Methods of Determining Reaction Mechanisms

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 INVERSIONSN
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