Mechanism of Organic Reactions

Paper Code: 
CHY 222
Contact Hours: 

To learn the concept of substitution, addition and elimination reactions and their reaction mechanism.

Unit I: 
Substitution v/s Elimination

Pre requisite: SN1 and SN2 mechanism of alkyl halides

Aliphatic Nucleophilic Substitution: SN1, SN2, mixed SN1 and SN2, ion pair and SN1 mechanism, SNi mechanism, SET mechanism; neighbouring group participation and anchimeric assistance; substitution at allylic and vinylic carbon atoms; ambident nucleophiles; effects of substrate structure, attacking nucleophile, leaving group and reaction medium on reactivity; regioselectivity.

Pre requisite: E1, E2 mechanism of alkyl halides, Hoffmann and Saytzeff rules.

Elimination Reaction:  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.

Self Study: hydrolysis of esters (acid and base catalyzed mechanism).

Unit II: 
Aromatic Nucleophilic Substitution

 SNAr, SN1, benzyne and SRN1 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.

Unit III: 
Electrophilic Substitution Reaction

(a) Aliphatic Electrophilic Substitution: bimolecular mechanism – SE2 and SEi; the SE1 mechanism, substitution by double bond shift; addition-elimination mechanism and cyclic mechanism; effect of substrates, leaving group and solvent polarity on the reactivity,

nitrogen electrophiles- aliphatic diazonium coupling, direct formation of diazo compounds, direct amination,  metalation with organometallic compounds, trans metalation with metal and metal halides.

(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).

Unit IV: 
Free radical Substitution Reactions

Long lived and short lived radicals, detection and characteristics of free radicals; neighbouring group participation and free radical rearrangements; mechanism at an aromatic substrate, reactivity for aliphatic, aromatic substrate at bridge head carbon atom, reactivity of the attacking radical, effect of solvent.

Important reactions involving free radicals – Wohl-Ziegler bromination, autooxidation, Sandmeyer and Gattermann reaction, Hunsdiecker reaction, Gomberg-Bachmann reaction,oxidation of aldehydes to carboxylic acid, coupling of alkynes .

Unit V: 
Addition to C-C and C-Hetero Multiple Bonds

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 double and triple bonds, hydrogenation of aromatic rings, hydroboration, Michael reaction, Sharpless asymmetric epoxidation.

Addition to C-Hetero Multiple Bonds: mechanism of metal hydride reduction of saturated and unsaturated carbonyl compounds, acids, esters and nitriles; Tollens reaction; addition of grignard reagents, organozinc and organolithium reagents to unsaturated  carbonyl system

Mechanism of condensation reactions involving enolates – Aldol, Knoevenagel, Claisen, Mannich, Benzoin, Perkin and Stobbe reactions.

Essential Readings: 
  1. Advanced Organic Chemistry: Reactions, Mechanisms and Structure; Fourth Edition; Jerry March; John Wiley and Sons Asia Private Limited, New Delhi, 2007
  2. Advanced Organic Chemistry Part A & B; Fourth Edition; Francis A. Carey and Richard J. Sundberg; Kluwer Academic/Plenum Publishers, New York, 2000.
  3. Principles of Organic Synthesis; Third Edition; R.O.C. Norman and J.M. Coxon; Nelson Thornes, United Kingdom, 2003.
  4. A Guidebook to Mechanism in Organic Chemistry, Peter Sykes, Sixth Edition; Orient Longman; New Delhi, 2002.
  5. Basic Principles of Organic Chemistry, First Edition; John D. Roberts and Marjorie C. Caserio; W. A. Benzamin Inc., New Delhi, 1964.
  6. Organic Chemistry .Vol. I and II ;Singh, Mukherjee, Kapoor , Wiley Eastern Limited ,New Delhi ,1993