Hammonds postulates

Hammond postulate, transition state geometry, chemistry

1. Hammond’s postulates DR.AMBEDKAR COLLEGE 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 (Reactant) (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 S N 1 mechanism of alkyl halides which proceeds through the formation of carbocation as an intermediate.

10. Application of the Hammond’s postulate to the S N 1 reaction Since CH 3 + is less stable than (CH 3 ) 3 C + E n [1] > E n [2] Reaction [1] is slower Fig. Energy diagram for carbocation formation in two different S N 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 S N 1 or an S N 2 mechanism. The alkyl halide : CH 3 X, RCH 2 X, R 2 CHX, OR R 3 CX 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 S N 1 Decreasing alkyl substitution favors S N 2 Increase rate of the S N 1 reaction Increasing rate of S N 2 reaction H H H C X R H H X C H R R X C R R R C X S N 1 Both S N 1 and S N 2 S N 2 methyl 1 2 3

13. Vinyl and aryl halides S N 1 or S N 2 reaction occur on sp 3 hybridized carbons. Vinyl and aryl halides. Which have a halogen attached to sp 2 hybridized carbon, do not undergo S N 1 or S N 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 S N 2 reaction. Weak nucleophile, such as H 2 O and ROH favor S N 1 reactions by decreasing the rate of any competing S N 2 reaction. Consider what happens when the 2 alkyl halide A, which can react by either mechanism, is treated with the strong nucleophile HO - or the weak nucleophile H 2 O OH - H 2 O (Strong nucleophile) (Weak nucleophile ) CH 3 Br Cis-1-bromo-4-methyl-cyclohexane

15. Effect of leaving groups A better leaving group increases the rate of both S N 1 and S N 2 reactions. Transition state of the S N 2 mechanism Transition state of the rate-determining step of the S N 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 H 2 O and ROH favor S N 1 reaction because the ionic intermediates (both cation and anions) are stabilized by solvation. Polar aprotic solvent favor S N 2 reactions 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.

18. Thank you