Free Radical Reactions: Chain Mechanisms and Selectivity

September 10, 2026

| 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…

The Partition Function: Connecting Molecules to Thermodynamics

September 9, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Statistical The Partition Function: Connecting Molecules to Thermodynamics One quantity that, once known, yields every thermodynamic property of the system. Understanding what it counts is more useful than memorising the formulas it generates. BSc & MSc · Physical Chemistry · Concept The short answer: The partition function sums a Boltzmann factor over every accessible state, so it measures how many states are effectively available at a given temperature. Energy, entropy and free energy all follow from it by differentiation, which is why it is the bridge between molecular properties and bulk thermodynamics. The definition and what it…

Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals

September 9, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Organometallics Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals The infrared stretching frequency of bound carbon monoxide is a direct readout of electron density at the metal, which makes it one of the most informative single measurements in the subject. BSc & MSc · Inorganic Chemistry · Concept The short answer: Carbon monoxide donates a lone pair to the metal and simultaneously accepts electron density from filled metal d orbitals into its own antibonding orbital. That back-donation weakens the C–O bond, lowering its stretching frequency — so the observed frequency measures how much back-donation is occurring….

The Distribution Law and Why Extraction in Portions Works Better

September 9, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Equilibria The Distribution Law and Why Extraction in Portions Works Better A short derivation that settles a practical question — is one large extraction better than several small ones, and by how much. BSc & MSc · Physical Chemistry · Concept The short answer: A solute distributes between two immiscible solvents in a fixed ratio at constant temperature. Working through the arithmetic shows that several small extractions remove far more solute than one extraction with the same total volume — which is why laboratory procedure always specifies portions. The law When a solute is shaken with two…

Aromatic Nucleophilic Substitution and the Benzyne Mechanism

September 9, 2026

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| 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…

Conformational Analysis of Cyclohexane

September 8, 2026

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| 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…

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