The chromophore of phenylephrine is not extended but its structure includes a phenolic hydroxyl group. The phenolic group functions as an auxochrome under both acidic and alkaline conditions. Under acidic conditions it has two lone pairs of electrons, which can interact with the benzene ring and under basic conditions it has three. Figure 4.11 shows the bathochromic and hyperchromic shift in the spectrum of phenylephrine, which occurs when 0.1 M NaOH is used as a solvent instead of 0.1 M HC1. Under acidic conditions the X max is at 273 and has an A (1 %, 1 cm) value of 110 and under alkaline conditions the X max is a 292 nm and has an A (1%, 1 cm) value of 182.
The types of shifts observed for procaine and phenylephrine can be exploited in order to achieve analysis of mixtures. Two examples of this are given later in the chapter.
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