Carboxylic Acids

INTERCHAPTER R Carboxylic Acids

A fruit market in Portugal. The carboxylic acids citric acid CH2COHCH2(COOH)3(s), malic acid, CH2CHOH(COOH)2(s), and tartaric acid, (CHOH)2(COOH)2(s), are all found in fruit. Citric acid is particularly abundant in lemons, limes, oranges, and other citrus fruits. It plays a major role in our metabolism and is added to food and beverages to provide an acidic or sour flavor. Malic acid is found in apples, pears, plums, and cherries and gives unripe fruit and some candies their sour taste. Tartaric acid is found mostly in grapes, bananas, and tamarinds and is one of the main acids present in wine. Esters, which are derived from the reaction of carboxylic acids and alcohols, are responsible for the characteristic odors of many fruits.

Organic molecules that have the functional group R-1. Carboxylic Acids Can Be Produced by O the Oxidation of Aldehydes C One important use of aldehydes is in the production OH of carboxylic acids: (abbreviated –COOH) are called carboxylic ac- R oxidizing agent R ids. Carboxylic acids occur both in nature and in C O → C O the laboratory. Vinegar, for example, is a dilute H HO solution of acetic acid, CH3COOH(aq); butanoic aldehyde carboxylic acid acid, CH3CH2CH2COOH(aq), is responsible for the rancid odor of spoiled butter; and hexanoic acid, For example, the two simplest carboxylic acids, formic acid (Figure R.1) and acetic acid (Figure R.2), CH3(CH2)4COOH(aq), is partially responsible for the characteristic odor of goats. In fact, hexanoic acid is often called caproic acid from the Latin name for goat, caper. The two simplest carboxylic acids are

O O

HOC H H3COC H methanoic acid ethanoic acid (formic acid) (acetic acid)

Methanoic acid and ethanoic acid are generally referred to by their common names, formic acid and acetic acid, respectively. Being acids, carboxylic acids donate a proton to O water to become carboxylate ions, C and react – O with bases to form carboxylate salts. Generally, carboxylic acids are weak acid. As we shall see below, carboxylic acids can be produced by the oxidation Figure R.2 Acetic acid is a clear, colorless liquid with a of aldehydes. Carboxylic acids react with alcohols pungent odor. Household vinegar is an approximately to form a class of often fragrant compounds called 5% acetic acid solution. The pure compound is esters and with polyalcohols to form oils and fats. commonly called glacial acetic acid because it freezes at 18°C. Acetic acid is used in the manufacture of plastics, pharmaceuticals, dyes, insecticides, photographic chemicals, acetates, and many other organic chemicals.

Figure R.1 Formic acid is responsible for the irritants in certain plants, such as stinging nettles. Formic acid is a colorless, fuming liquid that is very soluble in water. It is the major irritant in ant bites and was first isolated by the distillation of ants. R2 GENERAL CHEMISTRY, FOURTH EDItION | McQuarrie, Rock, and Gallogly

can be formed from formaldehyde and acetaldehyde, respectively, according to

O O – MnO4(aq) H C H (aq) H C OH (aq) H+(aq) methanal methanoic acid (formaldehyde) (formic acid)

and O O formic acid 2– Cr2O7 (aq) H C C H (aq) H C C OH (aq) 3 H+(aq) 3 ethanal ethanoic acid (acetaldehyde) (acetic acid)

The oxidizing agent in these reactions effectively inserts an oxygen atom between the carbon atom and the hydrogen atom in the aldehyde group. The –COOH group is called the carboxyl group and is the functional group that is characteristic of organic acids (carboxylic acids). We can also obtain carboxylic acids by the direct acetic acid oxidation of primary alcohols. In the oxidation, the aldehyde occurs as an intermediate species: The IUPAC name of a carboxylic acid is formed 2– Cr2O7 (aq) by replacing the -e from the end of the alkane name CH CH OH (aq) 3 2 H+(aq) of the longest chain of carbon atoms containing the a primary alcohol –COOH group with the suffix -oic acid. For example, the IUPAC name for formic acid, HCOOH(aq), O O 2– is methanoic acid, and that for acetic acid, Cr2O7 (aq) H3C C H (aq) H3C C OH (aq) CH COOH(aq), is ethanoic acid. Table R.1 lists the H+(aq) 3 an aldehyde a carboxylic acid Table R.1 Structural formulas and IUPAC names of some Being acids, carboxylic acids donate a proton to a wa- carboxylic acids ter molecule according to Structural formula IUPAC name RCOOH(aq) + H O(l) ⇌ H O+(aq) + RCOO–(aq) 2 3 O carboxylic acid carboxylate ion propanoic acid CH3CH2 C OH

Typical acid dissociation constants of carboxylic acids O are of the order of 10–3 M to 10–5 M (see Table 20.4). CH3 CH C OH 2-chloropropanoic acid For example, the pKa value of acetic acid is 4.74, or –5 Cl Ka = 1.8 × 10 M. The acidic nature of carboxylic acids is due to the relative stability of the carboxylate anion, O which can delocalize its negative charge over its two CH CH CH C OH oxygen atoms according to the two resonance forms: 3 2 3-methylbutanoic acid

CH3 O O – RC R C H Cl O 2,2,3-trichloropropanoic O – O Cl C C C OH acid H Cl (resonance stabilized carboxylate ion)

University Science Books, ©2011. All rights reserved. www.uscibooks.com R. Carboxylic Acids R3 structural formulas and the IUPAC names of some carboxylic acids for practice. Carboxylic acids are neutralized by bases just as any other acid is, to form water and a salt. For example,

CH3COOH(aq) + NaOH(aq) → acetic acid

NaCH3COO(aq) + H2O(l ) sodium acetate oxalic acid The salts of carboxylic acids are named from the acid by dropping the -oic acid ending and adding -oate to form the name of the anion. Thus, we have

NaHCOO KCH3COO sodium methanoate potassium ethanoate (sodium formate) (potassium acetate)

Just as we can have dialcohols, with two –OH groups per molecule, we can have diacids as well. Some common diacids are malonic acid

O O O O

HO C C OH OH C CH2 C OH ethanedioic acid propanedioic acid (oxalic acid) (malonic acid)

Oxalic acid is a white crystalline substance and is a relatively strong acid, with K = 0.053 M and K = a1 a2 5.2 × 10–5 M. It occurs in a number of plants, particularly rhubarb, buckwheat, and tea leaves. Malonic acid is a white crystalline substance that occurs in beetroot and is used extensively as an organic reagent. There is also a family of aromatic carboxylic acids, such as benzoic acid O O

C OH HO C C OH

O benzoic acid terephthalic acid

Benzoic acid is a colorless crystalline solid. Benzoic acid and its salts are used as a food preservative to inhibit the growth of mold, yeast, and some bac- teria. We’ll see in Interchapter S that terephthalic acid is one of the building blocks of the polymer terephthalic acid Dacron. R4 GENERAL CHEMISTRY, FOURTH EDItION | McQuarrie, Rock, and Gallogly

R-2. Carboxylic Acids React with Alcohols to their odors. Esters are named by first naming the Produce Esters alkyl group from the alcohol and then designating the acid, with its -oic ending changed to -oate. For Carboxylic acids react with alcohols in the presence example, of an acid catalyst to produce esters. The equation for the reaction is H H + + C2H5OH C O C OH2O R R ′ ′ HO C2H5O ROH + C O C OH+ O 2 ethanol methanoic acid ethyl methanoate HO RO (formic acid) (ethyl formate) alcohol carboxylic acid ester Such reactions are called esterification reactions. where R and R′ represent possibly different alkyl Let’s look at an esterification reaction equation more groups. As the carboxylic acid–alcohol reaction equa- closely. We write the equation as tion indicates, the general formula for an ester is R R R′ ′ ′ + + C O ROH C O HOH C O RO HO RO

where the R group comes from the alcohol. The The coloring used in this equation emphasizes that pleasant odors of flowers and fruits are due to esters. the water is formed from the –OH group of the acid Table R.2 lists some naturally occurring esters and and the hydrogen atom of the alcohol. This reaction

Table R.2 Various esters and their odors

Structure Name Odor

C2H5 O C H ethyl methanoate (ethyl formate) rum

C5H11 O C CH3 pentyl ethanoate (amyl acetate) bananas

C8H17 O C CH3 octyl ethanoate (octyl acetate) oranges

CH3 O C C3H7 methyl butanoate (methyl butyrate) apples

C2H5 O C C3H7 ethyl butanoate (ethyl butyrate) pineapples

C5H11 O C C3H7 pentyl butanoate (amyl butyrate) apricots, pears

University Science Books, ©2011. All rights reserved. www.uscibooks.com R. Carboxylic Acids R5 is quite unlike an acid-base neutralization reaction, The product of this reaction is a triester because it in which the water is formed by the reaction between contains three ester groups. Triesters are comprised a hydroxide ion from the base and a hydronium ion of glycerol and long-hydrocarbon-chain carboxylic from the acid. Alcohols do not react like bases. The acids, called fatty acids. Because the fatty acids have similarity in their chemical formulas, for example, long hydrocarbon chains, they are insoluble in water

CH3OH(l ) versus NaOH(s), is superficial. but soluble in organic solvents such as diethyl ether, We can verify this result experimentally by synthe- chloroform, and acetone. sizing the alcohol with oxygen-18, which we write as Complete hydrolysis of a fat or an oil yields three RO*H, where we use an asterisk to denote the oxygen- fatty acid molecules for every molecule of glycerol: 18 atom. When we carry out the esterification reaction, we find that the oxygen-18 ends up in the ester O and not in the water. We can illustrate this result sche- O CH2 O C R matically by writing + R′ C O CH 3H2O (l ) R′ R′ CH O C R′′ RO*H + C O HOH + C O 2 HO RO* O

CH OH RCOOH It’s easy to detect the incorporation of oxygen-18 in 2 the ester by using mass spectrometry. CH OH + R′COOH We have written all the above esterification equations with double arrows to emphasize that esteri- CH2OH R′′COOH fication reactions are reversible. The equilibrium constants for esterification reactions are usually not where R, R′, and R′′ are hydrocarbon chains. Of the large, so appreciable quantities of both reactants 50 or more fatty acids that have been obtained from and products are present at equilibrium. You will fats and oils, the three most abundant are learn more efficient methods of preparing carbo CH (CH ) COOH xylic acids and esters if you take a course in organic 3 2 14 hexadecanoic acid (palmitic acid) chemistry. CH3(CH2)16COOH octadecanoic acid (stearic acid)

CH3(CH2)7CH=CH(CH2)7COOH R-3. Fats and Oils Are Esters cis-9-octadecanoic acid (oleic acid) Fats and oils are called triglycerides, which are triesters of the trialcohol glycerol with long hydrocarbon- These three fatty acids are generally referred to by chain carboxylic acids, such as octadecanoic acid, their common names; there is no need to memorize CH3(CH2)16COOH(s) (generally referred to by its the formulas or names of these acids. common name stearic acid). We write the equation describing the reaction between glycerol and stearic acid as shown below:

CH2 O C (CH2)16 CH3 CH OH 2 O

CH OH + 3 HO C (CH2)16 CH3 CH O C (CH2)16 CH3 + 3H2O

CH2OH O O

CH2 O C (CH2)16 CH3

O glycerol stearic acid glycerol tristearate R6 GENERAL CHEMISTRY, FOURTH EDItION | McQuarrie, Rock, and Gallogly

palmitic acid

stearic acid

oleic acid

Note that palmitic acid and stearic acid have satu- are alleged to be a dietary factor in atherosclerosis; rated hydrocarbon chains, whereas oleic acid has an therefore, for health reasons, vegetable oils have be- unsaturated hydrocarbon chain. Triglycerides rich in come increasingly popular. saturated fatty acids are generally solids at room temperature and are called fats. Beef tallow is about 45% by mass saturated fatty acids. Triglycerides rich in TERMS YOU SHOULD KNOW unsaturated fatty acids are generally liquids at room temperature and are called oils. Vegetable oils such carboxylic acid R1 as corn oil, soybean oil, and olive oil are all over 85% carboxyl group R2 by mass unsaturated fatty acids. ester R4 For reasons of convenience and dietary prefer- esterification reaction R4 ence, oils often are converted to fats by hydrogenating triglyceride R5 some of the double bonds in the unsaturated triglyc- fatty acid R5 eride. The products are sold as shortening (Crisco), fat R6 margarine, and other food products. Saturated fats oil R6

QUESTIONS R-6. Write the structural formula for each of the following salts: R-1. Write the structural formula for each of the (a) sodium 2-chloropropanoate following carboxylic acids: (b) rubidium methanoate (a) propanoic acid (c) strontium 2,2-dimethylpropanoate (b) 2-methylpropanoic acid (d) lanthanum ethanoate (c) 3,3-dimethylbutanoic acid R-7. Complete and balance the following equations (d) 3-methylpentanoic acid and name the products: R-2. Give the IUPAC name of the following COOH + compounds: H (aq) (a) (aq) + CH3CH2OH(aq) →

CH3 Cl benzoic acid (a) (b) CH3C COOH (b) HOOC–COOH(aq) + CH3CH2CH2OH(aq) CH3CH CH2COOH H+(aq) CH → 3 oxalic acid + Cl CH Cl Cl H (aq) 3 (c) CH3COOH(aq) + CH3CHOHCH3(aq) → (c) CH CCC H COOH (d) Cl C C CH COOH 3 2 2 R-8. Complete and balance the following equations H H Cl Cl and name the products: (a) HOOC–COOH(aq) + KOH(aq) → R-3. Complete and balance the following equations: oxalic acid

(a) HCOOH(aq) + NaOH(aq) → (b) CH3(CH2)16COOH(aq) + NaOH(aq) → H+(aq) stearic acid (b) HCOOH(aq) + CH3OH(aq) → COOH (c) HCOOH(aq) + Ca(OH)2(aq) → (c) COOH(aq) + KOH(aq) → R-4. Complete and balance the following equations:

(a) CH3CH2COOH(aq) + NH3(aq) → phthalic acid H+(aq) (b) CH3CH2COOH(aq) + CH3OH(aq) → H+(aq) R-9. Animal fats and vegetable oils are triesters of (c) CH CH COOH(aq) + CH CH OH(aq) → 3 2 3 2 glycerol with long-chain acids called fatty acids. These esters are generally called glycerides. Write R-5. Complete and balance the following acid-base equations and name the salt formed in each case: a Lewis formula for the glyceride product in the following (a) CH CH COOH(aq) + KOH(aq) → 3 2 reaction: CH CH COOH 3 CH OH (b) (aq) + KOH(aq) → 2 CH3 3 CH3(CH2)14COOH + CHOH → 3 H2O + a glyceride hexadecanoic acid (c) Cl2CHCOOH(aq) + Ca(OH)2(aq) → (palmitic acid) CH2OH glycerol