The carbonyl group of an aldehyde or ketone may interact with an alcohol to form acetals and hemiacetals. Modern terminology only uses the terms acetals and hemiacetals, but you may sometimes see the older terms hemiketal, which is a type of hemiacetal, and ketal, a type of acetal. See Figure 3-6 for an illustration of these compounds.
FIGURE 3-6: Acetals, hemiacetals, hemiketals, and ketals.
The difference between a hemiacetal and a hemiketal is that in a hemiacetal R” = H, but in a hemiketal R” ≠ H. Similarly, the difference between an acetal and a ketal is that in an acetal R” = H, but in a ketal R” ≠ H.
Carboxylic acids
Carboxylic acids, along with phosphoric acid, are the most important biological acids. Carboxylic acids react with bases such as the amines to produce salts. The salts contain an ammonium ion from the amine and a carboxylate ion from the acid.
Carboxylic acids combine with alcohols to form esters and can indirectly combine with amines to form amides. Hydrolysis of an ester or an amide breaks the bond and inserts water. An acid, base, or enzyme is needed to catalyze hydrolysis. Under acidic conditions, you can isolate the acid and either the alcohol or the ammonium ion from the amine. Under basic conditions, you can isolate the carboxylate ion and either the alcohol or the amine.
Thiols and amines
Under mild oxidation, two thiols join to form a disulfide. Mild reducing conditions, catalyzed by enzymes or through the use of certain reducing agents, reverse this process. Such formation of disulfide linkages is important in the chemistry of many proteins, such as insulin.
It’s possible to replace all the hydrogen atoms from an ammonium ion,
Phosphoric acid
Phosphoric acid, H3PO4, may behave like a carboxylic acid and form esters. The esters have an organic group, R, replacing one, two, or three of the hydrogen atoms. The resultant compounds are monoesters, diesters, and triesters. The hydrogen atoms remaining in the monoesters and diesters are acidic.
pH and functional groups
Many of the biological functions of substances are pH-dependent. For this reason, knowing which functional groups are acidic, basic, or neutral is important. Neutral functional groups behave the same no matter what the pH is. Table 3-2 lists the functional groups; whether they’re acidic, neutral, or basic; and, for acidic and basic groups, their weakness level (medium, weak, or very weak). The weaker a substance is in terms of pH, the less likely it’s affected by its solution pH (except under extreme pH conditions, which normally don’t occur in biological systems).
TABLE 3-2 Acid-Base Properties of Biologically Important Functional Groups
| Functional Group | Acid, Base, or Neutral | Weakness Level |
|---|---|---|
| Monophosphate esters and diphosphate esters | Acid | Medium |
| Carboxylic acids | Acid | Weak |
| Phenols | Acid | Very weak |
| Thiols | Acid | Very weak |
| Amine salts | Acid | Very weak |
| Amines | Base | Weak |
| Carboxylate ions | Base | Very weak |
| Alcohols | Neutral | |
| Carboxylate esters | Neutral | |
| Ethers | Neutral | |
| Triphosphate esters | Neutral | |
| Thioethers | Neutral | |
| Disulfides | Neutral | |
| Amides | Neutral | |
| Ketones | Neutral | |
| Aldehydes | Neutral |
Same Content, Different Structure: Isomerism