1. 19.14 Mechanism of Acid-Catalyzed Esterification

2. Acid-catalyzed Esterification + CH 3 OH COHO H+H+H+H++ H2OH2OH2OH2O COCH 3 O Important fact: the oxygen of the alcohol is incorporated into the ester as shown. (also called Fischer esterification)

3. The mechanism involves two stages: 1)formation of tetrahedral intermediate (3 steps) 2)dissociation of tetrahedral intermediate (3 steps) Mechanism of Fischer Esterification

4. The mechanism involves two stages: 1)formation of tetrahedral intermediate (3 steps) 2)dissociation of tetrahedral intermediate (3 steps) Mechanism of Fischer Esterification COHOH OCH 3 tetrahedral intermediate in esterification of benzoic acid with methanol

5. First stage: formation of tetrahedral intermediate COHOH OCH 3 + CH 3 OH COHO H+H+H+H+ methanol adds to the carbonyl group of the carboxylic acid the tetrahedral intermediate is analogous to a hemiacetal

6. Second stage: conversion of tetrahedral intermediate to ester COHOH OCH 3 + H2OH2OH2OH2O H+H+H+H+ this stage corresponds to an acid-catalyzed dehydration COCH 3 O

7. Mechanism of formation of tetrahedral intermediate

8. Step 1 C O OH O + H CH 3 H

9. Step 1 C O OH O + H CH 3 H C O OH + H O CH 3 H

10. Step 1 C O OH + H carbonyl oxygen is protonated because cation produced is stabilized by electron delocalization (resonance) C OOH + H

11. Step 2 C O OH + H O CH 3 H

12. Step 2 C O OH + H O CH 3 H C OH OH O + CH 3 H

13. Step 3 O CH 3 H C OH OH O CH 3 H +

14. Step 3 O CH 3 H C OH OH O CH 3 H + O CH 3 H H + C OH OH O CH 3

15. Tetrahedral intermediate to ester stage

16. Step 4 C OH O OCH 3 H

17. Step 4 O CH 3 H H + C OH O OCH 3 H

18. Step 4 O CH 3 H H + C OH O OCH 3 H C OH O OCH 3 H H + O CH 3 H

19. Step 5 C OH O OCH 3 H H +

20. Step 5 C OH O OCH 3 H H + O H H+ C OH OCH 3 +

21. Step 5 C OH OCH 3 + C OH +

22. Step 6 C O OCH 3 + H O H CH 3 + O H H C O OCH 3

23. Activation of carbonyl group by protonation of carbonyl oxygen Nucleophilic addition of alcohol to carbonyl group forms tetrahedral intermediate Elimination of water from tetrahedral intermediate restores carbonyl group Key Features of Mechanism

24. 19.15 Intramolecular Ester Formation: Lactones

25. Lactones are cyclic esters Formed by intramolecular esterification in a compound that contains a hydroxyl group and a carboxylic acid function Lactones

26. Examples HOCH 2 CH 2 CH 2 COH O OO + H2OH2OH2OH2O 4-hydroxybutanoic acid 4-butanolide IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by -olide identify the oxygenated carbon by number

27. Examples HOCH 2 CH 2 CH 2 COH O HOCH 2 CH 2 CH 2 CH 2 COH O O O OO + + H2OH2OH2OH2O H2OH2OH2OH2O 4-hydroxybutanoic acid 5-hydroxypentanoic acid 4-butanolide 5-pentanolide

28. Common names O O OO  -butyrolactone  -valerolactone        Ring size is designated by Greek letter corresponding to oxygenated carbon A  lactone has a five-membered ring A  lactone has a six-membered ring

29. Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if the resulting ring is 5- or 6-membered. Lactones

30. Example 5-hexanolide (78%) O H3CH3CH3CH3C O CH 3 CCH 2 CH 2 CH 2 COH OO 1. NaBH 4 2. H 2 O, H +

31. Example 5-hexanolide (78%) via: O H3CH3CH3CH3C O CH 3 CCH 2 CH 2 CH 2 COH OO 1. NaBH 4 2. H 2 O, H + CH 3 CHCH 2 CH 2 CH 2 COH O OHOHOHOH