Distinction Tests of Functional Groups
Practical Organic Chemistry (POC) distilled. Master the visual color changes, the Hinsberg solubility logic, and the exact structures that answer the Iodoform test.
Module Focus: Identifying the Unknown
In the laboratory, you cannot "see" a molecule's structure. You must rely on specific chemical reactions that yield obvious physical changes—like the evolution of a gas, a distinct foul smell, or the formation of a brilliantly colored precipitate. In NEET, you will be frequently asked to choose the correct test to distinguish between two closely related isomers (e.g., Pentan-2-one vs. Pentan-3-one).
1. Tests for Alcohols and Phenols
Used to distinguish $1^\circ, 2^\circ, \text{and } 3^\circ$ alcohols. Based on $S_N1$ carbocation stability.
Reagent: Conc. $HCl$ + Anhydrous $ZnCl_2$
- $3^\circ$ Alcohol: White turbidity appears immediately.
- $2^\circ$ Alcohol: Turbidity appears after ~5 minutes.
- $1^\circ$ Alcohol: No turbidity at room temp. (Needs heat).
The definitive test for the phenolic $-OH$ group (distinguishes Phenol from Alcohols/Acids).
The color is due to the formation of an iron-phenol complex: $[Fe(OC_6H_5)_6]^{3-}$.
2. Tests for Aldehydes and Ketones
Aldehydes are easily oxidized (they have a free H on the carbonyl carbon), while ketones are resistant to mild oxidation. This is the basis of their distinction.
A general test for ALL carbonyl groups (both Aldehydes and Ketones).
Result: Yellow, Orange, or Red crystalline precipitate.
Reagent: Ammoniacal Silver Nitrate $[Ag(NH_3)_2]^+$
Oxidizes ALL aldehydes (Aliphatic & Aromatic) to carboxylates.
Result: Bright Silver Mirror on the test tube wall.
Reagent: Alkaline $Cu^{2+}$ complexed with Tartrate/Citrate.
Oxidizes ONLY Aliphatic aldehydes. Fails for Benzaldehyde!
Result: Brick-red precipitate of $Cu_2O$.
This test yields a Canary Yellow precipitate ($CHI_3$). It does NOT test for all ketones. It strictly tests for the presence of the Methyl Ketone group ($CH_3-CO-$) or a group that can be oxidized into it ($CH_3-CH(OH)-$).
- Acetaldehyde ($CH_3CHO$)
- Ethanol ($CH_3CH_2OH$)
- Propan-2-ol
- Acetophenone ($C_6H_5COCH_3$)
- Formaldehyde ($HCHO$)
- Methanol ($CH_3OH$)
- Propan-1-ol
- Pentan-3-one
3. Tests for Amines
Exclusively for Primary ($1^\circ$) Amines (both aliphatic and aromatic). The amine is heated with Chloroform ($CHCl_3$) and alcoholic KOH.
Reagent: Benzenesulfonyl chloride ($C_6H_5SO_2Cl$). Reacts with amines to form sulfonamides. The trick lies in the solubility of the product in alkali (NaOH).
Product has one acidic H attached to Nitrogen.
Soluble in Alkali
Product has NO acidic H attached to Nitrogen.
Insoluble in Alkali
No H to replace initially.
Does Not React
Exclusively for Primary Aromatic Amines (like Aniline). The amine is diazotized ($NaNO_2/HCl$ at $0-5^\circ\text{C}$), then coupled with an alkaline solution of $\beta$-naphthol.
4. Tests for Carboxylic Acids
Carboxylic acids react with weak bases like $NaHCO_3$ to release Carbon Dioxide gas.
Crucial Distinction: Phenol is too weak an acid and DOES NOT give effervescence with $NaHCO_3$. (Exception: Picric acid is strong enough to give it!).
NEET Grand Test: POC Distinction
15 High-Yield Questions testing visual indicators, reagent specificities, and solubility logic.
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