Subjects

Organic Chemistry · Heterocycles Five-Membered Heterocycles: Aromaticity and Reactivity Order Three rings with the same shape and very different reactivity. The difference tracks one property of the heteroatom. BSc & MSc · Organic Chemistry · Concept The short answer: Pyrrole, furan and thiophene are all aromatic six-electron systems in which the heteroatom donates its lone pair to the ring. Their reactivity toward electrophiles follows the heteroatom’s willingness to share that pair, so pyrrole is most reactive and thiophene least among the three toward electrophilic attack. Why they are aromatic Each ring has two carbon–carbon double bonds, contributing four π electrons….

The Thermodynamics of Mixing

September 14, 2026

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Thermodynamics The Thermodynamics of Mixing Why two liquids mix even when nothing energetic is gained, and what has to be true for them not to. BSc & MSc · Physical Chemistry · Concept The short answer: For an ideal solution the enthalpy of mixing is zero, so mixing is driven entirely by the entropy increase from having more accessible arrangements. Since that entropy term is always positive, ideal liquids always mix. Immiscibility therefore requires a positive enthalpy of mixing large enough to overcome it. The entropy of mixing Mixing increases the number of ways the molecules can…

Catalytic Cycles: Hydroformylation and the Wacker Process

September 14, 2026

Inorganic Chemistry · Catalysis Catalytic Cycles: Hydroformylation and the Wacker Process Industrial catalysis assembled from four elementary steps that repeat in different orders. Learn the steps and any cycle becomes readable. BSc & MSc · Inorganic Chemistry · Concept The short answer: Homogeneous catalytic cycles are built from a small set of elementary organometallic steps: oxidative addition, migratory insertion, reductive elimination and ligand substitution. Hydroformylation and the Wacker process are the two standard examples, and each is examined by asking which step does what. The elementary steps Step What changes Oxidation state Electron count Oxidative addition A bond adds across…

Organolithium and Organocuprate Reagents

September 14, 2026

Organic Chemistry · Reagents Organolithium and Organocuprate Reagents Three carbon nucleophiles that look interchangeable and are not. The differences in reactivity are exactly what makes each useful. BSc & MSc · Organic Chemistry · Method The short answer: Organolithiums are more reactive and more basic than Grignards, so they attack hindered substrates but tolerate fewer functional groups. Organocuprates are much softer and add conjugately to enones rather than at the carbonyl — which is the single most useful distinction among the three. The three compared Grignard Organolithium Organocuprate Reactivity Moderate High Moderate but selective Basicity Strong Very strong Weak Character…

Crystal Defects and Non-Stoichiometry

September 13, 2026

Inorganic Chemistry · Solid State Crystal Defects and Non-Stoichiometry A perfect crystal would have zero entropy of disorder, which thermodynamics forbids above absolute zero. Defects are not flaws but a requirement. BSc & MSc · Inorganic Chemistry · Concept The short answer: Point defects exist in every real crystal because they increase entropy enough to lower the free energy despite costing enthalpy. Schottky defects remove ion pairs and reduce density; Frenkel defects relocate an ion and leave density unchanged. Non-stoichiometric compounds arise when defects are accompanied by a change in oxidation state. Why defects must exist Defects are thermodynamically required,…

Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry

September 13, 2026

Organic Chemistry · Aromatic Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry One functional group that can be replaced by almost anything, which makes it the standard route to substituents that cannot be installed directly. BSc & MSc · Organic Chemistry · Method The short answer: An aromatic amine treated with nitrous acid at low temperature gives a diazonium salt. Nitrogen is an outstanding leaving group, so the diazonium group can be replaced by halide, hydroxyl, cyano, hydrogen and more — giving access to substitution patterns unreachable by direct electrophilic substitution. Preparation An aromatic primary amine is treated with…

Osmotic Pressure and the van’t Hoff Factor

September 13, 2026

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Solutions Osmotic Pressure and the van’t Hoff Factor The most sensitive of the colligative properties, and the only one practical for measuring the molar mass of very large molecules. BSc & MSc · Physical Chemistry · Concept The short answer: Osmotic pressure is the pressure that must be applied to stop solvent flowing through a semipermeable membrane into a solution. It is far larger than the other colligative effects at the same concentration, which is why it is the method of choice for polymers and proteins. The van’t Hoff factor corrects for dissociation or association of the…

Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra

September 13, 2026

Inorganic Chemistry · Spectra Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra Two diagrams for the same purpose, differing in whether they can handle a spin state change. BSc & MSc · Inorganic Chemistry · Concept The short answer: Both plot the energies of electronic terms against ligand field strength. Orgel diagrams cover weak-field high-spin complexes only. Tanabe–Sugano diagrams cover the whole range including the high-spin to low-spin crossover, and take the ground state as the horizontal axis so transition energies are read directly. What the diagrams show A free ion’s electronic states are described by term symbols. Placing that ion…

Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be

September 12, 2026

Organic Chemistry · Mechanism Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be An unexpected rate enhancement together with retention of configuration is the signature. Both point to the same cause. BSc & MSc · Organic Chemistry · Concept The short answer: A group elsewhere in the molecule can attack the reacting centre internally, forming a cyclic intermediate before the external nucleophile arrives. Because two inversions occur in sequence, the overall configuration is retained — and the reaction is much faster than the substrate structure alone would predict. The two observations that reveal it Neighbouring group participation…

Noble Gas Compounds: Why Xenon Reacts and Helium Does Not

September 12, 2026