Terpenes and the Isoprene Rule

September 19, 2026

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Natural Products Terpenes and the Isoprene Rule A structurally bewildering class of natural products that becomes tractable once you learn to find the five-carbon units. BSc & MSc · Organic Chemistry · Concept The short answer: Terpenes are built from isoprene units joined head to tail, so their carbon counts are multiples of five. Classifying by that count — mono, sesqui, di and so on — and locating the units in a drawn structure is the standard exercise, and it also predicts how the compound is biosynthesised. The isoprene rule Terpenes are assembled from five-carbon isoprene units….

The Clausius–Clapeyron Equation and Vapour Pressure

September 19, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Thermodynamics The Clausius–Clapeyron Equation and Vapour Pressure One equation linking vapour pressure, temperature and enthalpy of vaporisation, and three ways it is asked. BSc & MSc · Physical Chemistry · Method The short answer: Starting from the equality of chemical potentials across a phase boundary gives the Clapeyron equation for any transition. Assuming ideal vapour and neglecting the liquid volume gives the Clausius–Clapeyron form, which is linear in 1/T and yields the enthalpy of vaporisation from vapour pressure data. The general Clapeyron equation At a phase boundary the two phases are in equilibrium, so the chemical potential…

Nitrogen and Phosphorus: Same Group, Very Different Chemistry

September 19, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Main Group Nitrogen and Phosphorus: Same Group, Very Different Chemistry Two elements in the same group whose chemistry diverges sharply, and every divergence traces to size and to the availability of d orbitals. BSc & MSc · Inorganic Chemistry · Concept The short answer: Nitrogen forms strong multiple bonds and is limited to four bonds because it has no d orbitals. Phosphorus forms weaker multiple bonds but can expand its coordination beyond four. The result is that nitrogen exists as a triple-bonded diatomic while phosphorus exists as a singly bonded tetrahedral cluster. The two structural causes Nitrogen…

Michael Addition: Conjugate Addition and Where It Leads

September 18, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Carbonyl Michael Addition: Conjugate Addition and Where It Leads A soft nucleophile meeting a conjugated enone adds at the far end, not at the carbonyl. That single preference builds rings. BSc & MSc · Organic Chemistry · Concept The short answer: In an alpha-beta unsaturated carbonyl the beta carbon is electrophilic through conjugation. A stabilised enolate adds there rather than at the carbonyl, giving a 1,5-dicarbonyl product. Combining that with an intramolecular aldol gives the Robinson annulation, which builds a six-membered ring in one sequence. Why the beta carbon is electrophilic In an enone the carbonyl and…

Non-Aqueous Solvents: Chemistry Water Cannot Support

September 18, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Solvents Non-Aqueous Solvents: Chemistry Water Cannot Support Water levels every strong acid and every strong base. Escaping that limitation is the main reason other solvents are used. BSc & MSc · Inorganic Chemistry · Concept The short answer: A self-ionising solvent defines its own acid–base scale. Liquid ammonia is more basic than water, so it supports far stronger bases; anhydrous sulphuric acid is more acidic, so it supports far stronger acids. Solvents also differ in their ability to dissolve ionic compounds and to stabilise unusual oxidation states. Why water is limiting Water self-ionises, and that sets a…

Error Analysis and Significant Figures in Chemical Measurement

September 18, 2026

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| Chemical Science Exams, General Aptitude

Analytical Chemistry · Method Error Analysis and Significant Figures in Chemical Measurement A result reported without an idea of its uncertainty is not a measurement, and the arithmetic for handling that uncertainty is short. BSc & MSc · Analytical Chemistry · Method The short answer: Systematic errors shift every result in the same direction and are not reduced by repeating; random errors scatter around the true value and average out. Precision describes reproducibility, accuracy describes closeness to the true value, and the two are independent. Two kinds of error Systematic Random Effect on results Shifts all in the same direction…

Reduction Methods: Choosing a Reagent That Reduces Only What You Want

September 18, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Reagents Reduction Methods: Choosing a Reagent That Reduces Only What You Want The whole topic is selectivity. Two hydride reagents differ enough in reactivity that the choice between them decides the product. BSc & MSc · Organic Chemistry · Method The short answer: Lithium aluminium hydride is powerful and reduces almost every carbonyl derivative; sodium borohydride is milder and reduces aldehydes and ketones while leaving esters and acids untouched. Catalytic hydrogenation reduces alkenes and alkynes and can be poisoned deliberately to stop at the alkene. The two hydride reagents Substrate Lithium aluminium hydride Sodium borohydride Aldehyde Primary…

Actinides: Why They Behave So Differently From Lanthanides

September 17, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · f Block Actinides: Why They Behave So Differently From Lanthanides Two f-block series that look parallel on the periodic table and are not, because the 5f orbitals are far more accessible than the 4f. BSc & MSc · Inorganic Chemistry · Concept The short answer: The 5f orbitals extend further from the nucleus than the 4f orbitals do, so they participate in bonding. The result is a much wider range of oxidation states in the early actinides, more covalent character in their compounds, and complex formation behaviour unlike anything in the lanthanide series. The structural difference Both…

Electrochemical Cells: Notation, Sign Conventions and the Nernst Equation

September 17, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Electrochemistry Electrochemical Cells: Notation, Sign Conventions and the Nernst Equation Most errors in this topic are sign errors, and nearly all sign errors come from not writing the cell reaction down first. BSc & MSc · Physical Chemistry · Method The short answer: By convention the anode is written on the left and the cathode on the right, and the cell potential is the right-hand electrode potential minus the left. Writing the balanced cell reaction before applying the Nernst equation fixes both the sign and the value of n, which is where errors otherwise arise. Cell notation…

Combined Spectroscopy Problems: A Working Order

September 17, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Spectroscopy Combined Spectroscopy Problems: A Working Order Given four spectra of an unknown, the order in which you read them determines whether the problem takes five minutes or thirty. BSc & MSc · Spectroscopy · Method The short answer: Take the molecular formula from the mass spectrum and compute degrees of unsaturation. Use infrared to identify functional groups. Use carbon NMR for the number of distinct carbons. Use proton NMR last, for the detailed skeleton. Reading them in that order means each step narrows the possibilities for the next. The order, and why it matters Read the…

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