Reaction Mechanisms
Organic chemistry looks like thousands of unrelated reactions until you see that almost all of them are the same handful of moves. This book teaches the moves: the curly arrow and what makes one legal, the intermediates every mechanism passes through, then substitution, elimination, addition, carbonyl and aromatic chemistry in full — ending with how anybody knows a mechanism is right.
- 9 parts
- 89 sections
- ~183,151 words
- 9 free extracts — one per part
- CSIR-NET · GATE · IIT-JAM
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Foundations — How Mechanisms Work
Organic chemistry looks like thousands of unrelated reactions until you see that almost all of them are the same handful of moves, repeated. This part teaches those moves. It starts with the curly arrow — what it actually means, and the rules that make an arrow legal — and builds up to reading a reaction energy profile and predicting…
- The curly arrow — what it means and what makes one legal
- Finding the nucleophile and the electrophile — from orbitals, not from memory
- The electronic toolkit — induction, resonance, hyperconjugation and sterics
- Reaction coordinate diagrams
- Intermediates versus transition states
- The rate-determining step, and the Hammond postulate
- Kinetic versus thermodynamic control
- Acidity and basicity — the master variable
- Hard and soft acids and bases in organic reactivity
- The map — what the remaining eight parts do
Reactive Intermediates I — Carbocations & Carbanions
Almost every mechanism in this book passes through a species that cannot be put in a bottle. This part is about the two most important of them. Carbocations explain rearrangements, solvolysis rates and why Friedel–Crafts alkylation so often gives the wrong product; carbanions explain enolate chemistry and most of carbon–carbon bond…
- Geometry, hybridisation, and where the empty orbital points
- The stability ordering — and the reason behind every entry
- Making a carbocation, and what it does next
- Wagner–Meerwein 1,2-shifts — and predicting which group moves
- Ring expansion and ring contraction
- Bridged ions — the non-classical debate, and neighbouring-group participation
- Geometry, hybridisation and the inversion barrier
- What stabilises a carbanion — four mechanisms and one retracted explanation
- Enolates and organometallics — carbanions you can actually use
- Where this leads — a pointer to Part 3
Reactive Intermediates II — Radicals, Carbenes, Nitrenes & Arynes
The intermediates in this part behave nothing like ions. Radicals ignore most of what you learned about nucleophiles and electrophiles and follow their own selectivity rules. Carbenes come in two electronic flavours that give different stereochemistry from the same reaction. Nitrenes drive four different named rearrangements. Arynes make…
- Radical structure, geometry and stability
- Chain reactions — initiation, propagation, termination
- Selectivity — halogenation, NBS, and radical addition
- Carbene electronic structure — singlet versus triplet
- Making carbenes — α-elimination, diazo compounds, carbenoids
- Carbene reactions — cyclopropanation, insertion, Wolff
- Nitrenes — generation and structure
- Hofmann, Curtius, Lossen and Schmidt — one mechanistic family
- Arynes, ylides and radical ions
- Where this leads — a pointer to Part 4
Nucleophilic Substitution at Saturated Carbon
This is the most examined mechanism in organic chemistry and the one students think they already understand. The two-mechanism picture taught at undergraduate level is a simplification, and the exam knows it. This part builds the real picture: the continuous spectrum between the two extremes, the ion pairs that explain partial…
- The SN2 transition state, backside attack and Walden inversion
- Substrate structure — steric control, and the carbons that never react
- The nucleophile — nucleophilicity is not basicity
- The SN1 mechanism, the rate law, and the price of a carbocation
- Ion pairs — the mechanism between the two mechanisms
- Solvolysis, the Winstein–Grunwald treatment, and the borderline region
- Leaving-group ability and its correlation with conjugate-acid pKa
- Solvent effects on both mechanisms — the Hughes–Ingold analysis
- Ambident nucleophiles, allylic systems, and the decision procedure
Elimination Reactions
Elimination is where stereochemistry stops being decoration and starts determining the answer. An E2 reaction will not happen at all unless the hydrogen and the leaving group can reach the right geometry, which is why two diastereomers of the same compound can give different alkenes — or one of them nothing at all. This part covers the…
- The E2 mechanism and its rate law
- The anti-periplanar requirement and its orbital basis
- Zaitsev versus Hofmann — which alkene?
- The variable transition state, the E1–E2–E1cb spectrum, and kinetic isotope effects
- The E1 mechanism and its relationship to SN1
- The E1cb mechanism
- Syn eliminations and their cyclic transition states
- Bredt’s rule — alkenes that cannot be isolated
- Substitution or elimination? The complete decision procedure
- Where Part 5 leaves you, and what Part 6 does with it
Additions to C=C and C≡C
Every addition in this part is a competition between two things: which end of the double bond the electrophile attacks, and which face it attacks from. Get both right and you have the product. This part explains where Markovnikov’s rule comes from rather than asking you to memorise it, shows why bromination gives anti addition while…
- The electrophilic addition mechanism and its carbocation intermediate
- Markovnikov selectivity, derived from cation stability — and the rearrangements that prove the cation is real
- Bridged ions: halonium, mercurinium, and the anti stereochemistry they enforce
- Hydroboration–oxidation — concerted, syn, and anti-Markovnikov
- Oxymercuration–demercuration and catalytic hydrogenation
- Oxidative additions: epoxidation, dihydroxylation and ozonolysis
- Radical addition of HBr and the peroxide effect
- Conjugate addition to electron-poor alkenes, and the 1,2/1,4 question
- Alkynes — the same mechanisms with a worse intermediate
- Where this goes next
Carbonyl & Acyl Mechanisms
The carbonyl group is the single most important functional group in organic chemistry, and it does three quite different things depending on what is attached to it. It adds nucleophiles. It substitutes them, if there is a leaving group. And it makes the hydrogens next door acidic. This part takes each in turn, and gives ester hydrolysis…
- Nucleophilic addition to C=O — trajectory, electronics, sterics
- The addition reactions themselves — cyanohydrins, bisulfite, hydride, hemiacetals and acetals
- Nitrogen nucleophiles — imines, oximes, hydrazones, enamines, and the pH–rate profile
- The tetrahedral intermediate and the reactivity order of the acid derivatives
- Ester hydrolysis in full — the Ingold classification and the evidence
- Transesterification, amides, anhydrides and acid chlorides
- Keto–enol tautomerism and the acidity of the α-hydrogen
- α-Halogenation and alkylation
- Aldol and Claisen — one mechanistic idea
Aromatic Substitution Mechanisms
Aromatic rings react by mechanisms that exist nowhere else, because the ring insists on getting its aromaticity back. This part covers all four routes — the familiar electrophilic one, the addition–elimination that needs electron-withdrawing groups, the elimination–addition that goes through a triple bond inside a ring, and the radical…
- The two-step mechanism, the arenium ion, and the evidence that step 1 is rate-determining
- Making the electrophile: nitration, halogenation, sulfonation and Friedel–Crafts
- Activating and deactivating groups — and why the halogens break the pattern
- Ortho:para ratios, competing substituents and the order of a synthesis
- Nucleophilic aromatic substitution by addition–elimination: the Meisenheimer complex
- Benzyne and the SRN1 radical chain — substitution without activation
- Partial rate factors — measuring a directing effect instead of asserting it
- The Hammett equation, σ+ and σ−
- What a change in ρ tells you — and where this goes next
Determining a Mechanism — The Physical Organic Toolkit
The previous eight parts told you what the mechanisms are. This one tells you how anybody knows. Every mechanism in this book was established by experiment, and the exam frequently asks you to reason in that direction — here is the evidence, what does it prove? This part covers the standard tools, then works through real cases where the…
- Rate laws — what the order in each component does and does not tell you
- Catalysis — specific, general, nucleophilic and intramolecular
- Kinetic isotope effects — locating the atom that moves
- The Hammett equation — substituents as a probe of charge
- The Taft treatment — separating sterics from electronics
- Solvent effects and activation parameters
- Stereochemistry, crossover experiments and isotopic labelling
- Trapping and direct observation of intermediates
- Case studies — four disputes the evidence settled
- The nine parts at a glance
- Section-by-section index, A.1 to J.9
- What this book has built
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