Colour By Design Flashcards
Why does an object appear white?
Because all the wavelengths of visible light are reflected and are not absorbed so appear white
What happens to light that hits a transparent object?
All the wavelengths of light pass through it
What happens to light that hits a coloured object?
Some wavelengths of the light are absorbed by the object and some of the light is reflected. The colour of the light that is reflected is what colour the object appears to be.
What happens to light hitting a blue object?
Only blue light is reflected. All other colours of light have been absorbed.
The colour of ______ light that has been ______ causes the object to appear the _______ colour of the light that has been absorbed
Visible
Absorbed
Complimentary
Do atoms only absorb photons of visible light to excite electrons from the ground state?
No they can absorb any frequency of light
What colour does an object that absorbs photons of ultraviolet light appear?
It appears colourless
What is a conjugated system?
The electrons in the π bond which are part of a covalent double bond are spread out over the whole molecular system.
What is are 2 examples of a conjugated system?
Benzene, R-COO-
In a conjugated system, what are the electrons said to be?
Delocalised
How does a conjugated system affect the colour of light that the substance absorbs?
The difference between the energy of the ground state and excited state is smaller.
The more delocalised electrons the smaller the energy gap
What happens if the energy gap of the ground state and excited state is smaller?
Longer wavelengths of light are absorbed
What is a useful rule of thumb about the wavelengths of light for a delocalised system of electrons?
A delocalised system with 5 π-bonds in a conjugated system are likely to absorb visible light and be coloured
What is an Arene?
It is a ring shape that is stabilised by electron delocalisation
What is the structure of a benzene?
- Regular planar hexagon
- Bond angles are 120º
- All the carbon-carbon bonds are the same length. (smaller than a normal carbon-carbon bond but larger than a carbon-carbon double bond)
What is the delocalisation of a benzene ring?
It forms two doughnut shapes above and below the plane of the benzene ring molecule. The p-shell forms a figure of 8 where the top is above the plane and the bottom is below the plane. This then morphs into the doughnut shape
In a benzene ring, how are the electrons from a carbon distributed?
3 electrons form σ-bonds between the two carbons either side and the hydrogen.
The 4th electrons forms the π-bond
The more delocalised a structure has….
…..The more stable it is (generally)
How stable is benzene?
It is very stable
What is the name of the old (wrong) model for benzene? What did it look like?
Kekulé’s model
It had alternating double and single bonds
What disproved Kekulés model?
The energy released in the hydrogenation kekulés model was 3x that of a 6 carbon ring with 1 double bond. But the enthalpy change of the hydrogenation of benzene was not 3x and was in fact much smaller than kekulés model
How does benzene react with hydrogen and what is produced?
It only reacts under SPECIAL conditions (300º 30 atm)
C6H12 is produced
What is the shape of kekulés molecule?
It is not a hexagon but is instead a 6 sided shape with alternating smaller sides where the double bonds are and longer sides where the single bonds are.
Are arenes saturated?
No, they are unsaturated
How are simple arenes where a hydrogen has been replaced with an alkyl group named?
The name of the functional group attached to the arene followed by the name of the arene.
e.g. methyl benzene
What is the naming strategy for benzene where one of the hydrogens has been replaced with a alkene functional group?
Phenyl followed by the name of the functional group.
e.g. phenylethene
What is an aromatic molecule?
It is a molecule that contains a benzene ring
What is the name of the molecule: C6H5Cl?
chlorobenzene
What is the name of the molecule: C6H5NO2?
nitrobenzene
What is the name of the molecule: C6H5COOH?
Benzoic acid
What is the name of the molecule: C6H5SO3H
Benzene sulfonic acid
What are a lot of benzene based molecules used for?
They are used in medicines and for dyes
What type of a reaction does an alkene do when reacted with Br2? and what sort of product is formed?
It is an addition reaction
A saturated product is formed, all the double bonds are removed
What type of a reaction does a benzene do when reacted with Br2? what are the products?
It is an electrophilic substitution reaction
The product is bromobenzene and HBr
What are the reagent, conditions and the catalyst for the reaction of bromine with benzene?
Reagent: Br2
Catalyst: Iron filings or iron(III)bromide
Conditions: Reflux
What sort of molecule is attracted to the delocalised electron ring in benzene?
An electrophile
How does iron react with bromine?
2Fe + 3Br2 ==> 2FeBr3
How does iron(III)bromide react with bromine? and how does this help with the reaction between a benzene ring and bromine? what is the reaciton?
Br2 + FeBr3 ==> Br+ + FeBr4-
The Br+ then reacts with the benzene ring and an H+ ion is lost from the benzene ring.
This H+ then reacts with FeBr4- to form FeBr4H
What are the reagents, conditions and the catalyst for the reaction of benzene and chlorine?
Reagent: Cl2
Catalyst: AlCl3
Conditions: Anhydrouse (without water)
What is the reaction with chlorine and aluminium chloride?
Cl2 + AlCl3 ==> ClCl4- + Cl+
Why are halobenzenes useful in the process of making products?
Because it can be used to form intermediates in synthetic routes. The carbon with the halogen attached can attack other other carbon atoms to form a carbon-carbon bond and produce larger molecules.
What is a Friedel-crafts reaction?
It is a reaction where an organic molecule with a halogen is substituted into a benzene ring using an AlCl3 catalyst
What is alkylation? Explain it
This is a Friedel-crafts reaction where benzene is heated under reflux with a chloroalkane and anhydrouse aluminium chloride.
A substitution reaction occurs called alkylation and this causes a benzene with an alkene attached to form with HCl.
Benzene + H3C-CH2Cl =(AlCl3 & reflux)=> Benzene-CH2CH3 + HCl
What is the purpose of the Aluminium chloride in the Friedel-crafts?
It polarises the alkane so that it is able to react with the benzene ring. It causes the alkene to become ionised (charged).
CH3CH2Cl + AlCl3 ==> CH3CH2+ + AlCl4-
What is acylation? Explain it
This is the reaction where benzene is is treated with an aclychloride or acid anhydride and AlCl3 NOT UNDER REFLUX. This produces a benzene with a bond to a C with a =O group and R group.
Benzene + R-COCl =(AlCl3)=> Benzene-C(=O)R + HCl
Why are the Friedel-crafts reactions important?
Because they allow side chains to be added to benzene rings which can lead to reactions producing further molecules.
What is nitration? Explain it
This is the process of reacting a benzene ring and nitric acid with a sulfuric acid catalyse below 55ºC so that an NO2 group can join onto the benzene ring.
Benzene + HNO3 =(H2SO4 <55ºC)=> Benzene-NO2 + H20
The product is nitrobenzene
What happens if nitration occurs at higher temperatures?
There is further substitutions with 2 or 3 NO2 groups on the benzene ring.
What is the electrophile that reacts with the benzene ring? How does this happen? What is the this called?
NO2+ is the electrophile that reacts with the benzene. It is a attracted to the delocalised benzene ring.
The NO2+ is produced in the reaction below:
HNO3 + H2SO4 ==> NO2+ + HSO4- + H2O
This is called production in situ.
What is the importance of inserting a nitro group into a benzene?
This will modify the properties of the chromophore and change the wavelength of light that it absorbs making it coloured.
What is Sulfonication?
This is the process where sulfuric acid is reacted with benzene under reflux to produce benzenesulfonic acid.
Benzene + H2SO4 =(Reflux)=> Benzene-SO3H + H+
What is the structure of benzenesulfonic acid?
Benzene-S(=O)(=O)OH
The benzene is bonded with the sulfur. There are 2 =O groups attached to the sulfur group and another OH group attached to the sulfur.
What happens when Benzenesulfonic is in an alkaline solution?
Benzene-S(=O)(=O)OH + NaOH ==> Benzene-S(=O)(=O)O-Na+ + H2O
What is an azo group?
This is a N=N group between two R arene groups
What does the N=N group do to the delocalisation?
It extends the delocalisation between the two benzene rings and this causes the compounds to be coloured and therefore can be used as dyes.
What is an Azo dye?
This is 2 benzenes joined with an N=N group.
Benzene-N=N-Benzene
What is a Diazonium compound?
This is a molecule with a diazo group connected to a single benzene ring.
Benzene-N+≡N
Talk about the stability of diazonium compounds. Make one reason for unsuitability and one reason for stability
They are unstable because the diazonium groups are easily lost as forming nitrogen gas.
They are stable because it forms part of the delocalised structure.
How are diazonium compounds prepared?
They are made in ice-cold solutions because they are explosive.
