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Lecture 5

Lecture 5. Ionic reactions. Q. What is called chemical reactions? Ans. In a chemical reaction, pre-existing bonds are broken and new bonds are formed. Q. What are ionic reactions? Ans. If bond breaking and bond making take place in a heterolytic fashion

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Lecture 5

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  1. Lecture 5

  2. Ionic reactions Q. What is called chemical reactions? Ans. In a chemical reaction, pre-existing bonds are broken and new bonds are formed. Q. What are ionic reactions? Ans. If bond breaking and bond making take place in a heterolytic fashion the reaction is called ionic reaction. On the other hand, if bond breaking and bond making take place in a homolytic fashion, the reaction is called a free radical reaction.

  3. Ionic Reactions : 1) Nucleophilic substitution : 2) Eletrophilic substitution : 3) Nucleophilic addition : 4) Eletrophilic addition : 5) Rearrangements :

  4. Nucleophilic Substitution at a saturated carbon : Two types of discrete mechanism : SN1 SN2 Substitution Nucleophilic Bimolecular Substitution Nucleophilic unimolecular

  5. SN2 mechanism : 2nd order kinetics • Are all SN2 reactions follow 2nd order kinetics? • Ans. If Y- is in large excess as compared to R-X, ( eg. If solvent is • the nucleophile) then Pseudo – first order reaction Effect of structure and solvent : Which mechanism will be operative will depend on structure and solvent polarity.

  6. T.S. for SN2 Inversion of configuration (Walden inversion) Why backside attack? To ensure maximum overlapping leading to stability of T.S.

  7. Important Point to Remember : Inversion of configuration does not mean R going to S or vice versa. It means that bond formation takes place opposite to that of bond breaking….…..which leads to the inversion. it is like inversion of umbrella in a storm. Problem : Rate of racemization = 2 * rate of inversion or incorporations

  8. For SN1 : Racemization is expected Extent of inversion = extent of retention However, due to ion pair formation, more inversion then retention. Ion pair mechanism : More inversion then retention. Problem : Rate of solvolysis in EtOH : Explain?

  9. Lecture - 6

  10. Factors Affecting the Rates of SN1 and SN2 reactions : • The structure of the substrate • Concentration and Reactivity of Nucleophile (for bimolecular • reactions only) • 3) The effect of solvent. • 4) The nature of leaving group Effect of substrate structure : For SN2 methyl > primary > secondary >> tertiary (unreactive)

  11. Solvent effect For SN1 : Greater the ionizing ability of the solvent, faster will be the reaction. Dielectric constant reflects the ionizing ability. Rate of increase of [ Br- ] increases with increase in proportion of water. Dielectric constant Q. What will be the effect of solvent polarity on the following SN1 reaction?

  12. Effect of Nucleophile : Nucleophilicity of 1) A negatively charged nucleophile is always stronger than its conjugate acid. 2) In a group of nucleophiles in which nucleophilic atom is the same, nucleophilities parallel basicities. 3) In polar protic solvents, in a particular group, nucleophilicity increases as we go down the group. 4) In polar aprotic solvents, reverse trend follows :

  13. Hammond’s postulate : In a particular step, the T.S. resembles the entity (substrate, product or intermediate) that is closer in energy. In SN1, carbocation ( R+) is the intermediate. T.S. should resemble more of the carbocation. T.S. stability will be reflected in the stability of the carbocation.

  14. Reason : Steric effect Steric hindrance offered by R1/ R2/ R3 to the approach of Nucleophile. If R1 or R2 or R3 is bulky, this steric hindrance will slow down the reaction. For SN1 : Stability of carbocation determines the rate.

  15. SN2 Inversion of configuaration SN1 ideally Racemization Retention of configuaration : SNi ( substitution nucleophilic internal) However, it can’t be a simple SN2. Problem :

  16. Neighbouring Group Participation : “Retention” “Anchimeric asssitance” In presence of , carboxylate will be produced. Double Inversion = Retention Qn. Hydrolysis of EtS–CH2CH2 – Cl is 10 4 times faster than that of EtO–CH2CH2–Cl. Explain?

  17. Biological Nucleophilic Substitution :

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