Reactive intermediates

Chemistry B.Sc.-I Sem-Ist
Sant Gadge Baba Amravati University, Amravati,
Late Ku. Durga K. Banmeru Science College, Lonar

1. 1 Late Ku. Durga K. Banmeru Science College , LONAR DIST. BULDANA (Maharashtra), India . Section –II Unit-3 Electronic displacements “Reactive Intermediates” B. Sc. Ist year Sem-Ist Subject :- Chemistry

2. 2 Dr. Suryakant B. Borul (M.Sc., M.Phil. , Ph.D.) Head Of Department Department of Chemistry Late Ku. Durga K. Banmeru Science College, Lonar Teacher Profile

3. B) Reactive Intermediates Q.-What are reactive intermediates? Ans - Definition- “ The short lived fragments or species obtained in homolytic or heterolytic bond fission are called as reactive intermediates. ” Ex . Carbocations, Carbanions and Free radicals. The organic compounds undergoes heterolytic bond fission gives carbocations and carbanions .

4. Carbocations (Carbonium ions) Ans - “ The positively charge carries carbon atoms containing six electrons in its valence shell is called as carbocation or carbonium ion. ” Examples- Q.-Explain the terms Carbocations. OR What is carbocation? How it is formed?

5. Characteristics of Carbocations It is positive charge carries carbon atom. Six electrons in its valence shell. Carbocation carbon is in sp2 hybridization state. It is planner ion. It acts as electrophiles . It attack on electron rich species i.e. on nucleophiles. It is classified into primary, secondary, tertiary carbocation Stability order as – tertiary > secondary > primary > Methyl carbocation It is product of heterolytic bond fission of organic compounds.

6. Formation of Carbocations Heterolysis of Alkyl Halides- 2. Dehydration of Alcohols-

7. 3. Protonation of Alkenes- 4. Decomposition of Diazo salt-

8. Stability of Carbocations Stability of carbocation is influenced by- Inductive effect- The electron releasing group i . e. + effect such as alkyl groups stabilize the carbocation by dispersing the positive charge. Maximum number such group alkyl groups attached to positive charge carbon greater the dispersal of positive charge and increases the stability of carbocation. Thus tertiary carbocation is more stable than secondary carbocation, which in turn is more stable than primary carbocation.

9. -I effect due to electron withdrawing groups destabilize the carbocation. Tertiary Secondary Primary Methyl carbocation carbocation carbocation carbocation.

10. B. Resonance- Q- Explain the stability of Allyl carbocation and n- propyl carbocation. Ans - The carbocation which involved resonance are more stable than those not involve in resonance. In resonance delocalization of positive charge increases the stability of carbocation. Ex.- Allyl carbocation is more stable than n- propyl carbocation, because allyl carbocation involved in resonance while n- propyl carbocation do not involve in resonance.

11. n- Propyl carbocation- Allyl carbocation- Ex- Benzyl carbocation - Benzyl carbocation is stabilized by resonance. Thus, Allyl carbocation is more stable than n- propyl carbocation.

12. C. Hyperconjugation - Allyl group release electrons via hyperconjugation effect increases the stability of carbocation. Greater the number of hyperconjugative structures the greater its stability of carbocations . The decreasing order of stability of allyl carbocation as-

13. Recations of Carbocations Combination with carbocations - 2. Elimination of Proton-

14. 3. Addition to double bond to produce higher carbocation- 4. Rearrangement to produce more stable carbocation- 1 2

15. Carbanion Ans - “ The negatively charge carries carbon atoms containing eight electrons in its valence shell is called as carbanion . ” Examples- Q.-Explain the terms Carbanions . OR What is carbanion ? How it is formed?

16. Characteristics of Carbanions It is negative charge carries carbon atom. Eight electrons in its valence shell. Carbanion carbon is in sp3 hybridization state. It is electron rich center. It acts as nucleophiles. It attack on electron deficient species i.e. on electrophiles . It is classified into primary, secondary, tertiary carbanion Stability order as – Methyl > primary > secondary > tertiary carbocation It is product of heterolytic bond fission of organic compounds.

17. Formation of Carbanion Heterolysis - 2. Dehydration of Alcohols-

18. Stability of Carbocations Stability of carbocation is influenced by- Inductive effect- The electron releasing group i . e. + effect such as alkyl groups stabilize the carbocation by dispersing the positive charge. Maximum number such group alkyl groups attached to positive charge carbon greater the dispersal of positive charge and increases the stability of carbocation. Thus tertiary carbocation is more stable than secondary carbocation, which in turn is more stable than primary carbocation.

19. -I effect due to electron withdrawing groups destabilize the carbocation. Tertiary Secondary Primary Methyl carbocation carbocation carbocation carbocation.

20. B. Resonance- Q- Explain the stability of Allyl carbocation and n- propyl carbocation. Ans - The carbocation which involved resonance are more stable than those not involve in resonance. In resonance delocalization of positive charge increases the stability of carbocation. Ex.- Allyl carbocation is more stable than n- propyl carbocation, because allyl carbocation involved in resonance while n- propyl carbocation do not involve in resonance.

21. n- Propyl carbocation- Allyl carbocation- Ex- Benzyl carbocation - Benzyl carbocation is stabilized by resonance. Thus, Allyl carbocation is more stable than n- propyl carbocation.

22. C. Hyperconjugation - Alkyl group release electrons via hyperconjugation effect increases the stability of carbocation. Greater the number of hyperconjugative structures the greater its stability of carbocations . The decreasing order of stability of alkyl carbocation as-

23. The tertiary carbocation is more stable than secondary carbocation and secondary carbocation more stable than primary carbocation. The stability order as – Tertiary > Secondary > Primary > Methyl Ex-. t-Butyl > sec- Propyl > Ethyl > Methyl 3. The carbocations which gives maximum structure of hyperconjugation which are more stable.

24. t-butyl carbocation gives nine hyperconjugative structures sec- propyl carbocation gives six hyperconjugative structures 4 5

25. ethyl carbocation gives three hyperconjugative structures Thus t-carbocation is more stable than primary carbocation.

26. Recations of Carbocations Combination with carbocations - 2. Elimination of Proton-

27. 3. Addition to double bond to produce higher carbocation- 4. Rearrangement to produce more stable carbocation- 1 2

28. Carbanion Ans - “ The negatively charge carries carbon atoms containing eight electrons in its valence shell is called as carbanion . ” Examples- Q.-Explain the terms Carbanions . OR What is carbanion ? How it is formed? B) Reactive Intermediates

29. Characteristics of Carbanions It is negative charge carries carbon atom. Eight electrons in its valence shell. Carbanion carbon is in sp3 hybridization state. It is electron rich center. It acts as nucleophiles. It attack on electron deficient species i.e. on electrophiles . It is classified into primary, secondary, tertiary carbanion Stability order as – Methyl > primary > secondary > tertiary carbocation It is product of heterolytic bond fission of organic compounds.

30. Formation of Carbanion Heterolysis - 2. Abstraction of Proton- 2. Decomposition of anion-

31. Stability of Carbanion Stability of carbanion is influenced by- Inductive effect- The electron releasing group such as alkyl groups decrease stabilize the carbanion by intensifying the negative charge. The electron withdrawing group i . e. -I effect increase stabilize the carbanion by dispersal of the negative charge. The stability order as – Methyl > Primary > Secondary > Tertiary Ex-. Methyl > Ethyl > sec- Propyl > t-Butyl

32. B. Resonance- Carbanions stabilized by resonance due to delocalization of the negative charge. Ex.– benzyl carbocation is more stable than ethyl carbanion . Resonance structures of Benzyl carbanion

33. Reactions of Carbanion Combination with proton or cation - 2. Additions to multiple bonds to form an anion or new carbanion -

34. Free Radicals Ans - “ The highly reactive odd or unpaired electron containing charge less species is called as free radical. ” Examples- Q.-Explain the terms Free Radicals. OR What is Free Radical? How it is formed? B) Reactive Intermediates

35. Characteristics of Free radical It is charge less species. It carries odd or unpaired electron in its valence shell. The central carbon atom of free radicals is in sp2 hybridized state. It is attack on free radicals and combine with free radicals. It has planer structure. It is classified into Primary, Secondary, & tertiary free radicals. Stability order as Tertiary > Secondary > Primary free radicals.

36. Formation of Free Radicals Thermal Decomposition - i ii Tetraethyl lead Ethyl free radical Due to heating above molecule undergoes homolysis to form free rad icals

37. 2. Photochemical Decomposition - i ii In presence of light compound undergoes homolysis to from free radicals

38. Stability of Free Radicals 1. Resonance- According to resonance theory those molecule gives maximum resonating structures which are more stable. Free radicals get stability by resonance due to delocalization of the unpaired electron. Ex. Allyl free radicals– Benzyl free radical- Resonance structures of benzyl free radicals In above two examples benzyl free radicals more stable than allyl free radicals.

39. 2. Hyperconjugation Effect- The alkyl groups are electron releasing groups. The presence of alkyl group attached to free radical carbon increases the stability of free radical. Hence the decreasing order of stability of free radical is Presence of alkyl group increases the hyperconjugation structures .

40. Reactions of Free Radicals 1. Combination with other free radicals- 2. Addition to multiple bond to form a new free radicals- 3. Disproportionation -

41. 4. Substitution-

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