Hammond postulate, transition state geometry, chemistry

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.