Dyes and colour Flashcards
What are azo dyes made from?
Man made dyes, made up of an azo group (N=N) linking benzene rings together
Why are dyes colourful?
Light is absorbed by the delocalised electron, which varies depending what groups are used to make the dyes.
What type of reaction are azo dyes made from?
Coupling reaction
How do we make a diazonium salt?
React with phenylamine with nitrous acid Nitrous acid is made in situ.
NaNO2 + HCl -> HNO2 + NaCl
Temperature MUST BE KEPT BELOW 5 degrees celsius otherwise phenol will form.
How do we make the azo dye from the diazonium salt?
- Dissolve phenol in NaOH solution making sodium phenoxide.
- This is stood in ice then the diazonium salt is added.
- The azo dyes precipitates out immediately.
The phenol group acts as a coupling agent
Define Colourfast, and how does colourfastness change?
It can’t wash out easily or fade in light.
Coulourfastness depends on the strength of the bonding
What bonds form between the fibre and an amine group (-NH2), and determine if it will be colourfast.
Forms hydrogen bonds, which forms well with cotton and linen, due to them containing many OH groups.
Hydrogen bonds aren’t very strong so these aren’t very colourfast.
What bonds form with acidic groups (COOH)
Bind to alkaline -NH2 groups found in wool, silk, and nylon.
Ionic interactions hold the dye to the fibre, which is quite strong, so they’re fairly colourfast.
What are fibre reactive dyes?
Most permanent type of dye. They contain a functional group that will react with -OH or -NH2, forming a strong covalent bond.
What structures give dyes their colour?
Chromophores.
What features do chromophores have?
Usually contain features with high electron densities, such as C=C, lone pairs and benzene rings.
How can we change the colour of a dye.
We can add groups containg O or N lone pairs, as these become part of the delocalised system responsible for absorbing light.
How can we make dyes soluble in water.
We can add solubilising functional groups (usually ionic groups), which dissolve due to waters polar nature.
What happens when electrons absorb energy?
When electrons absorb exactly the correct amount of energy can move up energy levels, forming an excited state.
