Chapter 19 - Derivatization Reactions for Analytes with Various Functional Groups

Derivatization of Compounds with Alcohol, Epoxide, or Ether Groups

Alcohols contain a hydroxyl functional group OH with an active hydrogen. For GC analysis, the formation of hydrogen bonds of the OH may affect the separation process as well as the boiling point of the compound even when the rest of the molecule is not large. Also, water elimination at higher temperatures may be possible in certain alcohols, these cases typically leading to a mixture of compounds. The replacement of the active hydrogen with groups containing halogens also is used to enhance

Derivatization Reactions for Phenols

Phenols have OH functional groups, and most reactions that were described for alcohols also work for phenols. Some differences result from two characteristics of phenols. The first is the acidity of the hydrogen in the OH group, which has a pKa = 8–11 for phenols as compared to the pKa = 16–18 for alcohols (see Table 18.1.2). The second difference comes from the special chemical properties of the aromatic ring.

The derivatization of phenols is in many cases directed toward the replacement of the

Derivatization Reactions for Thiols and Sulfides

The thiols (mercaptans) containing SH group(s) on an aliphatic chain are stronger acids than the corresponding alcohols and have the pKa = 10–11. The thiols with the SH group on an aromatic ring are stronger acids than the corresponding phenols and have the pKa = 6–8. The replacement of the active hydrogens in the SH group is therefore the common derivatization procedure for thiols, and several reactions take place more easily for thiols than for the equivalent alcohols or phenols. This is, for

Derivatization of Compounds with Amino and other Nitrogen-Containing Groups

A wide variety of derivatization reactions are used for amine analysis. This is explained by the presence of many amines in nature, mainly in biological materials, and by the diversity of chemical reactions applicable to amines.

Amines are Lewis bases, and because nitrogen is not as electronegative as oxygen, amines have a stronger tendency to react with protons. Amines are much more basic than water, and their aqueous solutions have basic character. Basicity of amines is usually expressed by

Derivatization Of Aldehydes And Ketones

The derivatization of aldehydes and ketones is commonly done for better detectability. The separation by GC or HPLC usually does not raise special problems for carbonyl compounds. This can be attributed to the absence of hydrogen bonding, which aldehydes and ketones cannot form because of the lack of active hydrogens. Some aldehydes or ketones containing an α-H may generate enols, and then the active hydrogens are present. However, even when enols are generated, the equilibrium between the enol

Derivatization of Carboxylic Acids

The hydrogen in COOH group is able to dissociate in water and form enough H₃O⁺ ions to confer an acidic character to aqueous solutions of carboxylic compounds. The acidity constant K_a for the equilibrium R-COOH + H₂O ⇄ RCOO⁻H₃O⁺ depends on the nature of the radical R, but in general carboxylic acids are weak acids, as shown for several acids in Table 19.6.1. Trichloroacetic acid and trifluoroacetic acid are among the strongest organic acids.

In addition to their acidity, the hydrogen in the

Derivatization of Amides, Esters, and other Derivatives of Carboxylic Acids, and of Carbonic Acid

Organic acids form a number of functional derivatives. Some of these are common natural compounds such as the lipids, and others are used as drugs or pesticides. Table 19.7.1 lists several of these compounds.

Derivatives of carbonic acid H₂CO₃, which can be seen as HO-COOH or O=C(OH)₂, are also encountered in practice quite frequently. A list with a few derivatives of carbonic acid is given in Table 19.7.2.

Among the compounds from the classes given in Tables 19.7.1 and 19.7.2, probably the most

Derivatization of other types of Organic Groups and of Organometallic and Inorganic Compounds

Many organic groups were not captured in the series of derivatizations presented in this chapter. Among these are functional groups containing phosphorus, arsenic, boron or sulfur. The derivatization of dienes was presented only related to unsaturated acids (see Section 19.6). Also, organometallic compounds of mercury, tin, etc. and a number of inorganic compounds, which are derivatized for analysis using chromatography [761], were not yet discussed. Some selected derivatizations of these types

Derivatization of Carbohydrates

Carbohydrates (sugars or saccharides) form an important group of naturally occurring compounds. Based on the number of sugar units in their molecule, carbohydrates are classified as monosaccharides, oligosaccharides, or polysaccharides. The general formula for monosaccharides is C_nH_2nO_n (n = 3 for trioses, n = 4 for tetroses, n = 5 for pentoses, n = 6 for hexoses…). Monosaccharides contain a carbonyl group (aldehyde in aldoses or ketone in ketoses) and two or more -OH groups. Many derivatizations of

Derivatization of Amino Acids and Related Compounds

Amino acids form an important class of bifunctional compounds. There are numerous known amino acids, and certain α-amino acids are the building blocks from which proteins are constructed. These protein amino acids have the general formula R-CH(NH₂)-COOH with the exception of proline and hydroxyproline, which have secondary amino groups. With a few exceptions, amino acids have chiral molecules, the amino acids from proteins having L (S) configuration at the chiral α carbon:

The names and the

Derivatization of other Multifunctional Compounds

A large number of compounds, either natural or synthetic have more than one type of functional group. The combinations of alcohol and carbonyl in carbohydrates and amine and acid in amino acids were previously discussed in Sections 19.9 and 19.10, respectively. Other combinations of groups that are rather common in practice are OH and NH₂ in amino alcohols or amino phenols, C=O and COOH in keto acids, OH and COOH in hydroxyacids, etc. Compounds with significant biological role such as