Hammond postulate

The Hammond Postulate is a kinetics theory of physical chemistry .

It was formulated by George S. Hammond ( California Institute of Technology ) and John E. Leffler ( Florida State University ). Therefore it is also known in the Anglo-American area as the "Hammond-Leffler postulate".

The Hammond postulate describes the molecular geometries of the transition states in chemical reactions. According to the Hammond postulate, the energetically closer states in a chemical reaction are also geometrically, i.e. H. structurally more similar. From this it follows that in exothermic reactions the geometry of the transition state is more similar to the educt , since the energy difference between educt and transition state is smaller (early transition state) than the difference between transition state and product . In endothermic reactions, however, it is the other way round, since the transition state is energetically and thus geometrically closer to the product (later transition state).

Application example

With the help of the Hammond postulate, for example, the relative reactivity of the CH bonds in n -butane or isobutane (gas phase 27 ° C.) with respect to halogen atoms in radical substitution can be described; see the following table.

radical	Primary CH bond	Secondary CH bond	Tertiary CH bond
F. ${\displaystyle \cdot }$	1	1.2	1.4
Cl ${\displaystyle \cdot }$	1	3.9	5.1
Br 1) ${\displaystyle \cdot }$	1	32	1600

1) at 127 ° C

If one considers fluorine versus bromine, one can describe the relative reactivities as follows with the help of the Hammond postulate. The reactive F atom reacts exothermically with a RH bond (R = rest), the transition state is, according to Hammond's postulate, reactant-like.
The less reactive Br atom usually reacts endothermically to the F atom; according to Hammond's postulate, the activated complex is product-like here.
This means that the RH bond in the transition state R H F is only slightly stretched, but in the R H Br the RH bond is already largely split. It follows that RH bonds of different strengths have little effect on the rate of the reaction with the highly reactive F , but RH bonds of different strength have a great influence on the speed of the reaction with the less reactive Br . As a result, it can be concluded that F has a low selectivity due to its high reactivity rate, while Br has a high selectivity due to the great influence of different RH bonds on the reaction rate. It can be seen that the Hammond postulate accurately describes the relative reactivities. ${\displaystyle \cdot \cdot \cdot }$ ${\displaystyle \cdot \cdot \cdot }$ ${\displaystyle \cdot \cdot \cdot }$ ${\displaystyle \cdot \cdot \cdot }$
${\displaystyle \cdot }$ ${\displaystyle \cdot }$

See also

• Kinetics (chemistry)
• Transition State Theory
• Selectivity (chemistry)

Individual evidence

1. ^ Organikum , 23rd edition, WILEY-VCH, Weinheim 2009. ISBN 978-3-527-32292-3 , p. 198.

literature

• Hammond, GS (1955): A Correlation of Reaction Rates . J. Am. Chem. Soc., 77, 334-338.
• Leffler, JE (1953): Parameters for the Description of Transition States . Science, 117, 340-341.
• Organikum: 23rd edition , WILEY-VCH, Weinheim 2009 ISBN 978-3-527-32292-3 .