Qualitative Analysis Lab
Before using advanced instruments, chemists perform simple chemical tests for preliminary evidence about functional groups. These tests rely on characteristic reactions that produce an observable change, like a colour change or fizzing. Click the buttons to simulate adding a reagent and see the result.
Test for Unsaturation (C=C)
Distinguish an alkene from an alkane using bromine water. The bromine adds across the double bond, and the brown colour disappears.
Sample + Br₂(aq)
Brown solution
Test for Acidity (-COOH)
Carboxylic acids are acidic enough to react with carbonates, producing CO₂ gas. Alcohols are not.
Sample + Na₂CO₃(aq)
No reaction
Purity via Melting Point
Impurities disrupt a solid’s crystal lattice, causing it to melt at a lower temperature and over a wider range.
Pure Benzoic Acid
Sharp M.P: 122°C
Impure Sample
Range: 118-121°C
Quantitative Analysis: Redox Titration
Volumetric analysis, or titration, is a classic quantitative technique. Here, we simulate the redox titration of ethanedioic acid (H2C2O4) with potassium permanganate (KMnO4). The purple MnO4– ion acts as its own indicator. The endpoint is the first persistent pink/purple colour when all the acid is consumed. Use the slider to add titrant and find the equivalence point.
Redox Titration Simulator
Aliquot
Instrumental Analysis: HPLC
High-Performance Liquid Chromatography (HPLC) separates mixtures based on each component’s differential distribution between a mobile phase and a stationary phase. The area under a component’s peak is proportional to its concentration. We use a calibration curve, prepared from standards of known concentration, to find the concentration of an unknown sample.
HPLC Calibration Curve for Caffeine
Analyse Unknown Sample:
Enter the measured peak area from the chromatogram of an energy drink to determine its caffeine concentration.
Instrumental Analysis: Mass Spectrometry (MS)
Mass spectrometry “weighs” a molecule and its fragments. It provides the relative molecular mass ($M_r$) from the molecular ion peak ($M^+$) — the peak with the highest mass-to-charge ($m/z$) ratio. Other peaks represent fragments, providing clues about the molecule’s structure. Hover over the major peaks in the spectrum of pentan-3-one below to see their identity.
Mass Spectrum of Pentan-3-one (C5H10O)
Instrumental Analysis: Infrared (IR) Spectroscopy
Infrared spectroscopy identifies functional groups by detecting their characteristic bond vibrations. A trough on the spectrum means energy has been absorbed at that wavenumber. The VCE data book lists key absorption ranges. Click the buttons to highlight significant absorptions in the spectrum of propanoic acid.
IR Spectrum of Propanoic Acid (C3H6O2)
Instrumental Analysis: NMR Spectroscopy
Nuclear Magnetic Resonance (NMR) is the most powerful tool for mapping a molecule’s C-H framework. It reveals unique chemical environments, the number of atoms in each environment (for $^1$H-NMR), and their connectivity through spin-spin splitting. Explore the spectra of ethanol below.
NMR Spectra of Ethanol (CH3CH2OH)
$^{13}$C-NMR Spectrum
The 2 signals show there are 2 unique carbon environments.
$^1$H-NMR Spectrum
The 3 signals show 3 unique proton environments. Hover over peaks for details on integration and splitting.
Structure Elucidation Puzzles
It’s time to be a chemical detective. Use the combined spectral data to determine the structure of an unknown compound. This requires a logical, systematic approach, combining clues from all the analytical techniques.
Puzzle #1: Solve for C4H8O2
Evidence File:
- IHD (Degree of Unsaturation): 1
- IR Spectrum: Strong, sharp peak at ~1740 cm⁻¹ (ester C=O). No broad O-H peak.
- $^{13}$C-NMR: 4 unique signals (no symmetry).
- $^1$H-NMR:
- δ 4.1 (quartet, 2H)
- δ 2.0 (singlet, 3H)
- δ 1.2 (triplet, 3H)
What is the structure?
Based on the evidence, propose a structure for the compound.
Application: Medicinal Chemistry
Analytical techniques are critical in medicine. A drug’s effectiveness depends on its precise 3D shape and its ability to interact with biological targets like enzymes. Many drugs work by inhibiting enzymes.
Chirality in Drugs
Many drugs are chiral (have a carbon with 4 different groups). Biological systems, like enzyme active sites, are also chiral and often only one enantiomer (mirror image) produces the desired effect. The other can be inactive or harmful.
Enantiomers are like a pair of hands: mirror images, but not superimposable.
Competitive Inhibition
Many drugs are competitive inhibitors. They have a shape similar to the enzyme’s natural substrate, allowing them to bind to the active site and block the normal reaction. This is a key principle of rational drug design.
Enzyme + Inhibitor ⇌ [Enzyme-Inhibitor Complex]
❌ No Reaction Occurs
Factors Affecting Enzyme Activity
Enzyme function is highly sensitive to its environment. Changes in temperature and pH can disrupt the forces holding the enzyme’s 3D structure, causing it to denature and lose function. This graph shows typical activity curves.
