Carbocation Rearrangements: Predicting the Unexpected Product

September 15, 2026

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Organic Chemistry · Mechanism Carbocation Rearrangements: Predicting the Unexpected Product When the product has a skeleton different from the starting material, a carbocation rearranged. Knowing when to expect it is most of the skill. BSc & MSc · Organic Chemistry · Concept The short answer: A carbocation rearranges when a hydride or alkyl group can migrate from an adjacent carbon to give a more stable cation. The migration is concerted with nothing else and requires no reagent. Recognising that a more stable cation is available is what predicts the rearranged product. The stability ladder tertiary > secondary > primary >…

Five-Membered Heterocycles: Aromaticity and Reactivity Order

September 14, 2026

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Organic Chemistry · Heterocycles Five-Membered Heterocycles: Aromaticity and Reactivity Order Three rings with the same shape and very different reactivity. The difference tracks one property of the heteroatom. BSc & MSc · Organic Chemistry · Concept The short answer: Pyrrole, furan and thiophene are all aromatic six-electron systems in which the heteroatom donates its lone pair to the ring. Their reactivity toward electrophiles follows the heteroatom’s willingness to share that pair, so pyrrole is most reactive and thiophene least among the three toward electrophilic attack. Why they are aromatic Each ring has two carbon–carbon double bonds, contributing four π electrons….

Organolithium and Organocuprate Reagents

September 14, 2026

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Organic Chemistry · Reagents Organolithium and Organocuprate Reagents Three carbon nucleophiles that look interchangeable and are not. The differences in reactivity are exactly what makes each useful. BSc & MSc · Organic Chemistry · Method The short answer: Organolithiums are more reactive and more basic than Grignards, so they attack hindered substrates but tolerate fewer functional groups. Organocuprates are much softer and add conjugately to enones rather than at the carbonyl — which is the single most useful distinction among the three. The three compared Grignard Organolithium Organocuprate Reactivity Moderate High Moderate but selective Basicity Strong Very strong Weak Character…

Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry

September 13, 2026

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Organic Chemistry · Aromatic Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry One functional group that can be replaced by almost anything, which makes it the standard route to substituents that cannot be installed directly. BSc & MSc · Organic Chemistry · Method The short answer: An aromatic amine treated with nitrous acid at low temperature gives a diazonium salt. Nitrogen is an outstanding leaving group, so the diazonium group can be replaced by halide, hydroxyl, cyano, hydrogen and more — giving access to substitution patterns unreachable by direct electrophilic substitution. Preparation An aromatic primary amine is treated with…

Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be

September 12, 2026

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Organic Chemistry · Mechanism Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be An unexpected rate enhancement together with retention of configuration is the signature. Both point to the same cause. BSc & MSc · Organic Chemistry · Concept The short answer: A group elsewhere in the molecule can attack the reacting centre internally, forming a cyclic intermediate before the external nucleophile arrives. Because two inversions occur in sequence, the overall configuration is retained — and the reaction is much faster than the substrate structure alone would predict. The two observations that reveal it Neighbouring group participation…

Birch Reduction and Dissolving Metal Reactions

September 12, 2026

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Organic Chemistry · Reduction Birch Reduction and Dissolving Metal Reactions A reduction that stops exactly where you want it, and whose regiochemistry is decided by the substituent already on the ring. BSc & MSc · Organic Chemistry · Concept The short answer: A metal dissolved in liquid ammonia supplies solvated electrons that reduce an aromatic ring to a non-conjugated cyclohexadiene. Electron-donating substituents end up on a double bond carbon; electron-withdrawing substituents end up on a saturated carbon — and that opposite outcome is the standard question. The reagent An alkali metal dissolved in liquid ammonia gives a deep blue solution…

Mass Spectrometry: Reading Fragmentation Patterns

September 11, 2026

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Organic Chemistry · Spectroscopy Mass Spectrometry: Reading Fragmentation Patterns The molecular ion gives the mass. The fragments give the structure, and they fragment in predictable ways. BSc & MSc · Spectroscopy · Method The short answer: The molecular ion peak gives the molecular mass, and isotope peaks reveal certain elements immediately. Fragmentation follows rules based on which cation is most stable, so the losses observed identify the groups present. The nitrogen rule links an odd molecular mass to an odd number of nitrogens. What the spectrum shows The sample is ionised, and the resulting ions are separated by mass-to-charge ratio….

The Wittig Reaction: Making Alkenes Where You Want Them

September 11, 2026

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Organic Chemistry · Synthesis The Wittig Reaction: Making Alkenes Where You Want Them Its value is not that it makes alkenes but that it puts the double bond in an unambiguous position, which elimination cannot guarantee. BSc & MSc · Organic Chemistry · Method The short answer: A phosphorus ylide adds to a carbonyl, and the resulting four-membered intermediate collapses to give an alkene and a phosphine oxide. The double bond forms exactly where the carbonyl was, with no possibility of migration — which is what makes it superior to elimination for regiochemical control. Why it matters Elimination reactions make…

Free Radical Reactions: Chain Mechanisms and Selectivity

September 10, 2026

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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…

Aromatic Nucleophilic Substitution and the Benzyne Mechanism

September 9, 2026

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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…

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