$ Reactivity
Observation: Tests the ease of $\ce{C-Cl}$ bond cleavage.
- Benzyl chloride ($\ce{C6H5CH2Cl}$): The benzylic $\ce{C-Cl}$ bond breaks easily upon boiling with aq $\ce{KOH}$ to form the highly stable benzyl carbocation, releasing $\ce{Cl-}$ ions. Adding $\ce{AgNO3}$ yields a curdy white precipitate of $\ce{AgCl}$.
- Chlorobenzene ($\ce{C6H5Cl}$): The $\ce{C-Cl}$ bond has partial double bond character due to resonance and is attached to an $sp^2$ carbon. It is extremely strong and does not hydrolyze under these mild conditions. No white precipitate forms.
Q17. Distinguish: Alkane (e.g., Ethane) and Alkene (e.g., Ethene)
Tests for Unsaturation
Option 1 (Bromine in CCl4):
- Ethene: Undergoes electrophilic addition. The reddish-brown color of bromine is rapidly discharged (decolorized).
- Ethane: Saturated. No reaction; the reddish-brown color persists.
Option 2 (Baeyer's Reagent - cold, dilute, alkaline $\ce{KMnO4}$):
- Ethene: Oxidized to a diol. The bright purple color of $\ce{KMnO4}$ is decolorized, forming a brown precipitate of $\ce{MnO2}$.
- Ethane: The purple color persists.
Q18. Distinguish: But-1-yne and But-2-yne
Terminal vs Internal Alkyne Acidic $sp$ Hydrogen
Observation: Terminal alkynes have an acidic hydrogen attached to an $sp$-hybridized carbon.
- But-1-yne (Terminal): The acidic proton ($\ce{\equiv C-H}$) reacts with Ammoniacal Cuprous Chloride ($\ce{Cu2Cl2 + NH4OH}$) to form a red precipitate of copper acetylide. (If Tollens' is used, a white precipitate of silver acetylide forms).
- But-2-yne (Internal): Lacks a terminal acidic proton ($\ce{CH3-C\equiv C-CH3}$). No precipitate forms.
Part 6: Rapid-Fire Mixed Identifications
Q19. Distinguish: Chloroform ($\ce{CHCl3}$) and Carbon Tetrachloride ($\ce{CCl4}$)
Observation: Add a primary amine (like aniline) and ethanolic $\ce{KOH}$ to both.
Chloroform: Forms the highly reactive dichlorocarbene, proceeding to form phenyl isocyanide. Foul smell observed.
Carbon tetrachloride: Cannot form the carbene. No foul smell.
Q20. Distinguish: Phenol and Aniline
Azo-Dye Test: Aniline undergoes diazotization ($\ce{NaNO2/HCl}$, 273K) and coupling with $\beta$-naphthol to form a brilliant orange-red dye. Phenol does not undergo diazotization.
Ferric Chloride Test: Phenol gives a deep violet color with neutral $\ce{FeCl3}$. Aniline does not.
Q21. Distinguish: 1-Butanol and 2-Butanol
Iodoform Test: 2-Butanol contains the $\ce{CH3-CH(OH)-}$ group and gives a yellow precipitate with $\ce{I2/NaOH}$. 1-Butanol does not.
Lucas Test: 2-Butanol ($2^\circ$) gives turbidity in 5 mins. 1-Butanol ($1^\circ$) gives no turbidity at room temp.
Q22. Distinguish: Acetone and Acetaldehyde
Both give positive Iodoform tests (both have $\ce{CH3-CO-}$). We must use a mild oxidation test.
Acetaldehyde: Gives a silver mirror (Tollens') or red-brown ppt (Fehling's).
Acetone (Ketone): Negative for both.
Q23. Distinguish: Formaldehyde ($\ce{HCHO}$) and Acetaldehyde ($\ce{CH3CHO}$)
Both are aliphatic aldehydes, so both give positive Tollens' and Fehling's tests.
Acetaldehyde: Contains the $\ce{CH3-CO-}$ group. Gives a yellow ppt of $\ce{CHI3}$.
Formaldehyde: Lacks the methyl group. Negative Iodoform test.
Q24. Distinguish: Methanoic acid (Formic) and Ethanoic acid (Acetic)
As covered in Q11, Methanoic acid ($\ce{HCOOH}$) has an aldehydic hydrogen and gives a silver mirror. Ethanoic acid ($\ce{CH3COOH}$) does not.
Q25. Distinguish: Glucose and Fructose
Mild Oxidation of Aldoses
Both are reducing sugars (positive for Tollens/Fehling's). However, Bromine water is a very mild oxidizing agent.
Glucose (Aldohexose): The aldehyde group is oxidized to gluconic acid. The red-brown color of bromine water is decolorized.
Fructose (Ketohexose): Ketones cannot be oxidized by bromine water. No decolorization.
Q26. Distinguish: Starch and Glucose
Iodine Test: Starch gives a deep blue-black color with iodine solution due to amylose-iodine complex formation. Glucose gives no color change.
Fehling's Test: Glucose is a reducing sugar (red-brown ppt). Starch is a non-reducing polysaccharide (no reaction).
Q27. Distinguish: Benzylamine and Aniline
Both are primary amines (Carbylamine positive). Benzylamine ($\ce{C6H5CH2NH2}$) is essentially an aliphatic amine because the $\ce{NH2}$ is not directly on the ring.
Aniline: Forms stable diazonium salt $\rightarrow$ couples with $\beta$-naphthol $\rightarrow$ Orange-Red Dye.
Benzylamine: Forms unstable diazonium salt $\rightarrow$ decomposes to benzyl alcohol with brisk effervescence of $\ce{N2}$ gas. No dye.
Q28. Distinguish: Nitroethane and Ethyl Nitrite
Ethyl Nitrite ($\ce{CH3CH2-O-N=O}$): It is an ester of nitrous acid. It undergoes hydrolysis with boiling aqueous $\ce{NaOH}$ to yield ethanol and sodium nitrite.
Nitroethane ($\ce{CH3CH2-NO2}$): It is a nitroalkane. Due to the highly acidic $\alpha$-hydrogens, it simply dissolves in aqueous $\ce{NaOH}$ to form a soluble salt without undergoing hydrolysis.
Q29. Distinguish: Methylamine and Dimethylamine
Carbylamine: Methylamine ($1^\circ$) gives a foul smell. Dimethylamine ($2^\circ$) does not.
Hinsberg's: Methylamine forms a product soluble in $\ce{KOH}$. Dimethylamine forms a product insoluble in $\ce{KOH}$.
Q30. Distinguish: Formic acid and Oxalic acid
Observation: Behavior upon heating.
Oxalic acid ($\ce{HOOC-COOH}$): When heated to 150°C, it decomposes to yield Carbon dioxide ($\ce{CO2}$), Carbon monoxide ($\ce{CO}$), and water.
Formic acid ($\ce{HCOOH}$): Stable to moderate heat. However, it gives a positive Tollens' test (Silver mirror), whereas Oxalic acid does not act as a reducing agent towards Tollens'.
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