Colour by design Flashcards
What is a coupling reaction?
A reaction needed to make an Azo dye
The first step is to make a diazonium salt
The azo dye is then made by coupling the diazonium salt with an aromatic compound that is susceptible to electrophilic attack
How do you make a diazonium salt?
By reacting phenylamine with nitrous acid
Nitrous acid is unstable so it has to be made in situ from HCl and sodium nitrate
Nitrous acid then reacts with phenylamine and HCl to form benzenediazonium chloride
The temperature must be kept below 5c to prevent a phenol forming instead
How do you make an azo dye?
By coupling the diazonium salt with a phenol
The phenol has to be dissolved in NaOH solution to make sodium phenoxide solution
Then stand in ice and add chilled
benzendiazonium chloride
The azo dye will precipitate out of the solution immediately
Why is phenol a coupling agent?
Because the lone pairs on its oxygen increase the electron density of the benzene ring
This gives the diazonium ion, which is a weak electrophile, something to attack
What is a chromophore?
The structure in molecules that gives them their colour
They absorb specific wavelengths of light. Visible wavelengths not absorbed will be seen as a particular colour
They tend to have double or triple bonds, lone pairs or benzene rings. These components usually form a part of the delocalised electron system
Modifying the chromophore will change the frequency of light it absorbs and so change the colour
Why is water solubility important for dyes?
As the process of dying involves soaking material in a solution of the dye compound
Water is cheap, readily available and non-toxic
To increase solubility in water, functional groups such as ionic groups (e.g. sulfate ion) can be added
Water dissolves ionic substances because of it’s polar nature
How does delocalisation affect the energy needed to excite electrons?
In a delocalised system, many molecular orbitals are formed
The orbitals are in even closer in energy than in a double bond
The electrons in them absorb low frequency UV and visible light when they become excited
As delocalisation increases, more molecular orbitals form, less energy is needed to excite electrons and the frequency of light absorbed drops. This is why functional groups that extend delocalisation in chromophores cause a colour change
How is Fehling’s solution used?
Fehling’s is a blue solution of copper(II) ions dissolved in sodium hydroxide
If heated with an aldehyde the copper(II) ions are reduced to a brick-red precipitate of copper (I) oxide
No reaction with a ketone
How is Tollen’s reagent used?
To test for aldehydes and ketones
Heat
Aldehyde is silver mirror
Ketone no reaction
Aldehydes and ketones are flammable so heat in water bath not naked flame
How do you make Tollen’s reagent?
2cm3 of silver nitrate
A few drops of dilute sodium hydroxide
A light brown precipitate should form
A few drops of dilute ammonia solution until the brown precipitate dissolves completely
How do dyes attach themselves to fibres?
Functional groups like the amine group or hydroxy group allow dyes to form hydrogen bonds with fibre molecules. These dyes aren’t particularly colourfast because the hydrogen bonds aren’t strong enough
Acidic groups like sulfonic acid or carboxylic acid help dyes bind to alkaline -NH- links in fibres. H+ ions move from dye to fibre and ionic interactions hold them together. These bonds give better colourfastness than hydrogen bonds
Some dyes have ionic salt groups which allow the dye to bind to -NH- links
The salt groups dissociate when the dye is dissolved in water. To make -+NH2- groups acid needs to be added to provide H+ ions to the -NH- links. SO3- groups will then bind to the -+NH2- links
What are the most permanent types of dye?
Fibre reactive dyes
They have a functional group that will react with the -OH or -NH- group in the fibre to form strong covalent bonds
What are fatty acids?
Carboxylic acids
They have a long hydrocarbon chain with a carboxylic acid group at the end
Can be saturated or unsaturated
Bromine water can be used to distinguish
What are triglycerides?
Contain the ester functional group 3 times
They’re triglyceryl esters
Made by reacting glycerol with fatty acids
It’s a condensation reaction as the three -OH groups on the glycerol molecules link up to fatty acids and water is eliminated
How does the delocalised structure explain why benzene reacts via substitution rather than addition?
Cyclohexene reacts with bromine to form a colourless solution at room temp
Benzene does but very slowly so a FeBr2 catalyst is need under reflux forming HBr as a product
Cyclohexene electrophilic addition
Benzene electrophilic substitution of a H atom to preserve the stability of the delocalised electron system
What is a Lewis acid?
A compound or ionic species which is an electron acceptor
What is a Lewis base?
An electron pair donator
What is a Bronsted-Lowry base?
A species that can accept a proton
What is a Bronsted-Lowry acid?
A species that can donate a proton
MS - Explain why this compound appears red
The delocalised electrons get promoted to a higher energy level
E=hf/ E=hv (frequency absorbed depends on energy change)
Absorbs visible light
Transmits complementary colour
Delocalisation effects the wavelength absorbed
How does the enthalpy change of hydrogenation give evidence for delocalisation
Enthalpy change of hydrogenation of benzene is less exothermic than 3x cyclohexene
Therefore bonding in benzene is not 3C-C and 3C=C
It forms a delocalised electron system making it more stable
X-ray diffraction studies
Show that all the C-C bonds in benzene are the same length - between the length of a single and double bond
Each carbon donates an electron from it’s p-orbital. The p-orbitals combine to form a ring of delocalised electrons
