Subha Arjunan
• Mechanism of a reaction gives the information about the process, in
which reactants are transferred into products.
Mechanism provides the information about:
o whether the reaction proceeds through single or multistep
o the transition state(T.S) and/or intermediates and the structure of
the T.S and/or intermediate
o The bond formation and cleavage (if both occurs in a single step
mechanism gives their order of occurrence)
o The stereochemistry of the reaction
o Any selectivity shown by the reaction
o Whether the reaction is reversible or not
o If two or more products formed whether the reaction shows Kinetic
or Thermodynamic Control
Single Step Reaction
• Also called Concerted Reaction
• Conversion of reactants into
products involves only one
step (i.e) A
B
A
Reactant
B
Product
• The reactant may be more than
one. (Ex.) A+P
B
Multistep Reaction
• Conversion of reactants into
products involves more than
one step
(i.e) A
B
C
D
• Hydrolysis of t-butyl bromide
H2O
H3C
H3C
• Alkaline Hydrolysis of MeI
H3C
I
+
-
HO
H3C OH
Br
OH
Mechanism:
• Single step reaction has no
intermediate and has only one
transition state
Mechanism:
• This type of reaction has n-1
no. of intermediate and n no. of
transition state.
• n
number of steps in a
reaction. For ex., 2 step reaction
has 2 T.S and 1 intermediate
Hydrolysis of t-butyl bromide-Mechanism:
(Sn1 type reaction)
Hydrolysis of MeI Mechanism: (Single step reaction)
(Sn2 type reaction)
Note: Sn1 reaction involves two steps (multi step)
Sn2 reaction involves single step
Transition State
• T.S is highly unstable hence its
lifetime is very short (zero
lifetime). So T.S cannot be
isolated from the reaction
mixture.
Intermediate
• Intermediates are stable. Some
are even stable for years. So it
can be easily isolated. Life time
10-13 sec.
• T.S occurs at energy maxima in • Intermediates occur at energy
minima in the energy profile.
the energy profile.
• In T.S, bond formation or
cleavage occur only partially.
(refer previous slide)
• Intermediates have complete
bond formation and bond
cleavage.
Energy Profile:
• The energy profile renders the information about energies of the
reaction
• In an energy profile,
horizontal axis depicts the progress of the reaction, knows as
“REACTION COORDINATE”(R.C).
•
•
R.C represents the changes in bond orders, bond distances and
geometries when reactants are transformed into the products.
vertical axis depicts the free energy of the reaction
Energy Profile: (Free energy changes of a single step reaction)
Free energy changes of a multistep reaction:
Types of Organic Reactions:
Substitution
Reaction
Redox
Reactions
Addition
Reaction
Organic
Reactions
Rearrangement
Elimination
Reaction
Bond Making and Breaking:
(Bond cleavage amd formation may be Homolytic or Heterolytic)
fission will occur)
will produce radical. (If bond is weak or non polar,
homolytic
Homolytic bond formation occurs in the same way.
will produce ions. (electron move towards the atom which
is more electronegative than the other)
Electrophiles
• Electron Deficient species
• Electron Loving
• Since e- deficient it can
accept an e- pair from
nucleophile to obtain a stable
configuration
• Ex: Lewis
acids(BF3,AlCl3,ZnCl2),
positively charges species
Nucleophiles
• Electron Rich species
• Nucleus loving
• It can donate an e- pair to
Electrophiles
• Ex: neutral species having
lone pair (H2O, NH3),
Anions (-OH, Cl-)
Leaving Group
A functional group that is eliminated with electrons of the
σ bond in a reaction
Solvent
Used to separate and solvate the ions involved in the
reaction
Activation
energy or
Energy barrier
Minimum amount energy which molecules must have to
react
Rearrangement
Migration of an atom or a group of atoms from one atom
to another within a molecule(Intra molecular
rearrangement) or from one molecule to
another(Intermolecular rearrangement)
Catalyst
Substance which used to change (may accelerate or
decrease) the rate of the reaction
per
This represents the O-O bond. Ex: Hydrogenperoxide(HO-O-H)
References:
Organic Chemistry – L.G. Wade JR
Organic Chemistry – Clayden, Greeves, Warren and Wothers
Organic Chemistry (An Intermediate Text) – Robert V. Hoffman