2.83 Triphenylmethane dyes are among the first synthetic dyes developed for commercial use. A comparison of the structures of these compounds reveals that even small differences in structure can lead to large differences in color. The structures of three such dyes are shown below.

\begin{tabular}{|l|c|c|}
\hline
\multicolumn{1}{|c|}{Dye} & \multicolumn{1}{|c|}{Group 1} & \multicolumn{1}{|c|}{Group 2} \\
\hline
Basic Green 4 & [tex]$H$[/tex] & [tex]$CH_3$[/tex] \\
\hline
Basic Red 9 & [tex]$NH_2$[/tex] & [tex]$H$[/tex] \\
\hline
Basic Violet 4 & [tex]$N_2(C_2H_5)_2$[/tex] & [tex]$C_2H_5$[/tex] \\
\hline
\end{tabular}

(a) Draw resonance structures for Basic Green 4 that illustrate the delocalization of the positive charge.

(b) Determine whether Basic Green 4 or Basic Violet 4 is expected to have greater resonance stabilization. Justify your choice.



Answer :

Certainly! Let's tackle the problem step-by-step.

### Part (a): Draw Resonance Structures for Basic Green 4 to Illustrate Delocalization of the Positive Charge.

To start, we should analyze the structure of Basic Green 4 and identify key elements such as where the positive charge is located. Basic Green 4 has the following in its structure:
- A central carbon atom bonded to three phenyl groups.
- A positive charge delocalized over these phenyl groups.

Given the structure:
```
Ph
|
Ph-C^+
|
Ph
```
We can draw resonance structures that show the delocalization of the positive charge over the phenyl rings.

1. First Resonance Structure:
```
+ +
| |
C6H5-C-C6H5 <--> C6H5--C+--C6H5
|
C6H5
```
2. Second Resonance Structure:
```
+
|
C6H5--C--C6H5 <--> +--C--C6H5
| |
C6H5 C6H5
```
3. Third Resonance Structure:
```
+
|
C6H5--C--C6H5 <--> C6H5--C--C6H5
| +
C6H5
```

These resonance structures show how the positive charge can be delocalized over the three phenyl groups through a series of π-electron shifts.

### Part (b): Determine Whether Basic Green 4 or Basic Violet 4 is Expected to Have Greater Resonance Stabilization.

To determine which dye, Basic Green 4 or Basic Violet 4, has greater resonance stabilization, we compare the structures, specifically considering the substituents and their ability to donate or withdraw electrons through resonance.

Basic Green 4 (Malachite Green):
Structure:
- Phenyl groups with no additional electron-donating groups.
- Positive charge delocalized over the aromatic rings.

Basic Violet 4 (Crystal Violet):
Structure:
- Phenyl groups with additional electron-withdrawing groups \(\left(\mathrm{N_2(C_2H_5)_2} \right) \) on each phenyl ring.
- These substituents can donate electrons via resonance through hyperconjugation and inductive effect, stabilizing the positive charge further.

In this case, Basic Violet 4 is expected to have greater resonance stabilization. This is because the ethyl groups attached to the nitrogen atoms in the substituents on the phenyl rings can act as electron-donating groups through hyperconjugation and inductive effects. Consequently, this will delocalize the positive charge more effectively than in Basic Green 4, which lacks such strong electron-donating substituents.

Hence, Basic Violet 4, with its additional electron-donating groups, is more resonance-stabilized compared to Basic Green 4.