Carbonyl Compounds and Nucleophilic Addition

1. Carbonyl Compounds and Nucleophilic Addition

2. Carbonyl compounds Contain at least one carbonyl group. R = R’ or R  R’

3. propanal butanal pentanal Aldehydes  terminal carbonyl groups No need to specify the position of the carbonyl group

4. pentan-2-one pentan-3-one

5. cyclohexanone cyclohexanecarbaldehyde cyclohexylmethanal cyclohexylethanal

6. benzaldehyde 1-phenylethanone diphenylmethanone benzophenone

7. 4-oxopentanoic acid 5-oxopentanoic acid 4-oxopentanal

8. Physical Properties 1. Most simple aliphatic ketones and aldehydes are liquids at room temperature except methanal (b.p. = 21C) and ethanal (b.p. = 20.8C) Aliphatic aldehydes have an unpleasant and pungent smell Ketones and aromatic aldehydes have a pleasant and sweet odour

9. Physical properties of some aldehydes and ketones

10. 2. Less dense than water except aromatic members

11. Boiling point : - (similar molecular masses) carboxylic acid > alcohol > aldehyde, ketone > CxHy Presence of polar group Absence of –OH group

12. Solubility Small aldehydes and ketones show appreciable solubilities in water due to the formation of intermolecular hydrogen bonds with water

13. Solubility Ethanal and propanone are miscible with water in all proportions. Propanone(acetone) is volatile and miscible with water  Once used to clean quick-fit apparatus potentially carcinogenic

14. Solubility Methanal gas dissolves readily in water Aqueous solutions of methanal (Formalin) are used to preserve biological specimens Methanal(formaldehyde) is highly toxic

15. Industrial preparation By dehydrogenation (oxidation) of alcohols Further oxidation is prohibited Out-dated

16. Laboratory preparation • Oxidation of alcohols 1 alcohol  aldehyde  carboxylic acid 2 alcohol  ketone Further oxidation of aldehyde to carboxylic acid is prohibited by (i) using a milder O.A., e.g. H+/ Cr2O72

18. Laboratory preparation • Oxidation of alcohols 1 alcohol  aldehyde  carboxylic acid 2 alcohol  ketone Further oxidation of aldehyde to carboxylic acid is prohibited by (i) using a milder O.A., e.g. H+/ Cr2O72 (ii) distilling off the product as it is formed

19. 70C > T > 21C

20. Heating under reflux Ethanol  ethanoic acid

21. carboxylic acid High T 2 alcohol  ketone Further oxidation of ketone to carboxylic acid has not synthetic application since 1. it requires more drastic reaction conditions 2. it results in a mixture of organic products

22. 2. Reduction of acid chlorides The catalyst Pd or BaSO4 is poisoned with S to prevent further reduction to alcohol

23. oxidation reduction Carboxylic acid or acyl chloride Aldehyde Alcohol Aldehydes  Intermediate oxidation state  Preparation must be well controlled.

24. Friedel-Crafts acylation (Preparation of aromatic ketones)

25. 4. Decarboxylation of calcium salts Symmetrical ketones can be obtained by heating a single calcium carboxylate

26. 4. Decarboxylation of calcium salts Aldehydes can be obtained by heating a mixture of two calcium carboxylates Cross decarboxylation is preferred

27. NaOH(s) from soda lime CH3COONa(s) CH4 + Na2CO3 fusion NaOH(s) from soda lime + Na2CO3 fusion Decarboxylation of sodium salts gives methane or benzene.(p.30 and p.49)

28. ketone 5. Catalytic hydration of alkynes enol Keto-enol tautomerism

29. 5. Catalytic hydration of alkynes enol Keto-enol tautomerism aldehyde

30. 6. Ozonolysis of symmetrical alkenes

31. 1. O3 2. Zn dust / H2O Unsymmetrical alkenes give a mixture of two carbonyl compounds making subsequent purification more difficult.

32. Reactions of Aldehydes and Ketones • Nucleophilic Addition Reactions (AdN) • Condensation Reactions (Addition-Elimination) • Iodoform Reactions (Oxidation) • Oxidation Reactions • Reduction Reactions

33. Bonding in the Carbonyl Group The carbonyl carbon atom is sp2-hybridized sp2 – 2p head-on overlap   bond 2p – 2p side-way overlap   bond The  and  bonds in the C = O bond

34. The carbonyl group is planar(sp2-hybridized) and highly polarized due to (i) Polarization of  bond (inductive effect) (ii) Polarization of  bond (mesomeric effect)  +

35. 50%  + 50% Susceptible to nucleophilic attack

36. Bond Enthalpy (kJ/mol) : C=C (611) < 2 C–C (346) ( bond <  bond) C=O (749) > 2 C–O (358) ( bond >  bond) Due to polarization of the C-O  bond

37. Nucleophilic Addition Reactions (AdN) Acid-catalyzed More susceptible to nucleophilic attack

38. Nucleophilic Addition Reactions (AdN) Base-catalyzed Stronger nucleophile

39. (Non-polar) +  AdN vs AdE

40. Reaction mechanism Slow (r.d.s.) H+ and Nu added across the C=O bond Fast

41. 50% 50% 50% 50% H+ Q.52 A racemic mixture

42. Reactivity depends on two factors : - (i) Electronic effect (ii) Steric effect

43. (i) Electronic effect Electron-deficiency of carbonyl C  • Ease of nucleophilic attack  • Reactivity 

44. Decreasing reactivity > > Decreasing positive charge on carbonyl carbon

45. Carbonyl C is less positive due to delocalization of positive charge to the benzene ring. Or, The C-O bond has less  character  less mesomeric effect

46. > Reactivity : -

47. (ii) Steric effect No. /bulkiness of R groups at carbonyl C  • Steric hindrance  • Reactivity 

48. Decreasing reactivity > > Increasing steric hindrance

49. pentan-3-one

50. Reactivity of AdN : - • Aldehydes > ketones • Aliphatic > aromatic