Pharmaceutical Chemistry - Essay Example

A colored compound having a chromophore is known as chromogen. 2. Give an example of an auxochrome and explain how it influences the absorption spectra of the molecule. If we change the pH of the solution how will that influence the spectrum, explain for full credit. In a molecule of nitrobenzene, –NO2 or nitro group is the chromophore. Nitrobenzene absorbs at 400nm and is pale yellow in color. When nitrobenzene is conjugated with the amine group, it froms nitro aniline. The amine group is the auxochrome.

Nitroaniline absorbs light at 450nm and is a yellow compound. The longer resonance system decreases the energy gap between the ground state and excited state transition, thus producing visible color. This displacement to longer wavelength is known as bathochromic shift. The auxochrome (ie. amine group), is basic; it will gain a proton and will have a positive charge (electron withdrawing). That positive charge may activate the chromophore for a facile nucleophilic addition of hydroxide (or water) thus converting one pi orbital to a sigma orbital (loss of conguation and less stable pi system/more energy in that pi system) or the electron withdrawing effect pulls electrons away from sparsely populated pi-star orbitals (increases their energy) so the energy gap between pi and pi-star orbitals is larger.

Larger energy, means larger frequency, means shorter wavelength (red shift or shorter wavelength). At alkaline pH, the absorbing system may be intact and will show red shift, but in acidic solution the amino - H2N group is replaced by - +NH3, which is considerably less efficient as an auxochrome. Thus in acidic solutions, a hypo chromic effect occurs resulting into a blue shift and the solution would appear light in color. 1. Verify the UV/VIS spectrophotometer is in the spectral wavelength range of the analytical method for