Define the term chromophore.

The chromophore is the delocalised system of electrons within a dye molecule that absorbs visible light and so gives the molecule its colour.

Name three groupings commonly found within a chromophore.

The azo group –N=N–, carbon–carbon double bonds and benzene rings.

Why must a chromophore contain an extended delocalised system?

Only a large delocalised system has electronic energy levels close enough together for the gap between them to match the energy of visible-light photons.

What is meant by a functional group that modifies the chromophore?

A group that changes the extent of the delocalisation, shifting the wavelength absorbed and so altering the shade or the intensity of the colour.

Name two functional groups that modify a chromophore.

The –NH₂ group and the –OH group.

Explain how an –NH₂ group attached to the ring changes the colour of a dye.

The lone pair on the nitrogen joins the delocalised system and extends it, so the energy gap falls and light of longer wavelength is absorbed.

OCR A-Level Chemistry B H433

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Colour by Design

Topic overview How dyes attach to fibres Structure of a dye molecule Fats and oils Arenes and delocalisation Representations of benzene and enthalpy of hydrogenation Tests for the common functional groups Identifying a polyfunctional molecule from a set of test results Halogenation and nitration of benzene and the electrophiles involved The nitration mechanism, sulfonation and the Friedel-Crafts reactions Diazotisation and why it is done cold Coupling reactions, azo dyes and the route from benzene Oxidising an aldehyde and the three carbonyl tests Cyanohydrin formation and its use in synthesis Using the Data Sheet reactions to plan a route Choosing between routes on yield and atom economy Nucleophilic addition mechanism Classifying organic reactions and unfamiliar mechanisms Origins of colour in organic molecules How gas-liquid chromatography separates a mixture Retention time, peak area and the limits of the technique

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Colour by Design

Structure of a dye molecule

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the chromophore, the effect of electron-donating and electron-withdrawing groups, and the role of solubilising groups.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

What does an NH₂ group do to a conjugated system, why does an NO₂ group deepen the colour of an azo dye, and why can too many solubilising groups be a problem?

Process

Which groups let a dye hydrogen bond to a fibre, why is a dye molecule best seen as several parts, and why does adding a sulfonate group hardly change the colour?

Numbers

State the structural feature common to almost all azo dyes.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the term chromophore.
Model answer
The chromophore is the delocalised system of electrons within a dye molecule that absorbs visible light and so gives the molecule its colour.
2
Name three groupings commonly found within a chromophore.
Model answer
The azo group –N=N–, carbon–carbon double bonds and benzene rings.
3
Why must a chromophore contain an extended delocalised system?
Model answer
Only a large delocalised system has electronic energy levels close enough together for the gap between them to match the energy of visible-light photons.
4
What is meant by a functional group that modifies the chromophore?
Model answer
A group that changes the extent of the delocalisation, shifting the wavelength absorbed and so altering the shade or the intensity of the colour.
5
Name two functional groups that modify a chromophore.
Model answer
The –NH₂ group and the –OH group.
6
Explain how an –NH₂ group attached to the ring changes the colour of a dye.
Model answer
The lone pair on the nitrogen joins the delocalised system and extends it, so the energy gap falls and light of longer wavelength is absorbed.

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