Spectroscopy is the study of how matter interacts with electromagnetic radiation. In organic chemistry, three key techniques are used together to identify compounds and determine their molecular structure:
In MS, a sample is vaporised and bombarded with high-energy electrons, causing molecules to lose an electron and form a molecular ion (M⁺). The molecular ion and its fragments are separated by mass-to-charge ratio ().
| Feature | Information Provided |
|---|---|
| Molecular ion peak (M⁺) | Relative molecular mass of the compound |
| Fragmentation pattern | Carbon skeleton and functional groups |
| M+1 / M+2 peaks | Presence of isotopes (e.g. Cl / Cl gives M+2 peak of ~1/3 height of M) |
Example: Ethanol (, ) shows M⁺ at and a fragment at ( or ).
IR radiation causes vibrational transitions in covalent bonds (stretching and bending). Different functional groups absorb at characteristic wavenumbers, allowing identification.
| Functional Group | Wavenumber () | Appearance |
|---|---|---|
| (alcohol) | 3200–3550 | Broad |
| (carboxylic acid) | 2500–3300 | Very broad |
| 3300–3500 | Medium | |
| (carbonyl) | 1680–1750 | Strong, sharp |
| 1000–1300 | Medium |
The region below is the fingerprint region — unique to each molecule and used for definitive identification by comparison with a reference spectrum.
H NMR exploits the magnetic properties of hydrogen nuclei. In a strong magnetic field, protons in different chemical environments absorb radiofrequency radiation at different chemical shifts (), measured in ppm relative to TMS (tetramethylsilane) at .
| Feature | Information Provided |
|---|---|
| Number of signals | Number of different proton environments |
| Chemical shift () | Type of proton environment (functional group) |
| Integration (peak area) | Relative number of protons in each environment |
| Splitting pattern | Number of adjacent protons ( rule) |
Common chemical shifts:
In practice, these three techniques are used in combination:
Together they allow unambiguous structural determination of organic compounds.
Azo compounds are an important class of organic compounds containing the azo group linking two aryl (aromatic) groups.
The azo group acts as a chromophore — it absorbs visible light, giving azo compounds their characteristic intense colours (typically orange, red, or yellow).
Azo dyes are formed by coupling a diazonium salt with an aromatic coupling component (such as phenol or a naphthol).
Example: Coupling of benzenediazonium chloride with phenol
Conditions: NaOH(aq), 0–5°C
The product is an orange-red azo dye.
Why NaOH is needed: NaOH converts phenol to the phenoxide ion (), which is a stronger activating group, making the ring more reactive towards the weakly electrophilic diazonium ion.
Mechanism: This is an electrophilic aromatic substitution where the diazonium ion () acts as the electrophile, attacking the para position of the phenoxide ring.
Azo compounds are widely used as dyes because:
Applications include: textile dyes (e.g. Congo Red, Methyl Orange), food colorants, pH indicators, and biological stains.
Other azo dyes can be formed via a similar route — by coupling different diazonium salts with different aromatic amines or phenols. For example, coupling with naphthalen-2-ol (β-naphthol) produces a bright red dye.