The document discusses Hammond's postulates, which state that the transition state of a chemical reaction resembles the structure of the species (reactant or product) that is closer in energy. Specifically: 1) For exothermic reactions, the transition state resembles the reactants more than the products. 2) For endothermic reactions, the transition state resembles the products more than the reactants. 3) Hammond's postulates can be used to predict reaction mechanisms and explain factors that influence reaction rates.

1. HAMMOND’S POSTULATES
DR.AMBEDKARCOLLEGE
PRESENTED BY
SACHIN KENDRE
MSC SEM-I
FACULTY OF CHEMISTRY

2. TRANSITION STATE
• Transition state are highly unstable, in which only partial bonds are present. Due to unstability transition state cannot be
isolated.
• In T.S Nor all bonds are bonds are formed or all bonds are braked, due to which T.S. Is unstable.
• A2 + B2 2AB
• So we can say that,
Transition state :- unstable, non isolate compound formed as bonds are broken and made
(Reactan
t)
(Product
)
A A
B B
Bond break
Bond formation
A
B
A
B
+
A A
B B

3. HISTORY
• In 1955 George Hammond's, a young professor at low state university, postulated that
transition- state theory could be use to qualitatively explain the observed structure-
reactivity relationship. Notably, john e, Leffler of Florida state university proposed a similar
idea in 1953.

4. HAMMOND’S POSTULATE
• Hamond’s postulate ( or alternatively the Hammond‘s-Leffler postulate), is a
hypothesis in physical organic chemistry which describes the geometric structure
of the transition state in an organic chemical reaction.
• The Hammond’s postulate state that,
‘The transition state of a reaction resembles the structure of the species ( reactant or
product) to which it is closer in energy’
• It means known we can predict the geometric structure of a transition state by
comparing its energy to the species neighboring it along the reaction coordinate.

5. EXOTHERMIC REACTION
• If the reaction is exothermic the transition state is reached relatively early on
the reaction coordinate . bond breaking and bond forming has not occurred to
a large extent. And the structure of the transition state resembles the reactant
more than the product.
• Therefore, the transition state will be more geometrically similar to the
reactants than to the products.

6. EXOTHERMIC REACTION
• The transition state resembles the reactants.

7. ENDOTHERMIC REACTION
• Bond breaking (in the reactant) and bond formation (in the product) has
occurred to a large extent and the structure of the transition state is more like
that of the product than the reactant.
• So, according to Hammond’s postulate the structure of the transition state
would resemble the products more than the reactants.

8. ENDOTHERMIC REACTION
• The transition state resembles the
products.

9. SOME APPLICATION OF HAMMOND
POSTULATE,
 Easily explain the relationship between the rate of a reaction and the stability of
the products.
 Application on electrophilic addition reactions which proceed through the
formation of carbocation which formed by protonation of an alkene is an
endergonic step.
 Explain the selectivity of product formation during bromination and chlorination
of alkanes.
 Useful in rationalizing the SN
1 mechanism of alkyl halides which proceeds
through the formation of carbocation as an intermediate.

10. APPLICATION OF THE HAMMOND’S POSTULATE
TO THE SN
1 REACTION
• Since CH3
+ is less stable than (CH3)3C+
• En [1] > En [2]
• Reaction [1] is slower
Fig.
Energy diagram for carbocation
formation in two different SN
1
reactions

11. PREDICTING THE MECHANISM OF
NUCLEOPHILIC SUBSTITUTIONS
REACTIONS
• Four factors are relevant in predicting whether a given reaction is likely to proceed by an SN
1 or
an SN
2 mechanism.
• The alkyl halide : CH3X, RCH2X, R2CHX, OR R3CX
• The nucleophile: strong or weak
• The leaving group: good or poor
• The solvent: protic or aprotic

12. NATURE OF THE ALKYL HALIDE
• THE MOST IMPORTANT FACTOR IS THE IDENTITY OF THE ALKYL HALIDE.
• Increasing alkyl substitution favors SN
1
• Decreasing alkyl substitution favors SN
2
Increase rate of the SN
1 reaction
Increasing rate of SN
2 reaction
H
H
H
C X R
H
H
X
C
H
R
R
X
C
R
R
R C X
SN
1
Both
SN
1 and SN
2
SN
2
methyl 1
0
20 30

13. VINYL AND ARYL HALIDES
• SN
1 or SN
2 reaction occur on sp3 hybridized carbons.
• Vinyl and aryl halides. Which have a halogen attached to sp2 hybridized carbon, do not undergo SN
1 or
SN
2 reactions.
• Heterolysis of the C-X bond would form a highly unstable vinyl or aryl cation.
C C
X
X
C
C
H
H
H
Br
C
H
C
H
H Br-
+
A vinyl halide
A phenyl halide or aryl halide
sp hybridized
A vinyl carbocation
highly unstable

14. EFFECT OF THE NUCLEOPHILE
• Strong nucleophiles ( which usually bear a negative charge ) present in high concentration favor SN
2
reaction.
• Weak nucleophile, such as H2O and ROH favor SN
1 reactions by decreasing the rate of any
competing SN
2 reaction.
• Consider what happens when the 20 alkyl halide A, which can react by either mechanism, is treated
with the strong nucleophile HO- or the weak nucleophile H2O
OH
-
H2O
(Strong
nucleophile)
(Weak nucleophile)
CH3
Br
Cis-1-bromo-4-methyl-
cyclohexane

15. EFFECT OF LEAVING GROUPS
• A BETTER LEAVING GROUP INCREASES THE RATE OF BOTH SN
1 AND SN
2 REACTIONS.
Transition state of the
SN
2 mechanism
Transition state of the rate-
determining step of the SN
1
mechanism
A better leaving group is more able to accept the negative charge
Nu
-
C X
-
+
+
C X-
+
+
+

16. EFFECT OF SOLVENT
• Polar protic solvent like H2O and ROH favor SN
1 reaction because the ionic
intermediates (both cation and anions) are stabilized by solvation.
• Polar aprotic solvent favor SN
2reactions because nucleophiles are not well
solvated, and therefore, are more nucleophilic.

17. LIMITATION OF HAMMOND
POSTULATE
 The Hammond postulate makes a connection between rate
(kinetics) and equilibrium (thermodynamics) that has no
theoretical basis.
 Use of the Hammond postulate is based primarily on enthalpy
considerations and neglects the effects that activation entropies
can have on reaction rates.
 The Bronsted catalysis law ( an empirical linear free energy
relationship) is valid within carefully chosen groups of acid or
bases.