Identification of functional groups: tests, spectroscopy

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Analytical Techniques - Functional Group Identification

Analytical Techniques - Functional Group Identification

This document provides detailed notes on the identification of functional groups using various chemical tests and spectroscopic techniques, relevant for Cambridge A-Level Chemistry 9701.

Chemical Tests for Functional Groups

1. Alkanes

Alkanes are generally unreactive. However, combustion is a characteristic test.

  • Combustion: Alkanes burn with a clean, blue flame producing carbon dioxide and water.

2. Alkenes

Alkenes undergo addition reactions.

  • Halogenation: Addition of bromine or chlorine. The disappearance of the bromine/chlorine colour indicates the presence of an alkene.
  • Acid-Base Test: Alkenes react with dilute acids (e.g., HCl) to give a neutral solution. This is due to the protonation of the alkene.

3. Alkynes

Alkynes undergo addition reactions, similar to alkenes.

  • Halogenation: Addition of bromine or chlorine. The disappearance of the bromine/chlorine colour indicates the presence of an alkyne.
  • Acetylene Test: A test for the presence of a terminal alkyne. A drop of dilute sodium hydroxide is added to the solution. If a pink colour develops, it indicates the presence of a terminal alkyne. The reaction is: R-C≡CH + NaOH → R-C≡C-Na+ + H2O

4. Alcohols

Alcohols can be identified through various tests.

  • Oxidation: Alcohols can be oxidized to aldehydes or ketones depending on the degree of oxidation. Common oxidizing agents include:
    • KMnO4 (refluxing): Oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones. Tertiary alcohols are generally resistant to oxidation.
    • acidified potassium dichromate (K2Cr2O7, refluxing): Similar to KMnO4.
  • Reaction with Sodium Metal: Alcohols react with sodium metal to produce hydrogen gas. This is a characteristic test for alcohols. The reaction is: 2R-OH + 2Na → 2R-ONa + H2 (g)
  • Lucas Reagent Test: A reagent containing zinc chloride and hydrochloric acid (ZnCl2/HCl). Alcohols react with the Lucas reagent to form an alkyl chloride solution. The rate of cloudiness indicates the relative reactivity of the alcohol.

5. Aldehydes

Aldehydes have characteristic tests.

  • Oxidation: Aldehydes are easily oxidized to carboxylic acids using mild oxidizing agents like acidified potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4).
  • Cannizzaro Reaction: Aldehydes with no α-hydrogens undergo the Cannizzaro reaction in the presence of a strong base (e.g., NaOH). This involves disproportionation, where one molecule is oxidized to a carboxylic acid and another is reduced to an alcohol.

6. Ketones

Ketones are less reactive than aldehydes.

  • Oxidation: Ketones are resistant to oxidation under mild conditions. Strong oxidizing agents can cleave the carbon-carbon bond.

7. Carboxylic Acids

Carboxylic acids have characteristic tests.

  • Reaction with Sodium Bicarbonate: Carboxylic acids react with sodium bicarbonate (NaHCO3) to produce carbon dioxide gas. This is a characteristic test. The reaction is: R-COOH + NaHCO3 → R-COONa + H2O + CO2 (g)
  • Reaction with Sodium Hydroxide: Carboxylic acids react with sodium hydroxide to form salts and water.

8. Esters

Esters undergo hydrolysis and saponification.

  • Hydrolysis: Esters can be hydrolyzed with dilute acid or base to form carboxylic acids and alcohols.
  • Saponification: Esters react with sodium hydroxide to form soaps (salts of fatty acids) and alcohols.

9. Amines

Amines can be identified through various tests.

  • Reaction with Nitrous Acid: Primary amines react with nitrous acid (HNO2) to form diazonium salts, which can then undergo further reactions.
  • Reaction with Acetic Anhydride: Primary amines react with acetic anhydride to form amides.

Spectroscopic Techniques

1. Infrared (IR) Spectroscopy

IR spectroscopy measures the absorption of infrared radiation by molecules, providing information about the functional groups present.

Functional Group Characteristic IR Absorption (cm-1)
O-H (Alcohols, Carboxylic Acids) 3200-3600 (broad)
C=O (Aldehydes, Ketones, Carboxylic Acids, Esters) 1700-1750 (strong)
C-H (Alkanes, Alkenes, Alkynes) 2850-3000
C=C (Alkenes) 1600-1680 (strong)
C≡C (Alkynes) 2100-2260 (strong)
C-O (Alcohols, Esters) 1000-1300
N-H (Amines) 3200-3500 (broad)

2. Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy provides information about the number and types of hydrogen and carbon atoms in a molecule.

  • 1H NMR: Shows the number of different types of hydrogen atoms and their chemical environment (e.g., shielding).
  • 13C NMR: Shows the number of different types of carbon atoms in the molecule.

3. Mass Spectrometry (MS)

Mass spectrometry measures the mass-to-charge ratio of ions, providing information about the molecular weight and fragmentation pattern of a molecule.

  • Molecular Ion Peak: The peak corresponding to the molecular weight of the compound.
  • Fragment Ions: Peaks corresponding to ions formed by the cleavage of bonds in the molecule. The fragmentation pattern can provide clues about the functional groups present.

Conclusion

By combining chemical tests and spectroscopic techniques, it is possible to confidently identify the functional groups present in organic molecules. The choice of technique depends on the specific functional group being investigated and the available resources.