mangal kamble M pharmacy pharmaceutical chemistry department Dr. D.Y. Patil college of pharmacy Akurdi Pune.

1. Types of Reaction Mechanism & Methods of Determination
By
Mangal Kamble
M pharm First Year
Sub-Advanced Organic Chemistry-I
Dr. D.Y. Patil college of Pharmacy Akurdi, Pune.

2. ORGANIC REACTION &Mechanism
Organic reactions are chemical reactions involving organic
compounds. The basic organic chemistry reaction types are addition
reactions, elimination reactions, substitution reactions, pericyclic
reactions, rearrangement reactions and redox reactions.
A reaction mechanism is the step by step sequence of elementary
reactions by which overall chemical change occurs

3. Nucleophile
• A reagent which can donate an electron pair in a reaction is called
a nucleophile. The name nucleophile means nucleous loving and
indicates that it attacks regions of low electron density (positive
centres) in the substrate molecule.
• Nucleophiles are electron rich.
• They may be negative ions including carbanions or neutral
molecules with free electron pair.
• A nucleophile can be represented by a by general symbol Nu:
Examples
Cl-, Br-, I-, CN -, OH-, RCH2
-, NH3, RNH2, H2O, ROH

4. Electrophiles
• A reagent which can accept an electron pair in a reaction called
an electrophile.
• The name electrophile means electron-loving and indicates that it
attacks regions of high electron density (negative centres) in the
substrates molecule.
• Electrophiles are electron deficient.
• They may be positive ions including carbonium ions or neutral
molecules with electron deficient centres
• An electrophile can represented by E+
• Examples
• H+, Cl+, Br+, I+, NO2
+, R3C+, +SO3H, AlCl3, BF3

5. Addition reactions – Electrophile Addition
An electrophilic addition reaction is an addition reaction where, in a chemical
compound, a π bond is broken and two new σ bonds are formed. The substrate
of an electrophilic addition reaction must have a double bond or triple bond.
Types of Reactions
• Addition reaction
• Substitution reaction
• Elimination reaction
• Rearrangement reaction

6. • In step 1, the positively charged intermediate combines with (Y) that is
electron-rich and usually an anion to form the second covalent bond.
• Step 2 is the same nucleophilic attack process found in an SN1 reaction.
The exact nature of the electrophile and the nature of the positively
charged intermediate are not always clear and depend on reactants and
reaction conditions.

8. Elimination reaction –
Two or four atoms or groups attached to the adjacent carbon atoms in the substrate molecule
are eliminated to form a multiple bond. Elimination reactions are important as a method for
the preparation of alkenes. The two most elimination reactions are:
• Dehydration (-H2O) of alcohols, and
• Dehydrohalogenation (-HX) of alkyl halides.
Dehydration (-H2O) of alcohols
Two groups on adjacent atoms are lost as a double bond is
formed.

9. Dehydrohalogenation (-HX) of alkyl halides
involves the elimination of the halogen atom and of a hydrogen atom
from a carbon adjacent to the one losing the halogen. The reagent
required is a base, whose function is to abstract the hydrogen as a
proton.
Dehydrohalogenation is a b elimination or1,2-elimination reaction
because the carbon holding the halogen is designated as a or 1-carbon
and elimination involves the loss of b or 2-hydrogen from b or 2-
carbon adjacent carbon holding the halogen.

10. There are three fundamental events in these elimination
reactions:
• Removal of a proton
• Formation of the CC  bond
• Breaking of the bond to the leaving group

11. Substitution Reaction-
The reactions in which an atom or group of atoms in a molecule is replaced or
substituted by different atoms or group of atoms are called substitution Reaction.
For example

12. 3
Rearrangement Reactions-
A rearrangement reaction is a broad class of organic reactions
where the carbon skeleton of a molecule is rearranged to give a
structural isomer of the original molecule.
The reactions which proceed by a rearrangement or shifting of
atoms/groups from one position to another in the molecule to produce
a structural isomer of the original substance are called rearrangement
reaction.

13. Methods adopted for determining the mechanism of a reaction:
• Isotopic labelling technique
• product Analysis
• Identification of reaction intermediate
• Cross over experiments
• Stereo chemical evidences

14. 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.
• After completion of reaction the location of labelled isotopes is
eliminated in the product by Radioactivity.

17. Product Analysis
• 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

18. 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
Detection of Reaction intermediates-
Unstable intermediates are often detected by spectroscopic
method. e.g carbonium ion intermediates formed in SN1 and E1
reaction have been detected by NMR spectroscopy.

19. Isolation-
• some reasonably stable intermediates can be isolated by
stopping the reaction before completion or by using mild
condition.
e.g Hoffmann rearrangement reaction
1) amide amine
2) formation of isocyanate intermediate
3) isolation of isocyanate intermediate

21. Trapping-
• The transient or metastable intermediates formed in many
reaction can not be isolated or detected such intermediates can be
trapped .
• The reaction is carried out in the presence of some
comp(trapping agent) with which intermediate react, this is
called trappig method.

23. CROSSOVER EXPERIMENTS
• To study mechanism of molecular rearrangement reactions
• INTRAMOLECULAR rearrangement or
INTERMOLECULAR rearrangement
e.g.
A+B C+D
This types of reaction are intramolecular in nature
A+B C+D +E +F
This types of reaction are in intermolecular in nature

25. 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.

26. Reference
1)Types of reaction mechanism slideshare presented by Kandarp
Vyas
2) Advanced Organic Chemistry by Dr. Jagdamba
and Dr. Yadav
3) Conceptual Organic Chemistry by Arun Seti
4) http://youtube/G2pd9RRs
5) Organic Chemistry Paul.

27. Thank You