Conformational Analysis of Cyclohexane

September 8, 2026

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Conformation Conformational Analysis of Cyclohexane Draw the chair correctly and half of six-membered ring chemistry answers itself — reactivity, stability and stereochemical outcome all follow from it. BSc & MSc · Organic Chemistry · Concept The short answer: Cyclohexane adopts a chair conformation with no angle or torsional strain. Each carbon carries one axial and one equatorial position, and ring flipping interconverts them. Substituents prefer equatorial positions to avoid 1,3-diaxial interactions, and that single preference drives most conformational questions. Why the chair A planar hexagon would have bond angles of 120°, well above the tetrahedral 109.5°, and…

The HSAB Principle: Predicting Which Combinations Are Stable

September 8, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Acids and Bases The HSAB Principle: Predicting Which Combinations Are Stable A qualitative rule that predicts a surprising amount — which minerals occur together, which ligands bind which metals, and which reactions go. BSc & MSc · Inorganic Chemistry · Concept The short answer: Hard species are small, highly charged and not easily polarised; soft species are large, less charged and readily polarised. Hard acids prefer hard bases and soft acids prefer soft bases. The rule is qualitative but predicts stability, solubility and reaction direction remarkably well. What hard and soft mean Property Hard Soft Size Small…

Nuclear Chemistry: Decay Kinetics, Stability and Applications

September 8, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Nuclear Nuclear Chemistry: Decay Kinetics, Stability and Applications Radioactive decay is first-order kinetics applied to nuclei, so the mathematics is already familiar. What is new is what decides which nuclei decay at all. BSc & MSc · Inorganic Chemistry · Concept The short answer: All radioactive decay is first order, so half-life is independent of the amount present. Which mode a nucleus takes depends on its neutron-to-proton ratio relative to the band of stability: too many neutrons favours beta emission, too few favours positron emission or electron capture, and very heavy nuclei emit alpha particles. Decay kinetics…

Retrosynthetic Analysis: Thinking Backwards From the Target

September 8, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Synthesis Retrosynthetic Analysis: Thinking Backwards From the Target Synthesis questions become tractable when you stop asking what the starting material can do and start asking what the product could have come from. BSc & MSc · Organic Chemistry · Method The short answer: Work backwards from the target, breaking bonds at strategic points to give simpler precursors. Each disconnection must correspond to a real forward reaction. Identifying which bond to break comes from recognising the functional group relationships that known reactions produce. The basic move A disconnection is a bond broken on paper, in the reverse direction…

Photochemistry: Quantum Yield and the Jablonski Diagram

September 7, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Photochemistry Photochemistry: Quantum Yield and the Jablonski Diagram Absorbing a photon puts a molecule somewhere unusual. The Jablonski diagram maps every route back down, and quantum yield counts which route was taken. BSc & MSc · Physical Chemistry · Concept The short answer: Quantum yield is the number of molecules undergoing a process per photon absorbed. The Jablonski diagram organises the competing routes back to the ground state — fluorescence, internal conversion, intersystem crossing and phosphorescence — and their relative rates decide what is observed. The two basic laws Grotthuss–Draper law: only light that is absorbed can…

Lanthanides: The Contraction and Why Separation Is So Hard

September 7, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · f Block Lanthanides: The Contraction and Why Separation Is So Hard One structural fact — poor shielding by f electrons — explains the contraction, the similarity of the elements, and the difficulty of separating them. BSc & MSc · Inorganic Chemistry · Concept The short answer: Across the lanthanide series the 4f electrons shield the nuclear charge poorly, so the effective nuclear charge rises steadily and the ionic radius falls. That contraction makes the elements chemically almost identical, which is why they occur together and separating them requires methods exploiting very small differences. What the contraction is…

Protecting Groups: When to Use One and How to Choose

September 7, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Synthesis Protecting Groups: When to Use One and How to Choose A synthesis question that looks impossible usually becomes routine once you notice which group needs protecting. BSc & MSc · Organic Chemistry · Method The short answer: A protecting group temporarily converts a reactive functional group into an unreactive one so a reaction can be performed elsewhere. It must go on selectively, survive the intended reaction, and come off under conditions the rest of the molecule tolerates. Failing any of those three makes it useless. The problem being solved Most reagents are not perfectly selective. A…

Vibrational Spectroscopy: Force Constants and Anharmonicity

September 7, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Spectroscopy Vibrational Spectroscopy: Force Constants and Anharmonicity The harmonic oscillator explains where the band is. Anharmonicity explains everything the harmonic model gets wrong, including why molecules can dissociate at all. BSc & MSc · Spectroscopy · Concept The short answer: A vibrating bond behaves approximately as a harmonic oscillator with evenly spaced levels and a selection rule of one quantum. The frequency depends on the force constant and the reduced mass, so a stiffer bond or lighter atoms absorb at higher wavenumber. Real bonds are anharmonic, which produces converging levels and permits overtones. The harmonic oscillator Model…

Rotational Spectroscopy: Measuring a Bond Length From a Spectrum

September 6, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Spectroscopy Rotational Spectroscopy: Measuring a Bond Length From a Spectrum Line spacing gives the rotational constant, the rotational constant gives the moment of inertia, and that gives the bond length. Three steps, no ambiguity. BSc & MSc · Spectroscopy · Method The short answer: For a rigid diatomic rotor the energy levels are E = BJ(J+1), so successive transitions are separated by exactly 2B. Measure that spacing, extract B, convert to the moment of inertia and then to the bond length. The whole chain rests on the selection rule that J changes by one. The rigid rotor…

Carbohydrate Structure: Anomers, Mutarotation and Ring Forms

September 6, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Biomolecules Carbohydrate Structure: Anomers, Mutarotation and Ring Forms Most carbohydrate confusion comes from moving between the open-chain and cyclic representations. Fix that and the chemistry is straightforward. BSc & MSc · Organic Chemistry · Concept The short answer: A sugar cyclises when its own hydroxyl attacks its carbonyl, forming a hemiacetal and creating a new stereocentre at that carbon — the anomeric centre. The two configurations are anomers, and in solution they interconvert through the open-chain form, which is what mutarotation measures. Open chain to ring A monosaccharide contains a carbonyl and several hydroxyls. One hydroxyl, positioned…

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