Acids

The names of acids differentiate between (1) acids in which the H⁺ ion is attached to an oxygen atom of a polyatomic anion (these are called oxoacidsAn acid in which the dissociable ${\text{H}}^{+}$ ion is attached to an oxygen atom of a polyatomic anion., or occasionally oxyacids) and (2) acids in which the H⁺ ion is attached to some other element. In the latter case, the name of the acid begins with hydro- and ends in -ic, with the root of the name of the other element or ion in between. Recall that the name of the anion derived from this kind of acid always ends in -ide. Thus hydrogen chloride (HCl) gas dissolves in water to form hydrochloric acid (which contains H⁺ and Cl⁻ ions), hydrogen cyanide (HCN) gas forms hydrocyanic acid (which contains H⁺ and CN⁻ ions), and so forth (Table 2.8 "Some Common Acids That Do Not Contain Oxygen"). Examples of this kind of acid are commonly encountered and very important. For instance, your stomach contains a dilute solution of hydrochloric acid to help digest food. When the mechanisms that prevent the stomach from digesting itself malfunction, the acid destroys the lining of the stomach and an ulcer forms.

If an acid contains one or more H⁺ ions attached to oxygen, it is a derivative of one of the common oxoanions, such as sulfate (SO₄²⁻) or nitrate (NO₃⁻). These acids contain as many H⁺ ions as are necessary to balance the negative charge on the anion, resulting in a neutral species such as H₂SO₄ and HNO₃.

The names of acids are derived from the names of anions according to the following rules:

1. If the name of the anion ends in -ate, then the name of the acid ends in -ic. For example, because NO₃⁻ is the nitrate ion, HNO₃ is nitric acid. Similarly, ClO₄⁻ is the perchlorate ion, so HClO₄ is perchloric acid. Two important acids are sulfuric acid (H₂SO₄) from the sulfate ion (SO₄²⁻) and phosphoric acid (H₃PO₄) from the phosphate ion (PO₄³⁻). These two names use a slight variant of the root of the anion name: sulfate becomes sulfuric and phosphate becomes phosphoric.
2. If the name of the anion ends in -ite, then the name of the acid ends in -ous. For example, OCl⁻ is the hypochlorite ion, and HOCl is hypochlorous acid; NO₂⁻ is the nitrite ion, and HNO₂ is nitrous acid; and SO₃²⁻ is the sulfite ion, and H₂SO₃ is sulfurous acid. The same roots are used whether the acid name ends in -ic or -ous; thus, sulfite becomes sulfurous.

The relationship between the names of the oxoacids and the parent oxoanions is illustrated in Figure 2.20 "The Relationship between the Names of the Oxoacids and the Names of the Parent Oxoanions", and some common oxoacids are in Table 2.9 "Some Common Oxoacids".

Figure 2.20 The Relationship between the Names of the Oxoacids and the Names of the Parent Oxoanions

Many organic compounds contain the carbonyl groupA carbon atom double-bonded to an oxygen atom. It is a characteristic feature of many organic compounds, including carboxylic acids., in which there is a carbon–oxygen double bond. In carboxylic acidsAn organic compound that contains an −OH group covalently bonded to the carbon atom of a carbonyl group. The general formula of a carboxylic acid is ${\text{RCO}}_{\text{2}}\text{H}$. In water a carboxylic acid dissociates to produce an acidic solution., an –OH group is covalently bonded to the carbon atom of the carbonyl group. Their general formula is RCO₂H, sometimes written as RCOOH:

where R can be an alkyl group, an aryl group, or a hydrogen atom. The simplest example, HCO₂H, is formic acid, so called because it is found in the secretions of stinging ants (from the Latin formica, meaning “ant”). Another example is acetic acid (CH₃CO₂H), which is found in vinegar. Like many acids, carboxylic acids tend to have sharp odors. For example, butyric acid (CH₃CH₂CH₂CO₂H), is responsible for the smell of rancid butter, and the characteristic odor of sour milk and vomit is due to lactic acid [CH₃CH(OH)CO₂H]. Some common carboxylic acids are shown in Figure 2.21 "Some Common Carboxylic Acids".

Figure 2.21 Some Common Carboxylic Acids

Although carboxylic acids are covalent compounds, when they dissolve in water, they dissociate to produce H⁺ ions (just like any other acid) and RCO₂⁻ ions. Note that only the hydrogen attached to the oxygen atom of the CO₂ group dissociates to form an H⁺ ion. In contrast, the hydrogen atom attached to the oxygen atom of an alcohol does not dissociate to form an H⁺ ion when an alcohol is dissolved in water. The reasons for the difference in behavior between carboxylic acids and alcohols will be discussed in Chapter 8 "Ionic versus Covalent Bonding".

Bases

We will present more comprehensive definitions of bases in later chapters, but virtually every base you encounter in the meantime will be an ionic compound, such as sodium hydroxide (NaOH) and barium hydroxide [Ba(OH)₂], that contain the hydroxide ion and a metal cation. These have the general formula M(OH)_n. It is important to recognize that alcohols, with the general formula ROH, are covalent compounds, not ionic compounds; consequently, they do not dissociate in water to form a basic solution (containing OH⁻ ions). When a base reacts with any of the acids we have discussed, it accepts a proton (H⁺). For example, the hydroxide ion (OH⁻) accepts a proton to form H₂O. Thus bases are also referred to as proton acceptors.

Concentrated aqueous solutions of ammonia (NH₃) contain significant amounts of the hydroxide ion, even though the dissolved substance is not primarily ammonium hydroxide (NH₄OH) as is often stated on the label. Thus aqueous ammonia solution is also a common base. Replacing a hydrogen atom of NH₃ with an alkyl group results in an amineAn organic compound that has the general formula ${\text{RNH}}_{\text{2}}$, where R is an alkyl group. Amines, like ammonia, are bases. (RNH₂), which is also a base. Amines have pungent odors—for example, methylamine (CH₃NH₂) is one of the compounds responsible for the foul odor associated with spoiled fish. The physiological importance of amines is suggested in the word vitamin, which is derived from the phrase vital amines. The word was coined to describe dietary substances that were effective at preventing scurvy, rickets, and other diseases because these substances were assumed to be amines. Subsequently, some vitamins have indeed been confirmed to be amines.