Which one of the following statements is not correct according to quinonoid theory of indicators?

The benzenoid form has dark colour and quinonoid a light colour

The correct answer is option 4. The benzenoid form has dark colour and quinonoid a light colour.

Let us delve into the details of the quinonoid theory of indicators and evaluate each statement in that context.

Quinonoid Theory of Indicators: The quinonoid theory explains the color change of acid-base indicators based on the structural transformation between two different forms of the indicator molecule: the benzenoid form and the quinonoid form. Here's a detailed look at the relevant aspects:

1. All indicators are organic compounds, i.e., benzene ring derivatives: Most acid-base indicators are indeed organic compounds and many contain benzene rings or other aromatic structures. Aromatic rings are crucial for the indicator's ability to absorb visible light, which leads to the perceived color. Thus, this statement is correct.

2. One of the benzene rings in the indicator molecules exists in two forms, i.e., benzenoid form and quinonoid form: The quinonoid theory posits that indicators can switch between two structural forms. The benzenoid form typically has a single or double-bond structure within the benzene ring, while the quinonoid form involves a structure where two of the carbons in the ring are double-bonded to oxygens or other atoms, forming a quinonoid structure. This statement is correct.

3. One form of indicator exists in acidic medium and the other form exists in the basic medium: According to the quinonoid theory, the indicator changes its structure based on the pH of the medium. In an acidic medium, the indicator is in one form (often the benzenoid form), and in a basic medium, it shifts to the other form (often the quinonoid form). This structural change is responsible for the color change observed. Therefore, this statement is correct.

4. The benzenoid form has dark colour and quinonoid a light colour: This statement is not universally correct. The color associated with the benzenoid and quinonoid forms of an indicator depends on the specific structure of the indicator molecule. In many cases, the quinonoid form is more deeply colored because it often has a more extended conjugated system that can absorb light at different wavelengths. The exact colors can vary significantly between different indicators. For instance:

Phenolphthalein: Colorless (benzenoid form) in acidic conditions and pink (quinonoid form) in basic conditions.

Methyl Orange: Red (benzenoid form) in acidic conditions and yellow (quinonoid form) in basic conditions.

Detailed Analysis:

Benzenoid Form: Typically, this is the form that exists in acidic conditions. It usually has a structure where the aromatic ring retains its classic single-double alternating bond configuration.

Quinonoid Form: This form is typically present in basic conditions. The structure involves a transformation where two carbon atoms in the ring double bond with other atoms, forming a more extended conjugated system.

The color change occurs because the different electronic configurations in the benzenoid and quinonoid forms result in different wavelengths of light being absorbed, hence different colors being observed. 

Conclusion: The incorrect statement according to the quinonoid theory of indicators is: (4) The benzenoid form has dark colour and quinonoid a light colour. This is incorrect because the color associated with each form depends on the specific indicator and can vary; it is not a universal rule that benzenoid forms are dark and quinonoid forms are light. In fact, for many indicators, the quinonoid form is more deeply colored due to its extended conjugation.