Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry

September 13, 2026

| Chemical Science Exams, Organic Chemistry

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…

Conformational Analysis of Cyclohexane

September 8, 2026

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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…

Retrosynthetic Analysis: Thinking Backwards From the Target

September 8, 2026

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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…

Protecting Groups: When to Use One and How to Choose

September 7, 2026

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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…

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