Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry

September 13, 2026

| Chemical Science Exams, Organic Chemistry

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

| No Comments

| 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…

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

September 12, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

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…

Birch Reduction and Dissolving Metal Reactions

September 12, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Reduction Birch Reduction and Dissolving Metal Reactions A reduction that stops exactly where you want it, and whose regiochemistry is decided by the substituent already on the ring. BSc & MSc · Organic Chemistry · Concept The short answer: A metal dissolved in liquid ammonia supplies solvated electrons that reduce an aromatic ring to a non-conjugated cyclohexadiene. Electron-donating substituents end up on a double bond carbon; electron-withdrawing substituents end up on a saturated carbon — and that opposite outcome is the standard question. The reagent An alkali metal dissolved in liquid ammonia gives a deep blue solution…

Ionic Conductance, Kohlrausch’s Law and Transport Numbers

September 12, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Electrochemistry Ionic Conductance, Kohlrausch’s Law and Transport Numbers Conductance measurements answer questions that look inaccessible — the degree of dissociation of a weak acid, and the solubility of a nearly insoluble salt. BSc & MSc · Physical Chemistry · Concept The short answer: Molar conductivity rises as a solution is diluted, but for different reasons in strong and weak electrolytes. Kohlrausch’s law of independent migration lets the limiting conductivity of a weak electrolyte be constructed from ionic values, which then gives its degree of dissociation and dissociation constant. The quantities Conductivity is conductance per unit dimension of…

Mass Spectrometry: Reading Fragmentation Patterns

September 11, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Spectroscopy Mass Spectrometry: Reading Fragmentation Patterns The molecular ion gives the mass. The fragments give the structure, and they fragment in predictable ways. BSc & MSc · Spectroscopy · Method The short answer: The molecular ion peak gives the molecular mass, and isotope peaks reveal certain elements immediately. Fragmentation follows rules based on which cation is most stable, so the losses observed identify the groups present. The nitrogen rule links an odd molecular mass to an odd number of nitrogens. What the spectrum shows The sample is ionised, and the resulting ions are separated by mass-to-charge ratio….

Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes

September 11, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Solutions Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes Deviations from ideality are not an inconvenience — they are the reason azeotropes exist, and the sign of the deviation predicts which kind forms. BSc & MSc · Physical Chemistry · Concept The short answer: An ideal solution obeys Raoult’s law across the whole composition range, which requires the interactions between unlike molecules to match those between like ones. Where unlike interactions are weaker, the solution shows positive deviation and can form a minimum boiling azeotrope; where stronger, negative deviation and a maximum boiling azeotrope. Raoult’s law and…

The Wittig Reaction: Making Alkenes Where You Want Them

September 11, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Synthesis The Wittig Reaction: Making Alkenes Where You Want Them Its value is not that it makes alkenes but that it puts the double bond in an unambiguous position, which elimination cannot guarantee. BSc & MSc · Organic Chemistry · Method The short answer: A phosphorus ylide adds to a carbonyl, and the resulting four-membered intermediate collapses to give an alkene and a phosphine oxide. The double bond forms exactly where the carbonyl was, with no possibility of migration — which is what makes it superior to elimination for regiochemical control. Why it matters Elimination reactions make…

Free Radical Reactions: Chain Mechanisms and Selectivity

September 10, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Radicals Free Radical Reactions: Chain Mechanisms and Selectivity Radical reactions look unruly, but their selectivity follows a clear rule — the more selective reagent is the less reactive one. BSc & MSc · Organic Chemistry · Concept The short answer: A radical chain has initiation, propagation and termination steps, and the propagation steps regenerate the radical so one initiation produces many products. Selectivity depends on how exothermic the abstraction step is: a less reactive radical has a later, more product-like transition state and discriminates more between C–H bonds. The chain Initiation — a bond breaks homolytically, usually…

Aromatic Nucleophilic Substitution and the Benzyne Mechanism

September 9, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Aromatic Aromatic Nucleophilic Substitution and the Benzyne Mechanism Two completely different mechanisms produce the same overall transformation, and the conditions tell you which one operated. BSc & MSc · Organic Chemistry · Concept The short answer: The addition–elimination route needs strong electron-withdrawing groups ortho or para to the leaving group, and gives substitution at exactly that position. The benzyne route needs very forcing conditions, has no such requirement, and can give substitution at the adjacent position too — which is the evidence that distinguishes them. Why aromatic rings resist nucleophiles A benzene ring is electron rich, so…

1 of 6
1
2
3
…
6
Next »