(a) p-Nitrodimethylaniline gives a yellow solution in water which fades to colorless when made acidic. Explain. (b) p-Dimethylaminoazobenzene is bright yellow in aqueous solution ( \(\lambda_{\max } 4200 \mathrm{~A}\) ) but turns intense red \(\left(\lambda_{\max } 5300 \mathrm{~A}\right)\) if dilute acid is added. If the solution is then made very strongly acidic, the red color changes to a different yellow \(\left(\lambda_{\max } 4300 \mathrm{~A}\right)\) than the starting solution. Show how one proton could be added to p-dimethylamino-azobenzene to cause the absorption to shift to longer wavelengths and how addition of a second proton could shift the absorption back to shorter wavelengths.

(a) p-Nitrodimethylaniline demonstrates a schoolarly colour in water due to the delocalized electron cloud throughout the molecule, allowing it to absorb light in the visible region. When an acidic solution is added, a proton (H+) attaches to the nitrogen of the amino group, breaking up the delocalization of electrons and causing the molecule to absorb light outside the visible spectrum, thereby turning the solution colorless. (b) p-Dimethylaminoazobenzene in an aqueous solution changes from bright yellow to intense red when dilute acid is added due to the attachment of a proton (H+) to the nitrogen atom of the dimethylamino group. This proton addition increases the delocalization of the electron cloud, allowing the molecule to absorb light at a longer wavelength (\(\lambda_{\max} = 5300 \) Å). When a second proton is introduced and binds with the nitrogen atom of the azo group, this disrupts this extended delocalization, causing the molecule to absorb light at a shorter wavelength (\(\lambda_{\max} = 4300 \) Å), and thus creating a different yellow color. In all, the addition of protons to this molecule changes electron delocalization in the molecule, and therefore modifies the absorption wavelengths.