Methods of Determining Reaction Mechanisms - Andria D'Souza
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This document discusses various methods used to determine reaction mechanisms, including isotopic labeling techniques, stereochemical evidence, crossover experiments, identification of products, and identification of reaction intermediates. Isotopic labeling involves replacing atoms with isotopes like deuterium or carbon-13 to follow the reaction path. Stereochemical evidence from chiral reactants and products can indicate SN1 or SN2 mechanisms. Crossover experiments use non-identical reactants to study intramolecular vs intermolecular rearrangements. Identification of products and intermediates through spectroscopy, isolation, or trapping with reagents provides clues about reaction steps.
Methods of Determining Reaction Mechanisms - Andria D'Souza
- 1. METHODS OF DETERMINING REACTION MECHANISMS Andria D’Souza
- 2. What is a mechanism of a reaction?
- 3. What is a mechanism of a reaction? • Step by step description of the reaction • Information about a. position b. shifting and movement
- 4. • Detailed reaction mechanism cannot be elucidated • Only PROPOSED mechanism that is put forward i. energetically reasonable ii. account for the experimental observations iii. in agreement with what is known about analogous reactions
- 6. Methods adopted for determining the mechanism of a reaction: • Isotopic labelling technique • Stereo chemical evidences • Cross over experiments • Identification of product • Identification of reaction intermediate
- 7. ISOTOPIC LABELLING TECHNIQUE • In this method a single atom of any chemical compound is replaced by its isotopes. • 3H Tritium, 2H Deuterium, 13C, 14C, 15N and 18O are the most common isotopes which are frequently used.
- 10. STEREOCHEMICAL EVIDENCES • stereochemistry of the reactant and the product. • optically active substrate optically inactive product RACEMIC MIXTURE SN 1 mechanism • optically active substrate optically inactive product an inverted configuration : WALDEN INVERSIONSN 2 mechanism.
- 13. CROSSOVER EXPERIMENTS • To study mechanism of molecular rearrangement reactions • INTRAMOLECULAR rearrangement or INTERMOLECULAR rearrangement • two non identical reactants differ by one characteristic group
- 16. IDENTIFICATION OF REACTION INTERMEDIATES • Reaction intermediates are an important class of chemical species, which are quite helpful in understanding the mechanism of a chemical reaction. 1. Isolation 2. Detection 3. Trapping
- 17. ISOLATION: • These species by nature are unstable under normal reaction conditions, but they can be isolated under favourable conditions either in pure state or in their form of solution. • The Hoffmann rearrangement amide amine formation of isocyanate intermediate isolation of isocyanate intermediate
- 19. DETECTION: • Stable intermediates long lifetime high concentration detected by spectroscopic methods. • FUN FACT: George Olah of California University got the noble prize in 1994 for his work on cation. He was able to prepare a number of stable, long lived carbocations and recorder their 13C NMR spectra.
- 20. TRAPPING: • Intermediates that are not capable of being isolated, as they are not quite stable or are quite low in concentration, are characterised by trapping them with a suitable reagent. • These reagents form products with the intermediate which are particularly characteristic of the species.
- 21. • Base promoted solvolysis of chloroform when carried out in the presence of cyclohexene, the dichlorocarebene is trapped and an adduct, cyclopropane derivative is obtained
- 23. IDENTIFICATION OF PRODUCTS • basic framework: reactants, intermediates, products • elucidation of the reaction mechanism • identifying intermediates very short lifetime low concentration • UV, IR, ESR, 1H NMR, 13C NMR and Mass
- 24. • The reaction of o-chlorotoulene with a strong base not only gave the expected m-cresol but also, the unexpected o-cresol. The formation of both the products can only be explained by the formation of benzyne intermediate.
- 25. REFERENCES: • Advanced Organic Chemistry by Dr. Jagdamba and Dr. Yadav • Conceptual Organic Chemistry by Arun Seti