These functional groups are listed in Table 4. Most familiar carboxylic acids have an even number of carbon atoms. As we shall see in Chapter 7 "Lipids", these acids—called fatty acids—are synthesized in nature by adding two carbon atoms at a time. Table 4. How is the carboxyl group related to the carbonyl group and the OH group?
How is the amide group related to the carboxyl group and amines? The carboxyl group has a carbonyl group joined to an OH group. The amide group has a carboxyl group joined to an amino group.
Carboxylic acids occur widely in nature, often combined with alcohols or other functional groups, as in fats, oils, and waxes. They are components of many foods, medicines, and household products. Not surprisingly, many of them are best known by common names based on Latin and Greek words that describe their source. What you learn in this chapter about the chemistry of carboxylic acids will help you understand biochemistry Chapter 6 "Carbohydrates" through Chapter 11 "Metabolic Pathways and Energy Production".
The bites of some ants inject formic acid, and the stings of wasps and bees contain formic acid as well as other poisonous materials. The next higher homolog is acetic acid, which is made by fermenting cider and honey in the presence of oxygen.
Acetic acid is probably the most familiar weak acid used in educational and industrial chemistry laboratories. Pure acetic acid solidifies at For that reason, pure acetic acid sometimes called concentrated acetic acid came to be known as glacial acetic acid , a name that survives to this day. It is found in rancid butter and is one of the ingredients of body odor. By recognizing extremely small amounts of this and other chemicals, bloodhounds are able to track fugitives.
Models of the first four carboxylic acids are shown in Figure 4. Figure 4. Carboxylic acids feature a carbon atom doubly bonded to an oxygen atom and also joined to an OH group. The four acids illustrated here are formic acid a , acetic acid b , propionic acid c , and butyric acid d.
These letters refer to the position of the carbon atom in relation to the carboxyl carbon atom. The - e ending of the parent alkane is replaced by the suffix - oic and the word acid.
As with aldehydes, the carboxyl carbon atom is counted first; numbers are used to indicate any substituted carbon atoms in the parent chain. The LCC contains four carbon atoms; the compound is therefore named as a substituted butyric or butanoic acid.
Write the condensed structural formula for 4-bromomethylhexanoic acid. The straight-chain aldehyde with five carbon atoms has the common name valeraldehyde.
What is the common name of the corresponding straight-chain carboxylic acid? As we noted in Chapter 3 "Aldehydes, Ketones", the oxidation of aldehydes or primary alcohols forms carboxylic acids:. In the presence of an oxidizing agent, ethanol is oxidized to acetaldehyde, which is then oxidized to acetic acid.
This process also occurs in the liver, where enzymes catalyze the oxidation of ethanol to acetic acid. Acetic acid can be further oxidized to carbon dioxide and water.
Many carboxylic acids are colorless liquids with disagreeable odors. These acids are also produced by the action of skin bacteria on human sebum skin oils , which accounts for the odor of poorly ventilated locker rooms. The acids with more than 10 carbon atoms are waxlike solids, and their odor diminishes with increasing molar mass and resultant decreasing volatility.
Carboxylic acids exhibit strong hydrogen bonding between molecules. They therefore have high boiling points compared to other substances of comparable molar mass. The carboxyl group readily engages in hydrogen bonding with water molecules Figure 4.
The acids with one to four carbon atoms are completely miscible with water. Solubility decreases as the carbon chain length increases because dipole forces become less important and dispersion forces become more predominant. The carboxylic acids generally are soluble in such organic solvents as ethanol, toluene, and diethyl ether.
The first six are homologs. Notice that the boiling points increase with increasing molar mass, but the melting points show no regular pattern. Which compound has the higher boiling point—butanoic acid molar mass 88 or 2-pentanone molar mass 86? Would you expect butyric acid butanoic acid to be more or less soluble than 1-butanol in water? Water-soluble carboxylic acids ionize slightly in water to form moderately acidic solutions.
Their aqueous solutions exhibit the typical properties of acids, such as changing litmus from blue to red. In these reactions, the carboxylic acids act like inorganic acids: they neutralize basic compounds.
Carboxylic acid salts are named in the same manner as inorganic salts: the name of the cation is followed by the name of the organic anion. The name of the anion is obtained by dropping the - ic ending of the acid name and replacing it with the suffix - ate.
The salts of long-chain carboxylic acids are called soaps. We discuss the chemistry of soaps further in Chapter 7 "Lipids", Section 7. Write an equation for the reaction of decanoic acid with each compound. Decanoic acid has 10 carbon atoms. Write an equation for the reaction of benzoic acid with each compound. Some organic salts are used as preservatives in food products. They prevent spoilage by inhibiting the growth of bacteria and fungi. Calcium and sodium propionate, for example, are added to processed cheese and bakery goods; sodium benzoate is added to cider, jellies, pickles, and syrups; and sodium sorbate and potassium sorbate are added to fruit juices, sauerkraut, soft drinks, and wine.
Look for them on ingredient labels the next time you shop for groceries. How does the neutralization of a carboxylic acid differ from that of an inorganic acid? How are they similar?
What products are formed when a carboxylic acid is neutralized with a strong base? What additional product is formed when a carboxylic acid is neutralized with a carbonate or a bicarbonate?
Insoluble carboxylic acids often form soluble carboxylate salts. Both form a salt and water. If it were hydrogen atom, the compound would be a carboxylic acid. Esters feature a carbon-to-oxygen double bond that is also singly bonded to a second oxygen atom, which is then joined to an alkyl or an aryl group.
The esters shown here are ethyl acetate a and methyl butyrate b. Esters occur widely in nature. Unlike carboxylic acids, esters generally have pleasant odors and are often responsible for the characteristic fragrances of fruits and flowers. Once a flower or fruit has been chemically analyzed, flavor chemists can attempt to duplicate the natural odor or taste.
Both natural and synthetic esters are used in perfumes and as flavoring agents. Fats and vegetable oils are esters of long-chain fatty acids and glycerol. Esters of phosphoric acid are of the utmost importance to life. Although esters are covalent compounds and salts are ionic, esters are named in a manner similar to that used for naming salts. The group name of the alkyl or aryl portion is given first and is followed by the name of the acid portion.
The part of the molecule derived from the carboxylic acid in red has three carbon atoms. The ester is therefore butyl propionate or butyl propanoate. An alkyl group in green is attached directly to the oxygen atom by its middle carbon atom; it is an isopropyl group.
The part derived from the acid that is, the benzene ring and the carbonyl group, in red is benzoate. Start with the portion from the acid. Draw the pentanoate five carbon atoms group first; keeping in mind that the last carbon atom is a part of the carboxyl group. Then attach the ethyl group to the bond that ordinarily holds the hydrogen atom in the carboxyl group. From what carboxylic acid and what alcohol can isopropyl hexanoate be made? Formaldehyde, an aldehyde with the formula HCHO, is a colorless gas with a pungent and irritating odor.
Formaldehyde causes coagulation of proteins, so it kills bacteria and any other living organism and stops many of the biological processes that cause tissue to decay. Thus, formaldehyde is used for preserving tissue specimens and embalming bodies. It is also used to sterilize soil or other materials.
Formaldehyde is used in the manufacture of Bakelite, a hard plastic having high chemical and electrical resistance. It is made commercially by fermenting corn or molasses, or by oxidation of 2-propanol. Acetone is a colorless liquid. Among its many uses are as a solvent for lacquer including fingernail polish , cellulose acetate, cellulose nitrate, acetylene, plastics, and varnishes; as a paint and varnish remover; and as a solvent in the manufacture of pharmaceuticals and chemicals.
The odor of vinegar is caused by the presence of acetic acid, a carboxylic acid, in the vinegar. The odor of ripe bananas and many other fruits is due to the presence of esters, compounds that can be prepared by the reaction of a carboxylic acid with an alcohol.
Because esters do not have hydrogen bonds between molecules, they have lower vapor pressures than the alcohols and carboxylic acids from which they are derived see Figure 2. Figure 2. Esters are responsible for the odors associated with various plants and their fruits.
Both carboxylic acids and esters contain a carbonyl group with a second oxygen atom bonded to the carbon atom in the carbonyl group by a single bond. In a carboxylic acid, the second oxygen atom also bonds to a hydrogen atom. In an ester, the second oxygen atom bonds to another carbon atom. The names for carboxylic acids and esters include prefixes that denote the lengths of the carbon chains in the molecules and are derived following nomenclature rules similar to those for inorganic acids and salts see these examples :.
The hydrogen atom in the functional group of a carboxylic acid will react with a base to form an ionic salt:. The remaining molecules are undissociated in solution. We prepare carboxylic acids by the oxidation of aldehydes or alcohols whose —OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:. Esters are produced by the reaction of acids with alcohols. The simplest carboxylic acid is formic acid, HCO 2 H, known since It is partially responsible for the pain and irritation of ant and wasp stings, and is responsible for a characteristic odor of ants that can be sometimes detected in their nests.
Cider vinegar is produced by allowing apple juice to ferment without oxygen present. Yeast cells present in the juice carry out the fermentation reactions. The fermentation reactions change the sugar present in the juice to ethanol, then to acetic acid. Pure acetic acid has a penetrating odor and produces painful burns. It is an excellent solvent for many organic and some inorganic compounds, and it is essential in the production of cellulose acetate, a component of many synthetic fibers such as rayon.
The distinctive and attractive odors and flavors of many flowers, perfumes, and ripe fruits are due to the presence of one or more esters Figure 3. Palmitic and stearic acids are saturated acids that contain no double or triple bonds. Carboxylic acids contain the carboxyl functional group -COOH. Carboxylic acids end in ' -oic acid '. The carboxyl group will never have a position number in a carboxylic acid, as it is always on the end of the carbon chain. The basic rules of naming apply.
Short carboxylic acids are liquids and are soluble in water.
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