1. Infrared (IR) Spectroscopy
Covalent bonds vibrate naturally by stretching and bending. Each bond absorbs infrared radiation matching its natural vibrational frequency. Absorption peaks appear as dips on the transmittance scale:
| Bond Type | Functional Group | Wavenumber Range (\(\text{cm}^{-1}\)) | Peak Appearance & Diagnostics |
|---|---|---|---|
| O-H | Alcohols | 3230 - 3550 | Broad, smooth trough due to hydrogen bonding. |
| O-H | Carboxylic acids | 2500 - 3000 | Very broad, jagged trough overlapping C-H stretching. |
| C=O | Aldehydes, Ketones, Carboxylic acids, Esters | 1680 - 1750 | Sharp, intense, deep spike (unmistakable carbonyl peak). |
| C=C | Alkenes | 1620 - 1680 | Moderate sharp absorption. |
| C-H | Alkanes, Alkenes, Arenes | 2850 - 3100 | Sharp, multi-pointed absorption. |
2. The Fingerprint Region
The complex region of an infrared spectrum below \(1500\text{ cm}^{-1}\), caused by complex bending vibrations of the whole molecule.
While the functional group region (\(1500-4000\text{ cm}^{-1}\)) identifies specific bonds, the fingerprint region is unique to an individual compound. Unknown molecules are positively confirmed by computer comparison of their fingerprint spectrum against a library of known reference spectra.
3. Mass Spectrometry in Organic Analysis
In electron impact mass spectrometry, an intact organic molecule loses one electron to form a radical cation known as the molecular ion (\(M^{\bullet+}\)):
\[ \text{M} + e^- \rightarrow M^{\bullet+} + 2e^- \]
The \(m/z\) ratio of the molecular ion peak gives the relative molecular mass (\(M_r\)) of the intact molecule.
High-Resolution Mass Spectrometry
Standard low-resolution mass spectrometry measures \(m/z\) to the nearest integer. High-resolution spectrometers measure masses to four or five decimal places. This differentiates compounds that have identical integer \(M_r\) values:
- Propanal (\(\text{C}_3\text{H}_6\text{O}\)): Accurate mass = \(3(12.00000) + 6(1.00782) + 15.99491 = \mathbf{58.04183}\).
- Butane (\(\text{C}_4\text{H}_{10}\)): Accurate mass = \(4(12.00000) + 10(1.00782) = \mathbf{58.07820}\).
4. The [M+1] Peak and Carbon Counting
Directly to the right of the molecular ion peak \(M\), a tiny peak is observed at \([M+1]\). This peak arises from the natural \(1.1\%\) abundance of the carbon-13 isotope (\(^{13}\text{C}\)) in the molecule.
The number of carbon atoms (\(n\)) in the molecule can be deduced by comparing the peak heights:
\[ n = \frac{\text{Height of }[M+1]\text{ peak}}{\text{Height of }M\text{ peak}} \times \frac{100}{1.1} \]
5. Chemical Identification Tests Summary
| Functional Group | Test Reagent | Positive Observation |
|---|---|---|
| Alkene (\(\text{C}=\text{C}\)) | Bromine water (\(\text{Br}_2(aq)\)) | Orange to colourless. |
| Halogenoalkane (\(\text{R}-\text{X}\)) | Warm with \(\text{NaOH}(aq)\), acidify with \(\text{HNO}_3\), add \(\text{AgNO}_3(aq)\) | \(\text{AgCl}\) white ppt; \(\text{AgBr}\) cream ppt; \(\text{AgI}\) yellow ppt. |
| Alcohol (\(-\text{OH}\)) | Acidified potassium dichromate(VI) | Orange to green (\(1^\circ\) and \(2^\circ\) alcohols). |
| Aldehyde (\(-\text{CHO}\)) | Tollens' reagent (warm) | Silver mirror formed on glass. |
| Carboxylic acid (\(-\text{COOH}\)) | Sodium hydrogencarbonate (\(\text{NaHCO}_3(aq)\)) | Effervescence; gas turns limewater cloudy (\(\text{CO}_2\)). |
6. Practice Questions
Deduction:
- Absorption at \(1715\text{ cm}^{-1}\) indicates the presence of a carbonyl group (\(\text{C}=\text{O}\)). (1 mark)
- Absence of absorption between \(3200-3600\text{ cm}^{-1}\) confirms compound X is not an alcohol. (1 mark)
- Negative Tollens' test confirms X is a ketone, not an aldehyde. (1 mark)
- With formula \(\text{C}_3\text{H}_6\text{O}\), the only possible ketone is propanone (\(\text{CH}_3\text{COCH}_3\)). (1 mark)