Question

(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.

Short answer

(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.

Step-by-step solution

(a) Understanding the color change of p-nitrodimethylaniline in an acidic solution

p-Nitrodimethylaniline is an aromatic amine with a nitro group at the para position. In its basic form, it has a delocalized electron cloud across the entire molecule, which can absorb light in the visible region. This is what gives the yellow color to the molecule. However, when the solution becomes acidic, the lone pair of electrons on the nitrogen in the amine group is donated to a proton (H+) from the acidic solution forming NH3+. This causes the delocalization of the electrons to be disrupted. The electrons become localized, causing the molecule to absorb light in a different wavelength that is outside the visible region. As a result, the solution turns colorless.

(b) Addition of protons and color change in p-dimethylaminoazobenzene

p-Dimethylaminoazobenzene has two functional groups, the azo group and the dimethylamino group. We will focus on these two groups to explain the color change upon acidification.

Addition of one proton

When the first proton is added to p-dimethylaminoazobenzene, it can attach to the nitrogen atom of the dimethylamino group. The lone pair of electrons on this nitrogen interacts with a proton (H+) from the acidic solution to form NH(CH3)2+. This causes the electron cloud to be more delocalized and extends across both the azo and phenyl groups. The extended delocalization of electrons causes the molecule to absorb light at a much longer wavelength (\(\lambda_{\max} = 5300 \) Å), resulting in an intense red color.

Addition of second proton

When the second proton is added, it can attach to the nitrogen atom of the azo group. The lone pair of electrons on the azo nitrogen interacts with a proton (H+) from the acidic solution, forming N2H+. This disrupts the delocalization of the electrons within the azo group, and the molecule is no longer able to absorb light at the longer wavelength of 5300 Å. As a result, the absorption shifts back to a shorter wavelength (\(\lambda_{\max} = 4300 \) Å) and the solution's color changes to a different yellow. In summary, the addition of protons to p-dimethylaminoazobenzene causes changes in the delocalization of electrons within the molecule and, therefore, changes in the absorption wavelengths. The first proton addition causes an absorption shift to a longer wavelength (red), while the second proton addition reverts the absorption back to a shorter wavelength (different yellow).

Key concepts

p-Nitrodimethylaniline

p-Nitrodimethylaniline is an organic compound with a distinct yellow color due to its structure. The molecule contains a nitro group and an aromatic amine where the amine group is at the para position. This unique setup allows the molecule to have a delocalized electron system across the aromatic ring. This electron cloud is responsible for absorbing visible light, particularly in the yellow spectrum.

When this compound is added to an acidic solution, a protonation process occurs. The lone pair of electrons on the nitrogen within the amine group interacts with protons from the acid. As a result, the nitrogen is converted into an ammonium form (NH3+). The delocalization of electrons is interrupted, causing the molecule to absorb light at different, non-visible wavelengths. This change explains why the solution becomes colorless when it is acidic.

p-Dimethylaminoazobenzene

p-Dimethylaminoazobenzene is a fascinating compound known for its vivid color changes when reacting with acids. The molecule is composed of two primary functional groups: a dimethylamino group and an azo group. These groups contribute significantly to its optical properties.

Initially, in an aqueous solution, p-dimethylaminoazobenzene appears bright yellow because it absorbs wavelengths around 4200 Å. The presence of these functional groups facilitates a partial electron cloud delocalization, allowing ultraviolet to visible light absorption. Changes in environmental acidity can greatly affect this electron distribution and hence the observed color.

Protonation effects

Protonation refers to the addition of a proton (H+) to a molecule, and it can dramatically influence the color of aromatic compounds like p-Dimethylaminoazobenzene. As protons are added, they interact with electron-rich sites in the molecule, such as the nitrogen atoms in the dimethylamino or azo groups.

With the first protonation, the proton attaches to the nitrogen atom in the dimethylamino group. This interaction results in an electron delocalization that extends further across the compound, leading to absorption at a longer wavelength, and thus the color shifts to intense red ( λ_{max} = 5300 Å).

• This involves absorbing longer wavelengths, indicating more energy requirement resulting in the red color.

When a second proton is introduced, it attaches to the azo nitrogen. This disrupts the extended delocalization, causing a shift back to absorption at shorter wavelengths ( λ_{max} = 4300 Å), yielding a distinct yellow color.

Visible light absorption

Visible light absorption in aromatic compounds depends heavily on the extent of electron delocalization across the molecule. Compounds like p-Dimethylaminoazobenzene are excellent examples of how structural changes impact light absorption.

The color observed in solutions is due to the specific wavelengths of light that the molecule absorbs and reflects. For instance, when p-Dimethylaminoazobenzene is neutral, it absorbs at λ_{max} 4200 Å, showing a yellow color since other wavelengths are reflected. Upon adding protons:

• The absorption differs. A longer wavelength (5300 Å) creates a red color due to deeper energy transitions, while re-protonation to a shorter 4300 Å wavelength reflects a different yellow.
• A change in wavelength corresponds to a fundamental shift in the electronic configuration of the molecule.

Understanding how protonation modifies this absorption logic helps explain the vivid visual changes in color experienced by these compounds when subjected to varying acid concentrations.