Derivatization

Several characteristics are desirable for a derivatization reaction: • The reaction is reliable and proceeds to completion. Less unreacted starting material will simplify analysis. Also, this allows a small amount of analyte to be used. • The reaction is general, allowing a wide range of substrates, yet specific to a single functional group, reducing complicating interference. • The products are relatively stable, and form no degradation products within a reasonable period, facilitating analysis. Some examples of good derivatization reactions are the formation of esters and amides via acyl chlorides. ==Classical qualitative organic analysis==

Classical qualitative organic analysis usually involves reacting an unknown sample with various reagents; a positive test usually involves a change in appearance — color, precipitation, etc. These tests may be extended to give sub-gram scale products. These products may be purified by recrystallization, and their melting points taken. An example is the formation of 2,4-dinitrophenylhydrazones from ketones and 2,4-dinitrophenylhydrazine. By consulting an appropriate reference table such as in Vogel's, the identity of the starting material may be deduced. The use of derivatives has traditionally been used to determine or confirm the identity of an unknown substance. However, due to the wide range of chemical compounds now known, it is unrealistic for these tables to be exhaustive. Modern spectroscopic and spectrometric techniques have made this technique obsolete for all but pedagogical purposes. ==For HPLC==

Colorimetric detection after separation by liquid chromatography is an established method e.g. in amino acid analysis. So-called post-column derivatisation with Ninhydrin as the active reagent can be used in HPLC systems directly after the separation column. Secondary amines react with ninhydrin to a blue iminium salt, while primary amines form a yellow product with ninhydrin. This principle is especially common in amino acid analysators, specialized HPLC-systems for the detection of free amino acids, also because in feed industry the use of post-column derivatisation is mandatory. ==For gas chromatography==

Polar N-H and O-H groups on which give hydrogen bonding may be converted to relatively nonpolar groups on a relatively nonvolatile compound. The resultant product may be less polar, thus more volatile, allowing analysis by gas chromatography. Bulky, nonpolar silyl groups are often used for this purpose. ==Chiral derivatizing agent==

Chiral derivatizing agents react with enantiomers to give diastereomers. Since diastereomers have different physical properties, they may be further analyzed by HPLC and NMR spectroscopy. ==References==