1. 18 18-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 8e Bettelheim, Brown Campbell, & Farrell

2. 18 18-2 © 2003 Thomson Learning, Inc. All rights reserved Chapter 18 Carboxylic Acids Carboxylic Acids

3. 18 18-3 © 2003 Thomson Learning, Inc. All rights reserved Carboxylic Acids In this chapter, we study carboxylic acids, a class of organic compounds containing the carbonyl group. carboxyl group The functional group of a carboxylic acid is a carboxyl group, which can be represented in any one of three ways.

4. 18 18-4 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature IUPAC names For an acyclic carboxylic acid, take longest carbon chain that contains the carboxyl group as the parent alkane. e oic acidDrop the final -e from the name of the parent alkane and replace it by -oic acid. Number the chain beginning with the carbon of the carboxyl group. Because the carboxyl carbon is understood to be carbon 1, there is no need to give it a number.

5. 18 18-5 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature In these examples, the common name is given in parentheses. An -OH substituent is indicated by the prefix hydroxy-; an -NH 2 substituent by the prefix amino-.

6. 18 18-6 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature dioic acid aneanedioic acidTo name a dicarboxylic acid, add the suffix -dioic acid to the name of the parent alkane that contains both carboxyl groups; thus, -ane becomes -anedioic acid. The numbers of the carboxyl carbons are not indicated because they can be only at the ends of the chain.

7. 18 18-7 © 2003 Thomson Learning, Inc. All rights reserved

8. 18 18-8 © 2003 Thomson Learning, Inc. All rights reserved Nomenclature For common names, use, the Greek letters alpha (  ), beta (  ), gamma (  ), and so forth to locate substituents.

9. 18 18-9 © 2003 Thomson Learning, Inc. All rights reserved Physical Properties The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H. The polarity of these bonds determines the major physical properties of carboxylic acids.

10. 18 18-10 © 2003 Thomson Learning, Inc. All rights reserved Physical Properties Carboxylic acids have significantly higher boiling points than other types of organic compounds of comparable molecular weight. Their higher boiling points are a result of their polarity and the fact that hydrogen bonding between two carboxyl groups creates a dimer that behaves as a higher-molecular-weight compound.

11. 18 18-11 © 2003 Thomson Learning, Inc. All rights reserved Physical Properties Carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight.

12. 18 18-12 © 2003 Thomson Learning, Inc. All rights reserved Fatty Acids Fatty acids; long chain carboxylic acids derived from animal fats, vegetable oils, or phospholipids of biological membranes. More than 500 have been isolated from various cells and tissues. Most have between 12 and 20 carbons in an unbranched chain. In most unsaturated fatty acids, the cis isomer predominates; trans isomers are rare. Unsaturated fatty acids have lower melting points than their saturated counterparts.

13. 18 18-13 © 2003 Thomson Learning, Inc. All rights reserved Fatty Acids Table 18.3

14. 18 18-14 © 2003 Thomson Learning, Inc. All rights reserved Fatty Acids Unsaturated fatty acids generally have lower melting points than their saturated counterparts.

15. 18 18-15 © 2003 Thomson Learning, Inc. All rights reserved Fatty Acids Saturated fatty acids are solids at room temperature; the regular nature of their hydrocarbon chains allows them to pack together in such a way as to maximize interactions (by London dispersion forces) between their chains.

16. 18 18-16 © 2003 Thomson Learning, Inc. All rights reserved Fatty Acids In contrast, all unsaturated fatty acids are liquids at room temperature because the cis double bonds interrupt the regular packing of their hydrocarbon chains.

17. 18 18-17 © 2003 Thomson Learning, Inc. All rights reserved Soaps Natural soaps are sodium or potassium salts of fatty acids. They are prepared from a blend of tallow and palm oils (triglycerides). Triglycerides are triesters of glycerol. the solid fats are melted with steam and the water insoluble triglyceride layer that forms on the top is removed.

18. 18 18-18 © 2003 Thomson Learning, Inc. All rights reserved Soaps Preparation of soaps begins by boiling the triglycerides with NaOH. The reaction that takes place is called saponification (Latin: saponem, “soap”). Boiling with KOH gives a potassium soap.

19. 18 18-19 © 2003 Thomson Learning, Inc. All rights reserved Soaps micelles hydrophobic hydrophilic In water, soap molecules spontaneously cluster into micelles, a spherical arrangement of molecules such that their hydrophobic parts are shielded from the aqueous environment, and their hydrophilic parts are in contact with the aqueous environment.

20. 18 18-20 © 2003 Thomson Learning, Inc. All rights reserved Soaps When soap is mixed with dirt (grease, oil, and fat stains), soap micelles “dissolve” these nonpolar, water-insoluble molecules.

21. 18 18-21 © 2003 Thomson Learning, Inc. All rights reserved Soaps Natural soaps form water-insoluble salts in hard water. Hard water Hard water contains Ca(II), Mg(II) and Fe(III) ions.

22. 18 18-22 © 2003 Thomson Learning, Inc. All rights reserved Detergents The problem of formation of precipitates in hard water was overcome by using a molecule containing a -SO 3 - group ( sulfonic acid group) in the place of a -CO 2 - group. Calcium, magnesium and iron salts of sulfonic acids, RSO 3 H, are more soluble in water than salts of fatty acids. Following is the preparation of the synthetic detergent, SDS, a linear alkylbenzene sulfonate (LAS), an anionic detergent.

23. 18 18-23 © 2003 Thomson Learning, Inc. All rights reserved Detergents Among the most common additives to detergents are foam stabilizers, bleaches, and optical brighteners.

24. 18 18-24 © 2003 Thomson Learning, Inc. All rights reserved Acidity of RCOOH Carboxylic acids are weak acids: Values of K a for most unsubstituted aliphatic and aromatic carboxylic acids fall within the range 10 -4 to 10 -5 (pK a 4.0 - 5.0).

25. 18 18-25 © 2003 Thomson Learning, Inc. All rights reserved Acidity of RCOOH Substituents of high electronegativity, especially -OH, -Cl, and -NH 3 +, near the carboxyl group increase the acidity of carboxylic acids. Both dichloroacetic acid and trichloroacetic acid are stronger acids than H 3 PO 4 (pK a 2.1).

26. 18 18-26 © 2003 Thomson Learning, Inc. All rights reserved Acidity of RCOOH When a carboxylic acid is dissolved in aqueous solution, the form of the carboxylic acid present depends on the pH of the solution in which it is dissolved.

27. 18 18-27 © 2003 Thomson Learning, Inc. All rights reserved Reaction With Bases All carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH, and other strong bases to form water-soluble salts. They also form water-soluble salts with ammonia and amines.

28. 18 18-28 © 2003 Thomson Learning, Inc. All rights reserved Reaction With Bases Like inorganic acids, carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble sodium salts and carbonic acid. Carbonic acid then decomposes to give water and carbon dioxide, which evolves as a gas.

29. 18 18-29 © 2003 Thomson Learning, Inc. All rights reserved Fischer Esterification Fischer esterification Fischer esterification is one of the most commonly used preparations of esters. In Fischer esterification, a carboxylic acid is reacted with an alcohol in the presence of an acid catalyst, most commonly concentrated sulfuric acid. Fischer esterification is reversible. It is possible to drive it in either direction by the choice of experimental conditions (Le Chatelier’s principle).

30. 18 18-30 © 2003 Thomson Learning, Inc. All rights reserved Fischer Esterification In Fischer esterification, the alcohol adds to the carbonyl group of the carboxylic acid to form a tetrahedral carbonyl addition intermediate. The intermediate then loses H 2 O to give an ester.

31. 18 18-31 © 2003 Thomson Learning, Inc. All rights reserved Phosphoric Esters Phosphoric acid forms mono-, di-, and triphosphoric esters. In more complex phosphoric esters, it is common to name the organic molecule and then indicate the presence of the phosphoric ester by either the word "phosphate" or the prefix phospho-. Dihydroxyacetone phosphate and pyridoxal phosphate are shown as they are ionized at pH 7.4, the pH of blood plasma.

32. 18 18-32 © 2003 Thomson Learning, Inc. All rights reserved Decarboxylation Decarboxylation Decarboxylation: the loss of CO 2 from a carboxyl group. Almost all carboxylic acids, when heated to a very high temperature, will undergo thermal decarboxylation. Most carboxylic acids, however, are resistant to moderate heat and melt and even boil without undergoing decarboxylation. An exception is any carboxylic acid that has a carbonyl group on the carbon  to the COOH group.

33. 18 18-33 © 2003 Thomson Learning, Inc. All rights reserved Decarboxylation Decarboxylation of a  -ketoacid. The mechanism of thermal decarboxylation involves (1) redistribution of electrons in a cyclic transition state, (2) followed by keto-enol tautomerism.

34. 18 18-34 © 2003 Thomson Learning, Inc. All rights reserved Decarboxylation An important example of decarboxylation of a  - ketoacid in biochemistry occurs during the oxidation of foodstuffs in the tricarboxylic acid (TCA) cycle. Oxalosuccinic acid, one of the intermediates in this cycle, has a carbonyl group (in this case a ketone)  to one of its three carboxyl groups.

35. 18 18-35 © 2003 Thomson Learning, Inc. All rights reserved End Chapter 18