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Read the following question and answer as per the direction given below : 

(a) Statement I is true; Statement II is true; Statement II is a correct explanation of Statement I. 

(b) Statement I is true; Statement II is true; Statement II is not the correct explanation of Statement I. 

(c) Statement I is true; Statement II is false. 

(d) Statement I is false; Statement II is true.

 Statement I : Aniline on reaction with NaNO2/HCl at OoC followed by coupling with B-naphthol gives a dark blue coloured precipitate.

Statement II : The colour of the compound formed in the reaction of aniline with NaNO2/HCl at 0oC followed by coupling with p-naphthol is due to the extended conjugation.

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Best answer

The correct option.(a) Statement I is true; Statement II is true; Statement II is a correct explanation of Statement I.

Explanation:

Statement II is correct. Statement I appears correct in the light of statement II. What is the colour of a dye that dye is not included in JEE syllabus but what is the cause and understanding behind a colour could be essentially expected.

Concept : Usually a certain proportion of the light is absorbed and the rest reflected, the substance has the colour of the reflected light.

If only a single band is absorbed, the substance has the complementary colour

The wavelength of photon absorbed depends on the energy difference between the excited and ground states. The smaller the difference, the longer being the wavelength.

The larger the number of electrons involved in resonance, the smaller is the energy difference between the ground and excited state. Hence, the more extended the conjugation in a molecule and the greater the contribution of charged structures, the longer is the wavelength of the photon required to excite the molecule.

Especially, the presence of + I / + M and - I / - M groups in the conjugated system will both extend the conjugation and increase the contributions of charged structures to the resonance hybrid. Therefore, in these circumstances, the wavelength should become progressively longer.

Depending on this discussion it is apparent that the concerned resultant azo dye will absorb higher wavelength.

Wavelength Colour absorbed Complimentary colour 

490 - 500   blue-green  red. 

580 - 595   yellow       blue

605 - 750   red           blue-green

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