Carboxylic Acids Chapter 18

Carboxylic Acids In this chapter, we study carboxylic acids, another 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.

Nomenclature IUPAC names For an acyclic carboxylic acid, take the longest carbon chain that contains the carboxyl group as the parent alkane. e oic acid Drop 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.

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-.

Nomenclature dioic acid aneanedioic acid To 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.

Nomenclature

For common names, use, the Greek letters alpha (  ), beta (  ), gamma (  ), and so forth to locate substituents.

Physical Properties

Carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight.

larger K a increased [ H 3 O + ] stronger acid A – + H 3 O + [HA][HA] [A–][A–][H3O+][H3O+] K a = HA + H 2 O

larger K a increased [ H 3 O + ] stronger acid A – + H 3 O + [HA][HA] [A–][A–][H3O+][H3O+] K a = HA + H 2 O

increased [ H 3 O + ] stronger acid A – + H 3 O + [HA][HA] [A–][A–][H3O+][H3O+] larger K a K a = HA + H 2 O

larger K a increased [ H 3 O + ] stronger acid A – + H 3 O + [HA][HA] [A–][A–][H3O+][H3O+] K a = HA + H 2 O

RCOOH + H 2 ORCOO – + H 3 O + [ RCOOH ] [ RCOO – ][H3O+][H3O+] K a =

RCOOH + H 2 ORCOO – + H 3 O + [ RCOOH ] [ RCOO – ][H3O+][H3O+] K a =

acids > phenols ~ thiols > water ~ alcohols K a % ionized [ H 3 O + ], M pH ~1  10 7 ~100 ~  10 –  10 –  10 –  10 –  10 –  10 –  10 –  10 – HCl HOAc PhOH EtSH EtOH HOH Comparative acidities of 0.1 M aqueous solutions of representative acids HA 1.8  10 –  10 –

Fatty Acids Table 18.3 The Most Abundant Fatty Acids in Animal Fats, Vegetable Oils, and Biological Membranes.

Fatty Acids Unsaturated fatty acids generally have lower melting points than their saturated counterparts.

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.

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.

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. 21

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. 22

Soaps micelles hydrophobic hydrophilic Figure 18.2 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. 23

Soaps Figure 18.3 When soaps and dirt, such as grease, oil, and fat stains are mixed in water, the nonpolar hydrocarbon inner parts of the soap micelles “dissolve” the nonpolar substances. 24

Soaps Natural soaps form water-insoluble salts in hard water. Hard water Hard water contains Ca 2+, Mg 2+, and Fe 3+ ions. 25

Detergents The problem of formation of precipitates in hard water was overcome by using a molecule containing a sulfonate (-SO 3 - ) group in the place of a carboxylate (-CO 2 - ) group. Calcium, magnesium and iron salts of sulfonic acids, RSO 3 H, are more soluble in water than are their salts of fatty acids. Following is the preparation of the synthetic detergent, SDS, a linear alkylbenzenesulfonate (LAS), an anionic detergent. 26

Detergents Among the most common additives to detergents are foam stabilizers, bleaches, and optical brighteners. 27

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.

Decarboxylation Decarboxylation of a  -ketoacid. The mechanism of thermal decarboxylation involves (1) redistribution of electrons in a cyclic transition state followed by (2) keto-enol tautomerism.

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.