Supramolecular Chemistry: Crown Ethers, Cryptands and Host–Guest Binding

September 4, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Entrance Exams Supramolecular Chemistry: Crown Ethers, Cryptands and Host–Guest Binding Chemistry beyond the covalent bond — where selectivity comes from the size of a hole and the number of contacts, not from making or breaking bonds. BSc & MSc · Inorganic Chemistry · Concept The short answer: Supramolecular chemistry studies assemblies held together by non-covalent forces. A crown ether binds an alkali metal ion whose radius matches its cavity, which is how selectivity is achieved without any covalent chemistry. Cryptands enclose the ion in three dimensions and bind far more strongly still. The extra stability of a…

Asymmetric Synthesis: How Enantioselectivity Is Actually Achieved

September 4, 2026

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| Chemical Science Exams, Organic Chemistry

Stereochemistry · Entrance Exams Asymmetric Synthesis: How Enantioselectivity Is Actually Achieved Two enantiomers have identical energies, so no achiral reagent can ever prefer one. Every method in this area is a way of breaking that symmetry. BSc & MSc · Organic Chemistry · Concept The short answer: Enantiomeric products come from enantiomeric transition states, which have identical energies, so an achiral system must give a racemate. Introducing something chiral makes the two competing transition states diastereomeric instead, and diastereomers differ in energy. Whether the chirality comes from an auxiliary, a reagent or a catalyst, that is the single mechanism behind…

Grignard Reagents: What They Make and What Destroys Them

September 4, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Reagents Grignard Reagents: What They Make and What Destroys Them One reagent class that builds carbon skeletons, and one failure mode that ruins more exam answers than any other. BSc & MSc · Organic Chemistry · Method The short answer: A Grignard reagent is a carbon nucleophile, effectively a carbanion. It attacks carbonyl carbons to build new carbon–carbon bonds, and the product depends entirely on which carbonyl it attacks. It is destroyed instantly by any acidic hydrogen, which is the constraint that governs how it can be used. What it is An alkyl or aryl magnesium halide,…

X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern

September 4, 2026

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| Chemical Science Exams, Physical Chemistry

Solid State · Entrance Exams X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern From a row of peaks on a diffractogram to a lattice type and a unit cell edge — the calculation examiners actually set. BSc & MSc · Physical & Solid State Chemistry · Method The short answer: Bragg’s law relates the angle at which a crystal diffracts to the spacing between its planes. For a cubic crystal the plane spacing depends on the Miller indices in a simple way, so the ratio of sin²θ values across the peaks reveals the lattice type directly — and the…

The Arrhenius Equation: Getting Activation Energy From Data

September 3, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Kinetics The Arrhenius Equation: Getting Activation Energy From Data Two forms of one equation, and knowing which to use is the difference between a clean answer and an arithmetic mess. BSc & MSc · Physical Chemistry · Method The short answer: The rate constant varies with temperature as k = A exp(−Ea/RT). Taking logarithms gives a straight line of slope −Ea/R against 1/T, which is how activation energy is measured. With only two temperatures, the two-point form avoids plotting altogether. The equation and what each part means k = A e−Ea/RT The exponential term is the fraction…

Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive

September 3, 2026

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| Chemical Science Exams, Physical Chemistry

Biophysical Chemistry · Entrance Exams Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive Three inhibition types, distinguished not by a definition to memorise but by what happens to Km and Vmax — and by where the lines cross. BSc & MSc · Chemical Kinetics · Concept The short answer: A competitive inhibitor binds the free enzyme at the active site, so more substrate overcomes it — apparent Km rises, Vmax is unchanged. An uncompetitive inhibitor binds only the enzyme–substrate complex, lowering both. A non-competitive inhibitor binds either form equally, lowering Vmax while leaving Km alone. On a Lineweaver–Burk plot each gives a…

Gibbs Free Energy: Why It Is the Criterion for Spontaneity

September 3, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Thermodynamics Gibbs Free Energy: Why It Is the Criterion for Spontaneity Entropy decides spontaneity, but only for the universe. Gibbs energy repackages that so the system alone is enough. BSc & MSc · Physical Chemistry · Concept The short answer: The second law says the entropy of the universe must increase. Rewriting that condition for a system at constant temperature and pressure gives ΔG < 0. So Gibbs energy is not a new law — it is the second law expressed in variables you can actually measure inside the system. The problem Gibbs energy solves The second…

Carbenes and Nitrenes: Structure, Spin State and Reactions

September 3, 2026

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| Chemical Science Exams, Organic Chemistry

Reactive Intermediates · Entrance Exams Carbenes and Nitrenes: Structure, Spin State and Reactions Whether a carbene adds to an alkene with retention or scrambles the stereochemistry is decided by one thing: singlet or triplet. BSc & MSc · Organic Chemistry · Concept The short answer: A carbene has six valence electrons on carbon and two non-bonding electrons that can be paired in one orbital or unpaired in two. Paired means singlet, and singlet carbenes add to alkenes in one step with complete retention of alkene geometry. Unpaired means triplet, which must react in two steps through a diradical, and rotation…

Molecular Orbital Theory for Diatomics: Bond Order and Magnetism

September 3, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Bonding Molecular Orbital Theory for Diatomics: Bond Order and Magnetism MO diagrams answer three questions at once — is the molecule stable, how strong is the bond, and is it paramagnetic. BSc & MSc · Inorganic Chemistry · Concept The short answer: Atomic orbitals combine to give bonding and antibonding molecular orbitals. Fill them by the aufbau principle, then bond order is half the difference between bonding and antibonding electrons. Unpaired electrons mean paramagnetism — which is where MO theory succeeds and valence bond theory fails. The core idea When two atomic orbitals of comparable energy and…

Cross-Coupling Reactions: Suzuki, Heck, Sonogashira and Stille

September 3, 2026

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| Chemical Science Exams, Inorganic Chemistry, Organic Chemistry

Organometallics · Entrance Exams Cross-Coupling Reactions: Suzuki, Heck, Sonogashira and Stille Four named couplings that share one palladium cycle — and differ only in what supplies the second carbon fragment. BSc & MSc · Organometallic Chemistry · Concept The short answer: All four couplings run on the same three-step palladium cycle. Oxidative addition and reductive elimination are common to every one of them; what distinguishes Suzuki, Sonogashira and Stille is only which organometallic delivers the partner group. The Heck is the exception — it has no transmetalation at all, and that single difference explains its different substrate scope and its…