Tag: Reaction Mechanism

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 >…

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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…

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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…

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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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Organic Chemistry · Carbonyl Aldol Condensation and Enolate Chemistry Almost every carbon–carbon bond formed at a carbonyl runs through an enolate. Learn the enolate and a dozen named reactions collapse into one idea. BSc & MSc · Organic Chemistry · Concept The short answer: A hydrogen alpha to a carbonyl is acidic because the resulting anion is resonance stabilised as an enolate. That enolate is a nucleophile, and it attacks another carbonyl to give a beta-hydroxy carbonyl. Heating then eliminates water to give the conjugated enone, which is the condensation step. Why the alpha hydrogen is acidic A hydrogen on…

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Organic Chemistry · Mechanism E1 and E2 Elimination: Saytzeff, Hofmann and Which Alkene Forms Elimination questions almost always come down to one thing: which of two possible alkenes is the major product, and why. BSc & MSc · Organic Chemistry · Concept The short answer: E2 is concerted and requires the leaving group and the beta hydrogen to be anti-periplanar. E1 goes through a carbocation. Saytzeff orientation gives the more substituted alkene and is normal; Hofmann orientation gives the less substituted one and arises with bulky bases or charged leaving groups. The two mechanisms E2 is a single concerted step….

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