Organic Acids and Bases

Organic acids and organic bases are acids and bases that contain a carbon skeleton. Within these categories are a number of classes of neutral proton acids and bases (that is, uncharged acids and bases.) The first part of this section examines the three main types of

neutral organic proton acids to see why they are acids and why they have widely different acid strengths. The second part looks at the two main types of neutral organic bases. The last part looks at positively charged carbon acids and negatively charged carbon bases.

Three main types of neutral organic Brønsted-Lowry acids are carboxylic acids, phenols, and alcohols. Each of these three functional groups has an —OH group. Each is acidic because of the electronegativity difference between the oxygen and the hydrogen involved in the O—H bond. The differences in acid strength of the three functional groups are due to the differences in stability of the conjugate base. The most acidic of the three groups are the carboxylic acids. Carboxylic acids are characterized by the presence of the carboxyl group:

 

Carboxylic acids are among the most acidic of the neutral organic acids, but they are rather weak acids. For example, the pKa of acetic acid, a common carboxylic acid, is 4.8, indicating that only a small portion of the molecules of acetic acid ionize in an aqueous solution. In contrast, mineral acids, such as HCl, with a pKa of –7.0, and HNO3, with a pKa of –5.2, completely ionize in aqueous solutions. Although carboxylic acids are weaker than mineral acids, they are the strongest of the neutral organic acids that you will study.

The reason for the relative strength of the carboxylic acids is the conjugate base is resonance-stabilized, which makes it a weak base.

 

 In the carboxylate ion the negative charge spreads over the two oxygen atoms as a resonance hybrid. This reduces the energy of the anion and makes the carboxylic acid more acidic.
Another way of visualizing the reason for the acid strength of carboxylic acids is to look at the molecular orbital system of the carboxylate ion. The carboxylate ion includes three p orbitals that contain a total of four electrons. The overlap of these three p orbitals results in a three-centered π molecular orbital system.

The carbon is joined to each oxygen atom by the equivalent of ½ of a π bond. Each oxygen atom bears ½ of the negative charge.

 

The second main type of neutral organic acids are the phenols. Phenols are much less acidic than carboxylic acids. An —OH group attached to an aromatic ring is characteristic of phenols:

 

Phenol has a pKa of 10.0 in aqueous media, indicating that in water only a very small portion of it ionizes.

 

Phenols are moderately strong organic acids because their conjugate bases are resonance-stabilized. The aromatic ring is involved in resonance, which stabilizes the negative charge.

 

However, this stabilization is less significant than it is for carboxylic acids for two reasons: the resonance stabilization of the phenolate ion disrupts the aromaticity of the aromatic ring, and the resonance stabilization places a negative charge on the carbon atoms, which, when compared to oxygen, are not very electronegative.
The third type of neutral organic acids are alcohols. An —OH group attached to an alkyl group characterizes an alcohol:

 R---OH

Alcohols are much less acidic than phenols. In fact, most alcohols have an acid strength slightly lower than that of water.

 

 

A typical alcohol has a pKa of 15 to 18 in aqueous media, indicating only a very small amount of ionization. Alcohols have such a low acidity because there is no resonance stabilization of the conjugate base.
This section discusses only two of the many types of neutral organic bases: amines and ethers. The primary characteristic of neutral organic bases is they contain one or more pairs of nonbonding electrons. These pairs of electrons are available to donate to a Lewis acid or to accept a proton when the base is acting as a Brønsted-Lowry base. The more available the pair of electrons, often called a lone pair, the stronger the base. Any molecule with a lone pair of electrons can act as a base.
The most common of the organic bases are the amines. Amines are derivatives of ammonia (NH3) and most are weak bases in aqueous media.

The pKa of methyl ammonium ion is 10.6 meaning that the methyl ammonium ion is a relatively weak acid. Thus, methylamine is a moderately strong base.
Amines are stronger bases than other neutral organic bases because the nonbonding pair of electrons on the nitrogen is more available than nonbonding pairs of electrons on other neutral organic bases. The atoms that are found in these other neutral organic bases are oxygen, sulfur, or the halogens. Nitrogen holds its electrons less tightly than these other atoms, so its compounds are the stronger bases. Figure 5.3 illustrates the structure of an amine.

Figure 5.3. Structure of the amine nitrogen.
Ethers, the second type of neutral organic bases, have the general structure ROR′. Ethers are weak bases in aqueous media. In fact, they are so weak that they do not appreciably protonate, or accept a proton, even in 1 M HCl. The pKa of the conjugate acid of ethyl ether is –3.8. A pKa of this magnitude indicates that water is a better base than is an ether.
In nonaqueous media, ethers are good Lewis bases, forming stable complexes with Lewis acids. The ability to form stable complexes is extremely important in organic reactions. For example, in organic synthesis, chemists widely use the complex of BH3 with the cyclic ether tetrahydrofuran:

The third category of organic acids and bases discussed in this section are the positively charged acids and the negatively charged bases. Positively charged acids are electron-deficient. That is, they are organic acids that contain a carbon without an octet of electrons. The most significant electron-deficient organic acid is the carbocation (formerly called a carbonium ion). Carbocations are very reactive reaction intermediates, so chemists seldom observe them directly. A carbocation is a Lewis acid because, without a full octet of electrons, it is electron-deficient and "needs" electrons. As a result of this need for electrons, it reacts with the first available Lewis base—although it prefers a hard one because it is a hard acid. As Figure 5.4 shows, the positively charged carbon forms three sp2 hybridized bonds in a plane with an empty p orbital perpendicular to that plane. Chapter 12 examines nucleophilic substitution reactions that involve carbocations.

The negatively charged organic base discussed in this section is the carbanion. A carbanion has bonds to three other atoms and one pair of nonbonding electrons. The structure of a carbanion is much like the structure of an amine (See Figure 5.5). Because carbon is not very electronegative, it holds these nonbonding electrons loosely. Thus, a carbanion is a strong base. (Chapters 19 and 20 cover carbanion reactions extensively.)

Now that you have seen the various types of organic acids and bases, Section 5.5 examines the factors that modify the strength of the specific acids and bases.