SN1 & SN2 mechanism

Subsititution Nucleophilic reaction mechanism

1. SN1 & SN2 Mechanism Dr . L. Sakthikumar M.Sc., M.Phil., ph.d Assistant professor in Chemistry, Saiva Bhanu Kshatriya College, Aruppukottai – 626101 , Tamilnadu E-mail : [email protected] 1

2. NUCLEOPHILIC SUBSTITUTION REACTIONS NUCLEOPHILES The word nucleophile comes from nucleus , the positive part of an atom, plus - phile from Greek word philos meaning to love . A nucleophile is any negative ion or any neutral molecule that has at least one unshared electron pair . Eg : 2

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4. MECHANISM OF S N 2 REACTION 4

5. The S N 2 reaction is a Substitution, Nucleophilic , bimolecular reaction. The transition state involves both the nucleophile and the substrate, it accounts for the observed second-order reaction rate. The mechanism for SN2 reaction was p roposed by Edward D. Hughes and Sir Christopher Ingold (the University College, London) in 1937. 5

6. The nucleophile attacks the carbon bearing the leaving group from the back side . The orbital that contains the electron pair of the nucleophile begins to overlap with an empty ( antibonding ) orbital of the carbon bearing the leaving group . The bond between the nucleophile and the carbon atom is forming, and the bond between the carbon atom and the leaving group is breaking at the same time . Because bond formation and bond breaking occur simultaneously in a single transition state, the SN2 reaction is a concerted reaction . The configuration of the carbon atom becomes inverted during SN2 reaction . The reaction is a exothermic reaction where the free energy value G is negative. 6

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8. A free-energy diagram for a hypothetical SN2 reaction that takes place with a negative value of G ° 1 . Most of the S N 2 chemical reactions occur much more rapidly at higher temperatures. 2. A reaction with a lower free energy of activation will occur very much faster than a reaction with a higher one . 8

9. THE STEREOCHEMISTRY OF SN2 REACTIONS 9

10. MECHANISM OF S N 1 REACTION The SN1 reaction is a Substitution , Nucleophilic and Unimolecular . The reaction of tert -butyl chloride with sodium hydroxide in a mixture of water and acetone to form tert -butyl alcohol is a good example for SN1 reaction. The rate of formation of tert -butyl alcohol is dependent only on the concentration of tert -butyl chloride and independent of the concentration of hydroxide ion. The rate equation for this substitution reaction is first order with respect to tert -butyl chloride. 10

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12. A free-energy diagram for the SN1 reaction of tert -butyl chloride with water 12

13. STEREOCHEMISTRY OF SN1 REACTIONS 13

14. FACTORS FAVOURING SN1 &SN2 14

15. THANK YOU 15