Mechanism of Organic Reactions

Aliphatic Nucleophilic Substitution: S_N1, S_N2, mixed S_N1 and S_N2, ion pair and S_N1 mechanism, S_Ni mechanism, neighbouring group participation; substitution at allylic and vinylic carbon atoms, ambident nucleophiles, effect of substrate structure, nucleophile, leaving group, reaction medium on the reactivity. Regioselectivity.

Elimination Reactions:  E2, E1, E1CB and E2C (syn elimination) mechanisms, E1 – E2 – E1CB spectrum, orientation of the double bond, effect of substrate structure, attacking base, leaving group and reaction medium on reactivity. Mechanism and orientation in pyrolytic elimination.

(a) Aromatic Nucleophilic Substitution: S_NAr, benzyne and S_RN1 mechanism, effect of substrate structure, leaving group and attacking nucleophiles on reactivity. Typical reactions –  Bucherer reaction, Rosenmund, Von-Braun reaction, von-Richter, Sommelet-Houser and Smiles rearrangement.

(b) Aromatic Electrophilic Substitution: Arenium ion mechanism, orientation and reactivity,  energy profile diagrams. Directive influence and its explanation in different substitutions. o/p ratio, ipso attack, substitution reactions involving diazonium ions, Vilsmeir-Haack reaction., Friedel-Craft reaction: alkylation, arylation (Scholl reaction), acylation (ring closer, Haworth reaction, Hoesch reaction).

Aliphatic Electrophilic Substitution: bimolecular mechanism– SE2 and SEi, the SE1 mechanism, substitution accompanied  by double bond shifts, halogenation, sulphonation,  aliphatic diazonium coupling, direct amination, metalation with organometallic compounds, trans metalation with metal and metal halides, insertion by nitrenes, insertion by carbenes, acylation at an aliphatic carbon. Effect of substrates, leaving group and solvent polarity on the reactivity.

Free radical substitution mechanisms, mechanism at an aromatic substrate, neighbouring group participation and free radical rearrangements,  reactivity for aliphatic and aromatic substrates, reactivity at bridgehead,  reactivity of the attacking radical and effect of solvent on reactivity.

Important reactions involving free radicals: Sandmeyer and Gattermann reaction, Hunsdiecker reaction, Gomberg-Bachmann reaction, coupling of alkynes.

Addition to C-C multiple bond: mechanistic and stereochemical aspects of addition reaction involving electrophiles, nucleophiles and free radical, regio and chemo selectivity, orientation and reactivity, addition to cyclopropane ring, hydrogenation of aromatic rings.

Addition to C-Hetero Multiple Bonds: mechanism of metal hydride reduction of saturated and unsaturated carbonyl compounds, acids, esters and nitriles, addition of Grignard reagents, organozinc, organocopper and organolithium reagents to carbonyl and  unsaturated  carbonyl systems.

Formation of enolates, kinetic and thermodynamic control, 1, 2 vs 1, 4  addition, HSAB principle, reactions of enolate anions with electrophiles: O and C alkylation. Enolate condensation reactions: synthetic applications of inter and intramolecular Aldol condensations, Claisen, Dieckmann, Knoevenagel, Stobbe’s condensations. Stereoselective enolate reactions: diastereoselection, Zimmermann-Traxler model, Evans mode. Baylis-Hillmann reaction, Robinson annulation.