The Trans Effect: Predicting the Product of Square Planar Substitution

September 4, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Reaction Mechanism The Trans Effect: Predicting the Product of Square Planar Substitution A synthesis question in coordination chemistry usually reduces to one thing — knowing which ligand directs the incoming group to the position opposite itself. BSc & MSc · Inorganic Chemistry · Concept The short answer: In square planar complexes, certain ligands strongly labilise the position trans to themselves. Arranging synthesis steps so the right ligand is present at the right time is how specific geometric isomers are made deliberately rather than as a mixture. What the effect is In a square planar complex, a ligand…

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…

E1 and E2 Elimination: Saytzeff, Hofmann and Which Alkene Forms

September 3, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Mechanism E1 and E2 Elimination: Saytzeff, Hofmann and Which Alkene Forms Elimination questions almost always come down to one thing: which of two possible alkenes is the major product, and why. BSc & MSc · Organic Chemistry · Concept The short answer: E2 is concerted and requires the leaving group and the beta hydrogen to be anti-periplanar. E1 goes through a carbocation. Saytzeff orientation gives the more substituted alkene and is normal; Hofmann orientation gives the less substituted one and arises with bulky bases or charged leaving groups. The two mechanisms E2 is a single concerted step….

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