Understanding Carboxylic Acids: Structure, Nomenclature, and Reactions

1. Carboxylic Acids

2. Structure and Bonding

3. – •• •• R O R O + •• •• •• C C O O •• •• •• •• H H Electron Delocalization

4. – – •• •• •• R O R O R O + •• •• •• •• •• C C C + O O O •• •• •• •• •• H H H Electron Delocalization • stabilizes carbonyl group

5. Formic acid is planar O H C O 120 pm H 134 pm

6. Carboxylic Acid Nomenclature

7. O HCOH O CH3COH O CH3(CH2)16COH NOMENCLATURE • systematic IUPAC names replace "-e" ending of alkane with "oic acid" Systematic Name methanoic acid ethanoic acid octadecanoic acid

8. O HCOH O CH3COH O CH3(CH2)16COH NOMENCLATURES • common names are based on natural origin rather than structure Systematic Name Common Name methanoic acid formic acid ethanoic acid acetic acid octadecanoic acid stearic acid

9. O CH3CHCOH O OH CH3(CH2)7 (CH2)7COH C C H H NOMENCLATURE Systematic Name Common Name 2-hydroxypropanoicacid lactic acid (Z)-9-octadecenoicacid oleic acid

10. Physical Properties

11. O O H CCH3 H3CC O H O Hydrogen-bonded Dimers • Acetic acid exists as a hydrogen-bonded dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other.

12. Hydrogen-bonded Dimers • Acetic acid exists as a hydrogen-bonded dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other.

13. H O O H H3CC H O H O H Solubility in Water • carboxylic acids are similar to alcohols in respect to their solubility in water • form hydrogen bonds to water

14. O RC O d+ – •• •• O O •• •• •• RC RC •• – O O •• •• •• •• Greater acidity of carboxylic acids is attributedstabilization of carboxylate ion by inductive effect of carbonyl group – resonance stabilization of carboxylate ion

15. Dicarboxylic Acids

16. O O HOC COH O O HOCCH2COH O O HOC(CH2)5COH Dicarboxylic Acids pKa • one carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation Oxalic acid 1.2 Malonic acid 2.8 Heptanedioic acid 4.3

17. Reactions of Carboxylic Acids • Acidity • Reduction • Esterification • Reaction with Thionyl Chloride

18. Reactions of Carboxylic Acids • a-Halogenation • Decarboxylation • But first we revisit acid-catalyzed esterificationto examine its mechanism. New reactions in this chapter

19. Acidity of Carboxylic Acids • Most carboxylic acids have a pKa close to 5.

20. O CH3COH Carboxylic acids are weak acids • but carboxylic acids are far more acidic than alcohols CH3CH2OH Ka = 1.8 x 10-5 pKa = 4.7 Ka = 10-16 pKa = 16

21. O CH2COH X X Ka pKa H 1.8 x 10-5 4.7 2.5 x 10-3 2.6 F Cl 1.4 x 10-3 2.9 Substituent Effects on Acidity • electronegative substituents increase acidity

22. Salts of Carboxylic Acids

23. O O Carboxylic acids are neutralized by strong bases • equilibrium lies far to the right; K is ~ 1011 • as long as the molecular weight of the acid is not too high, sodium and potassium carboxylate salts are soluble in water + + RCOH HO– RCO– H2O strongeracid weakeracid

24. O O CH3(CH2)16CO Micelles • unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles inwater ONa sodium stearate(sodium octadecanoate) – Na+

25. O Micelles • sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail" • the polar end is "water-loving" or hydrophilic • the nonpolar tail is "water-hating" or hydrophobic • in water, many stearate ions cluster together to form spherical aggregates; carboxylate ions on the outside and nonpolar tails on the inside ONa polar nonpolar

26. A micelle

27. O O R2CCOH R2CCOH H X a -Halogenation of Carboxylic Acids • analogous to a-halogenation of aldehydes and ketones • key question: Is enol content of carboxylic acids high enough to permit reaction to occur at reasonable rate? (Answer is NO) + + X2 HX

28. O O Br2 CH3CH2CH2COH CH3CH2CHCOH P Br (77%) Reactions • a-Halogen can be replaced by nucleophilic substitution

29. O RCOH O O O CH3COH HOCCH2COH Decarboxylation of Carboxylic Acids Simple carboxylic acids do not decarboxylatereadily. + RH CO2 But malonic acid does. 150°C + CO2

30. Carboxylic Acid DerivativesNucleophilic Acyl Substitution

31. O RC X Acyl Halides • name the acyl group and add the word chloride, fluoride, bromide, or iodide as appropriate • acyl chlorides are, by far, the most frequently encountered of the acyl halides

32. O CH3CCl O H2C CHCH2CCl O F CBr Acyl Halides acetyl chloride 3-butenoyl chloride p-fluorobenzoyl bromide

33. O O RCOCR' Acid Anhydrides • when both acyl groups are the same, name the acid and add the word anhydride • when the groups are different, list the names of the corresponding acids in alphabetical order and add the word anhydride

34. O O CH3COCCH3 O O C6H5COCC6H5 O O C6H5COC(CH2)5CH3 Acid Anhydrides acetic anhydride benzoic anhydride benzoicheptanoic anhydride

35. O O O O CH3COCCH3 O O O O Some anhydrides are industrial chemicals Aceticanhydride Phthalicanhydride Maleicanhydride

36. O RCOR' Esters • name as alkyl alkanoates • cite the alkyl group attached to oxygen first (R') • name the acyl group second; substitute the suffix -ate for the -ic ending of the corresponding acid

37. O CH3COCH2CH3 O CH3CH2COCH3 O COCH2CH2Cl Esters ethyl acetate methyl propanoate 2-chloroethylbenzoate

38. O CH3COCH2CH2CH(CH3)2 Esters are very common natural products • also called "isopentyl acetate" and "isoamyl acetate" • contributes to characteristic odor of bananas 3-methylbutyl acetate

39. Lactones • Lactones are cyclic esters • Formed by intramolecular esterification in acompound that contains a hydroxyl group anda carboxylic acid function

40. O O HOCH2CH2CH2COH O Examples • IUPAC nomenclature: replace the -oic acidending of the carboxylic acid by-olide • identify the oxygenated carbon by number + H2O 4-hydroxybutanoic acid 4-butanolide

41. O O O O Common names a b a • Ring size is designated by Greek letter corresponding to oxygenated carbon • A g lactone has a five-membered ring • A d lactone has a six-membered ring b g g d g-butyrolactone d-valerolactone

42. O RCNH2 Amides having an NH2 group • identify the corresponding carboxylic acid • replace the -ic acid or -oic acid ending by - amide.

43. O CH3CNH2 O (CH3)2CHCH2CNH2 O CNH2 Amides having an NH2 group acetamide 3-methylbutanamide benzamide

44. O O RCNHR' RCNR'2 Amides having substituents on N • name the amide as before • precede the name of the amide with the name of the appropriate group or groups • precede the names of the groups by the letter N- (standing for nitrogen and used as a locant) and

45. O CH3CNHCH3 O CN(CH2CH3)2 O CH3CH2CH2CNCH(CH3)2 CH3 Amides having substituents on N N-methylacetamide N,N-diethylbenzamide N-isopropyl-N-methylbutanamide

46. RC N Nitriles • add the suffix -nitrile to the name of the parent hydrocarbon chain (including the triply bonded carbon of CN) • or: replace the -ic acid or -oic acid name of the corresponding carboxylic acid by -onitrile • or: name as an alkyl cyanide (functional class name)

47. CH3C N C6H5C N CH3CHCH3 C N Nitriles ethanenitrileor: acetonitrileor: methyl cyanide benzonitrile 2-methylpropanenitrileor: isopropyl cyanide