Pharmaceutical Chemistry · Short notes

Functional Group Analysis: Short Notes & Cheat Sheet

Qualitative tests for functional groups, quantitative functional-group assays and characteristic IR absorption bands.

Short notes

Read these before you attempt the topic's MCQs.

Where functional groups absorb in the IR

Read an IR spectrum from left (high wavenumber) to right: first O–H and N–H, then C–H, triple bonds, C=O, and finally the fingerprint region.

Qualitative tests

Aldehydes: Tollens' reagent gives a silver mirror; Fehling's and Benedict's solutions give a red precipitate with aliphatic aldehydes and reducing sugars.
Carbonyls in general: 2,4-dinitrophenylhydrazine (2,4-DNP) gives a yellow–orange precipitate.
Methyl ketones and ethanol give the iodoform test (yellow precipitate of CHI₃).
Alcohols: the Lucas test turns cloudy at once for tertiary, in about 5 minutes for secondary, and not at room temperature for primary alcohols.
Phenols give a violet colour with neutral ferric chloride.
Amines: the Hinsberg test separates primary, secondary and tertiary amines. Primary aromatic amines give a coloured azo dye after diazotisation.
Proteins give a violet biuret test; amino acids give a purple colour with ninhydrin (proline gives yellow). Carbohydrates give a violet ring in the Molisch test.

Quantitative assays

Nitrite (diazotisation) titration assays primary aromatic amines such as sulfonamides and procaine, using starch–iodide paper as an external indicator.
Non-aqueous titration assays weak bases with perchloric acid in glacial acetic acid (crystal violet indicator), and weak acids with sodium methoxide.
Karl Fischer titration measures water.
Kjeldahl digestion measures nitrogen. The Zeisel method measures methoxy groups.
Oils and fats: acid value (free acid), saponification value (esters), iodine value (unsaturation) and hydroxyl value.

IR spectroscopy of functional groups

O–H stretch gives a broad band at 3200–3600 cm⁻¹; N–H appears at 3300–3500 cm⁻¹ (primary amines show two peaks).
C–H stretch: just above 3000 cm⁻¹ for aromatic and alkene C–H, just below 3000 cm⁻¹ for aliphatic C–H.
Triple bonds (C≡N, C≡C) absorb at about 2100–2260 cm⁻¹.
C=O gives a strong band near 1700 cm⁻¹: ester about 1735, ketone about 1715, amide about 1650–1690 cm⁻¹.
Below about 1500 cm⁻¹ is the fingerprint region, unique to each compound and used for identification against a reference spectrum.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Test → group

Test	Positive for
Tollens' (silver mirror)	Aldehydes
Fehling's / Benedict's	Aliphatic aldehydes, reducing sugars
2,4-DNP	Aldehydes and ketones
Iodoform	Methyl ketones, ethanol
Lucas	Distinguishes 1°, 2°, 3° alcohols
Ferric chloride	Phenols (violet)
Hinsberg	Distinguishes 1°, 2°, 3° amines
Ninhydrin	Amino acids (purple)
Biuret	Proteins / peptide bonds
Molisch	All carbohydrates

Quantitative methods

Method	Measures
Karl Fischer	Water content
Kjeldahl	Nitrogen
Nitrite titration	Primary aromatic amines
Perchloric acid (non-aqueous)	Weak bases
Iodine value	Unsaturation of oils
Saponification value	Esters (fats)
Zeisel	Methoxy groups

Memory aids

Lucas: Tertiary is Turbid at once: Tertiary alcohols cloud immediately, secondary in about 5 minutes, primary not at room temperature.

Exam traps

Benzaldehyde gives Tollens' test but not Fehling's test (aromatic aldehydes do not reduce Fehling's).
Ethanol gives a positive iodoform test even though it is not a methyl ketone.
Nitrite titration needs an external indicator (starch–iodide paper), not an internal one.
The fingerprint region is below 1500 cm⁻¹, not in the high-wavenumber region.

Revised? Now test yourself.

Attempt the Functional Group Analysis MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.

Attempt Functional Group Analysis MCQs