Colour by Design - Azo dyes
19 important questions on Colour by Design - Azo dyes
What can be said about nitrous acid? How is it made?
It is unstable and is made by reacting NaNO2 with dilute HCl (aq)
How are azo compounds made?
By reacting a 'coupling' agent with a diazonium
What is the general formula for diazonium ion?
R-N+*triple bond*N
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What can be said about diazonium ions? (refer to structure of their formation)
They are stable; however, they have to be made in solutions at temperatures below 5 degrees Celsius.
They are weak electrophiles - they will attack phenols and aromatic amines (calles coupling agents), both of which have electron rich benzene rings.
What are the regents and conditions for diazotisation?
- phenylamine
- dilute HCl
- sodium nitrate (III)
- temp below 5 degrees Celsius
What is the reaction called when an -N=N- bond is formed? What is the compound formed called?
Coupling reaction - the compound formed is an azo compound
What do coloured compounds tend to have compared to colourless compounds?
They tend to have double bonds and conjugation - less energy is needed to excite electrons as the electrons are spread over the molecule (delocalised)
Why is the temperature of the diazonium salt solution best kept below 10 degrees Celsius?
It has limited stability and so will start to break down at warmer temperatures
What is modified if different functional groups are attached to the chromophore?
The properties of the molecule e.g. its colour, as it extends delocalisation
What property is modified if -SO3-Na+ is attached to the chromophore?
The solubility of the dye in water is improved due to the ionic group - it can form ionic bonds with protein fibres such as wool.
What property is modified if -NH2 or -NR2 is attached to the chromophore?
Modern azo dye manufacturing plants nearly always have an ice-making plant nearby. Explain why.
The diazonium salt is only stable below 5 degrees celsius. Adding crushed ice is the cheapest way of cooling.
What is the role of the OH radical in the decomposition of benzoic acid?
It initiates the process and catalyses the reaction.
Explain why unsaturated oils are use as the medium for oil plants.
- they harden and dry
- they form cross links
How would you test an oil for unsaturation?
Add bromine - if it is unsaturated then there will be a colour change from orange to colourless
What feature of the bonding in benzene causes it to undergo substitution reactions rather than addition?
Its delocalisation
Name the type of reaction in which a dye is formed from a diazonium compound.
Coupling
What type of bonding would make a dye more likely to be washed out easier?
Hydrogen bonding i.e. a dye which has hydrogen bonding will bond to water more readily than to fibre.
As time goes by why would a pigment not fade?
It is not affected by sunlight/oxygen/water (just need to remember one)
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