Colour by Design
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- Created by: LBCW0502
- Created on: 18-02-17 13:44
Why are rocks and minerals often coloured?
They are coloured due to the presence of transition metal ions within compounds in the rock or mineral
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What are biological pigments?
Coloured organic molecules produced naturally by specific organisms (e.g. orange-red beta carotene molecule in plants/algae and provides vivid pink feathers of flamingos)
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What is a conjugated system?
A conjugated system has alternating double and single bonds, allowing the overlap of p-orbitals
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What does the term 'delocalised' mean?
Electrons not associated with a particular pair of atoms, but are able to spread out over several atoms. Extensive delocalisation affects the size of the energy gap between electron energy levels in the molecule.
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How does the number of delocalised electrons affect the energy gap between electron energy levels in a molecule?
The greater the number of delocalised electrons, the smaller the energy gap
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Why do white opaque solids appear white in sunlight (or white light)?
None of the wavelengths of incident light are absorbed by the surface of the object - they are all reflected. Our brain perceives a mixture of all the wavelengths of visible light as white
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What happens when white light shines through transparent substances?
None of the wavelengths of incident light are absorbed - they are all transmitted
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Why do some objects appear coloured?
Objects appear coloured because wavelengths corresponding to particular colours are absorbed by substances in the object. For solutions, it is the transmitted light you see, for solids it is the reflected light you see
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What are complementary colours?
Complementary colours are two colours which combined together produced white light
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What happens to electrons in coloured organic molecules?
A substance absorbs radiation in the visible region of the spectrum, energy absorbed causes changes in electronic energy and electrons are excited from their normal state (ground state) to an excited state
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Why do some electronic transitions lead to colourless compounds?
Compound absorbs UV radiation but not visible light (appears colourless). More energy is needed to excite an electron
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What does the term 'excited state' mean?
An electron is in a higher than normal energy level and this can be in an atom, molecule or ion
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Give one example of a delocalised system
Benzene ring
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Which factors affect the colour of organic molecules?
The more electrons that are delocalised in a conjugated system, the smaller the energy gap. Smaller energy gaps result in the absorption of longer wavelengths of light absorbed (visible region)
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Delocalised systems with pi-bonds in the conjugated system are likely to absorb light in which region of the spectrum?
Visible region
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What is the empirical formula for benzene?
C1H1
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What is the molecular formula for benzene?
C6H6
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Is benzene unreactive?
Yes
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Describe Kekulé's model of benzene
A six-membered ring of carbon atoms with alternating single and double bonds
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What was Kekulé's dream?
A snake biting its own tail (findings also came from Josef Loschmidt four years before) - [inspiration or plagiarism?]
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What is the modern model of the structure and bonding of benzene based on?
Evidence from techniques such as X-ray diffraction (produce a contour map)
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What are the features of a benzene molecule?
Regular/planar hexagon, all bond angles are 120 degrees and all the carbon-carbon bonds are the same length (less than C-C but greater than C=C)
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Describe the key features of the delocalised model (1)
Each carbon atom has four outer electrons, which can be used to form bonds
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Describe the key features of the delocalised model (2)
Three of these electrons are used to form single sigma bonds to either carbon or hydrogen, leaving one p-electron on each carbon atom
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Describe the key features of the delocalised model (3)
Instead of overlapping in pairs to form three separate pi-bonds, these six pi-electrons delocalise and spread out evenly so that they are shared by all six carbon atoms in the ring
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Describe the key features of the delocalised model (4)
These electrons form a delocalised charge cloud, above and below the plane of the molecule
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How do you represent delocalisation in the structural formula for benzene?
A circle is usually drawn inside the hexagon (in the delocalised model - electron density in all C-C bonds would be uniform all the way around the ring)
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How does delocalisation affect the stability of the benzene ring?
The more delocalised a structure is the more stable the molecule becomes
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How does the stability of the benzene ring affect the reactivity of the molecule?
Benzene ring does not readily undergo reactions that would disrupt the delocalisation of the electrons such as addition reactions
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Describe the reaction needed to obtain thermochemical data for benzene
Benzene reacts with hydrogen at 300 degrees Celsius and 30 atm in the presence of finely divided nickel as a catalyst (addition reaction to form cyclohexane - hydrogenation)
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What is the enthalpy change of hydrogenation for cyclohexene?
-120 kJ mol-1
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What is the enthalpy change of hydrogenation for benzene?
-208 kJ mol-1
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What was the predicted enthalpy change of hydrogenation for benzene using the Kekulé model?
-360 kJ mol-1
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What can be concluded from the evidence obtained from thermochemical data?
The delocalised model is a much better description of the bonding in benzene than the Kekulé model
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What was the issues with the molecular geometry in the Kekulé model?
Benzene is a planar molecule with bond angles of 120 degrees. But single/double bonds would have different lengths which would mean that the structure would not be a regular hexagon
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What is a dye?
A soluble, coloured organic molecule that is able to bind to a substrate such as a fibre and impart colour to it
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What is alizarin?
A bright red compound extracted from the root of the madder plant. The bright colour of the alizarin molecule was responsible for products as varied as cricket ball casing and Turkish carpets
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The structure of alizarin is based on which two compounds?
Naphthalene or anthracene (might contain fused benzene rings)
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Which reaction is used to convert aromatic compounds to their parent arene?
Reduction - using heat and zinc dust
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What is produced when alizarin is reduced?
Anthracene (similarities in structure)
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Describe the structure of alizarin
Two benzene rings joined together by a quinone group
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Why can the modern chemical industry find it hard to precisely mimic the colour of naturally occurring dyes?
Natural dyes are often a mixture of several closely related molecules
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What is an arene?
Aromatic compounds containing C and H only (also known as aromatic hydrocarbons)
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What does the ending '-ene' suggest?
The compound is unsaturated (e.g. alkene)
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What is a phenyl group?
A group of atoms with the formula C6H5. It is related to benzene in the same way that a methyl group is related to methane
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How many delocalised electrons are there in napthalene?
10
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How many delocalised electrons are there in anthracene?
14
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What is an aromatic compound?
Compounds which contain one or more benzene rings
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What are the features of an aromatic conjugated system?
Planar, cyclic and possesses 4n + 2 delocalised pi-electrons (e.g. 6, 10, 14 etc.) - Aromatic systems can include rings where one or more of the atoms is not a carbon e.g. nitrogen atoms in bases which make up DNA
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What are feedstocks?
Reactants that go into a chemical process
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What is a synthetic dye?
A compound made artificially by chemical reactions
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What is an electrophile?
A positive ion, or a molecule with a full or partial positive charge, that will be attracted to a negatively charged region and react by accepting a covalent bond
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How does delocalisation affect the behaviour of an alkene?
Alkenes react with electrophiles in addition reactions and the product is a saturated molecule
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How does delocalisation affect the behaviour of benzene?
Benzene undergoes substitution reactions and the product still contains an unsaturated benzene ring (electrophilic substitution)
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Why are reactions slow in the reaction mechanism?
The first step of the reaction mechanism disrupts the delocalised electron system (requires lots of energy). Often the presence of a catalyst is required to create an electrophile reactive enough to attack the benzene ring
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Describe the reagents and conditions for bromination
Benzene reacts with liquid bromine in the presence of a catalyst (iron fillings or iron(III) bromide). Heat is required
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What are the products of bromination?
1-bromobenzene (bromine is decolourised and HBr fumes are given off)
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Describe the role of the iron catalyst (1)
Iron reacts with bromine to form iron (III) bromide. This interacts with a neighbouring bromine molecules - a Br- ion is removed from it to form a Br+ electrophile
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Describe the role of the iron catalyst (2)
Br+ bonds to a carbon atom in the benzene ring and an H+ ion is lost from the ring. H+ reacts with the FeBr4- to produce HBr and FeBr3 catalyst is regenerated
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What are halogen carriers?
Substances that are able to remove halogen atoms from molecules in this way are known as halogen carriers
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Describe the reagents and conditions for chlorination
Benzene reacts with chlorine in the presence of a catalyst (aluminium chloride) at room temperature and anhydrous conditions (aluminium chloride reacts violently with water)
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What is alkylation?
A reaction that introduces an alkyl group into a molecule
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Describe the reagents and conditions for alkylation
Benzene is heated under reflux with chloroethane and anhydrous aluminium chloride (ethylbenzene is formed). AlCl3 helps to form a reactive electrophile to attack the benzene ring. CH3CH2+ reacts with benzene ring, forming ethylbenzene
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What is an acyl group?
An acyl group of atoms has the structure R-C=O, usually formed from a carboxylic acid RCOOH or an acyl chloride RCOCl
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What is acylation?
A reaction which an acyl group is introduced into a molecule
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Describe the reagents and conditions for acylation
Benzene is treated with an acyl chloride (or an acid anhydride) and AlCl3 under anhydrous conditions. An acyl group is attached to the benzene ring
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What is the significance of Friedel-Crafts reactions?
They provide a way of adding carbon atoms to the benzene ring and building up side chains (useful to synthetic chemists)
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What is a chromophore?
Part of molecule which gives the molecule its colour
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Describe nitration
Benzene reacts with a mixture of c.nitric acid and c.sulfuric acid (nitrating mixture) and the temperature is kept below 55 degrees Celsius (product is nitrobenzene)
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What happens when the temperature is increased during nitration?
Further substitution of the ring takes place to give the di- and tri- substituted compounds
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What is the electrophile which reacts with the benzene ring during nitration?
NO2 + (formed in the nitrating mixture by the reaction of sulfuric acid and nitric acid)
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How does the presence of nitro groups affect the molecule?
Nitro groups on the benzene ring modify properties of the chromophore. It will change the wavelength of light that it absorbs and the molecule will have a different colour
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Describe sulfonation
Benzene reacts with c.sulfuric acid and is heated under reflux for several hours (benzenesulfonic acid is formed)
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What is the electrophile in sulfonation?
SO3 carries a large partial positive charge on the sulfur atom (binds to benzene)
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Describe the significance of sulfonation reactions
Benzenesulfonic acid is a strong acid and forms salts in alkaline solution. The sulfonate group is able to form ionic salts - sulfonation provides a way of forming more soluble derivatives of aromatic compounds
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How are dyes made more soluble?
Add sulfonic acid groups to the benzene ring
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Azo compounds contain which functional group?
Azo functional group, R1-N=N-R2
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What is a coupling reaction?
A reaction between a diazonium compound and a phenol or aromatic amine to produce an azo dye
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Compare arene groups as variable groups with alkyl groups as variable groups
If R1 and R2 are arene groups, azo compounds are more stable than if R1 and R2 are alkyl groups. N=N group is stabilised by extended delocalised system involving the arene groups. Stability enables highly coloured aromatic azo compounds to be dyes
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Diazonium compounds contain which functional group?
Diazo group
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Why are diazonium compounds usually unstable?
Diaozonium group is easily lost, forming nitrogen gas
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What happens when the diaozonium group is attached to a benzene ring?
Electrons in the N≡N bond will become part of the delocalised system in the benzene ring. This stabilises it enough to allow diazonium compounds to be formed as intermediates
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Why are diazonium compounds prepared in ice-cold solutions
The solids are explosive - must be used immediately
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What is diazotisation?
A reaction in which an amine group is converted into a diazonium salt
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Describe diazotisation
Aromatic diazonium compounds are prepared from aromatic amines (such as phenylamine). Aromatic amine is dissolved in dilute HCl. A cold solution of sodium nitrite (sodium nitrate (III)), NaNO2 is added. Temperature is below 5 degrees Celsius
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What is the actual reactant in the diazotisation reaction?
Unstable acid - HNO2 (nitrous acid) which reacts with the amine functional group
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What is the electrophile in a coupling reaction?
Diazonium ion
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What do coupling agents usually contain?
Phenol or amino groups attached to the benzene ring as the lone pairs of electrons from these groups increase the electron density in the benzene ring and increase its reactivity towards electrophiles
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Describe the reagents needed for a coupling reaction
A solution of a coupling agent is made up, an ice-cold solution of the diazonium salt is added to it. A coloured precipitate of the azo compound immediately forms
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How is a coupling agent (containing phenol) prepared?
The coupling agent is usually dissolved in alkali to enable the reaction to occur
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Diazonium salts can also couple with which other type of compound?
Aromatic amines (such as phenylamine)
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How can the properties of an azo compound (e.g. colour, solubility) be modified?
Alter the structure of the diazonium compound and coupling agents used in the coupling reaction as well as introducing -OH, NH2 and SO3- groups (affects bond to fibres)
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What is cellulose?
A natural polymer found in plant cells, consisting of chains of glucose molecules joined together
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What is a fibre-reactive dye?
A molecule containing a reactive group of atoms that can bond covalently to molecules of the fibre
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Describe dyes with ionic bonding
Acid dyes (SO3- groups when dissolved in water) are applied in acidic conditions to fibres - attracted to NH3+ groups on polyamide chain or side groups in proteins
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What is mordanting?
Groups on the fabric and the dye form covalent bonds to the metal, forming chelate complex ions. Metal ions used as mordants include Al3+ and Cr3+
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What is a chelate?
A complex ion in which the metal ion is bonded to two or more atoms in the same molecule
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What is required for hydrogen bonding to occur between a dye molecule and polymer molecule?
Hydrogen bonding must be strong enough so it will not be broken in the presence of water
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Which type of dye molecules attach to fibres through hydrogen bonding?
Linear molecules - they align themselves and attach in several places. Such dyes are known as direct dyes
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What are disperse dyes?
Dyes which have very few polar groups and will be insoluble in water (dispersed in water)
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What happens when a mixture of dye and water is added to fabric?
The small dye molecules are able to diffuse into the fabric and are held there by weak ID-ID forces between dye and fabric
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What are the features of a dye molecule in an ionic bond?
SO3- groups
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What are the features of a polymer molecule in an ionic bond?
NH3+ groups in acidic solution
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Give examples of fabric dyed by ionic interactions
Nylon, wool and silk
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What are the features of a dye molecule in a covalent bond?
Presence of reactive groups e.g. triazine derivatives
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What are the features of a polymer molecule in a covalent bond?
OH or NH2 groups
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Give examples of fabric dyed by covalent interactions
Cotton and cellulose
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What are the features of a dye molecule in a hydrogen bond?
Several NH2 groups and a linear molecule
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What are the features of a polymer molecule in a hydrogen bond?
Frequent OH groups
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Give an example of fabric dyed by hydrogen interactions
Cotton
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What are the features of a dye molecule in an ID-ID bond?
Few polar groups, small molecule
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What are the features of a polymer molecule in an ID-ID bond?
No OH or NH groups
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Give an example of fabric dyed by ID-ID interactions
Polyester
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What is a triglyceride?
An ester molecule formed by the reaction of one molecule of glycerol (propan-1,2,3 triol) and three fatty acid molecules
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What is the physical state of oils at room temperature?
Liquids (store of chemical energy)
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What is the physical state of fats at room temperature?
Solids (store of chemical energy)
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Why do fats/oils not mix with water?
The presence of the long, non-polar fatty acid chains in fats and oils means that these substances do not mix with water
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Name some common fatty acids
Palmitic acid, stearic acid, oleic acid, linoleic acid
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Describe the hydrolysis of oils/fats
Heating oil or fat with concentrated sodium hydroxide solution to give glycerol and the sodium salts of the oil/fat
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How are free fatty acids released from the sodium salts?
Add dilute mineral acid, such as hydrochloric acid
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Food manufacturers are now required by law to include details of what?
Proportion of fats, sugars and other food components on the labels of the products that are sold to the public
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Natural oil or fat contain a mixture of what?
A mixture of triglycerides
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Describe the shape and structure of a saturated triglyceride
Chains packed closely together with stronger intermolecular bonds. Saturated triglyceride is a solid at room temperature
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Describe the shape and structure of an unsaturated triglyceride
Chains cannot pack closely due to the presence of double bonds, causing the molecules to 'kink'. There are weaker intermolecular bonds. Unsaturated triglyceride is a liquid (oil) at room temperature
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Describe the term partially hydrogenated
Fats still contain one or more double bonds after hydrogenation
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Which isomerism will artificial fats take the form of?
E-isomerism
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Which isomerism will natural fats take the form of?
Z-isomerism
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Describe some of the health worries about trans fats
Trans fatty acids present in partially hydrogenated fats were a serious health risk due to contributing to building up LDL cholesterol in the blood, leading to the development of heart disease and strokes
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What is gas-liquid chromatography?
Fats are hydrolysed into fatty acids and converted into methyl esters. Volatile compounds vaporise in the GLC
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Why are esters of trans and cis fatty acids used in GLC?
They will have different boiling point and will be detected at different retention times
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What is HPLC?
High-performance liquid chromatography. Uses high pressure but lower temperatures and relies more on solubility differences rather than boiling point differences to separate molecules
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What is the mobile phase in GLC?
Unreactive gas such as nitrogen (carrier gas)
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What is the stationary phase in GLC?
Small amount of liquid with a high boiling point, held on a finely divided inert porous solid support
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How is the material packed?
Into a long thin tube called a column. The column is coiled inside an oven
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Describe how the GLC works
Sample if injected into the gas stream before entering the column. The components of the mixture are carried through the column in a stream of gas
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How are the components separated?
Each component has a different affinity for the stationary phase compared to the mobile phase. Some will dissolve in stationary phase and some will remain in gaseous mobile phase. Each component emerges from the column at different times
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Which compounds emerge first?
Compounds that favour the mobile phase are carried only more quickly than those in stationary phase. The most volatile compounds usually emerge first
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What is Kpc?
Partition coefficient (component in stationary phase / component in mobile phase). The greater the partition coefficient, the greater the affinity the component of the sample has for the stationary phase
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What does a chromatogram show?
Recorder response against time which has elapsed since the sample was injected onto the column (can be connected to MS)
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What is retention time?
The time that a compound is held in a column, under given conditions (characteristics of the compound)
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Why is it important to maintain constant conditions?
Small changes in conditions such as temperature can affect the observed retention time
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What does the area under each peak indicate?
The relative amount of each component in the mixture (if peaks are very sharp, their relative heights can be used)
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Which two features are demonstrated in the synthesis of aspiring and dyes?
Modification of functional groups and modification of carbon skeleton (carbon skeleton also modified using carbonyl groups)
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How does acidified potassium dichromate solution test for the presence of primary or secondary alcohols?
Chromate ions are reduced and turn from orange to green (oxidised to aldehydes or ketones)
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Describe the content of Fehling's solution?
Fehling's A contains weak oxidising agent (Cu2+). Fehling's B contains an alkaline solution containing a bidentate ligand (solution made fresh in laboratory)
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How can Fehling's test be used to distinguish between aldehydes and ketones?
In an aldehyde, Cu2+ ions are reduced to Cu+ ions, forming a brick red precipitate (Cu2O - copper (I) oxide). There is no reaction with ketones so the solution remains blue
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How is Tollens' reagent made?
Aqueous silver nitrate is added to sodium hydroxide (forming a brown precipitate). Dilute ammonia solution is added (brown precipitate dissolves and solution turns colourless)
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How is Tollens' reagent used to test for an aldehyde?
Tollens' reagent is warmed with an aldehyde in a test tube (using a water bath). A silver mirror forms on the inside of the test tube.
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Describe the chemical detail of what happens when an aldehyde is added to Tollens' reagent
Tollens' reagent is formed by dissolving silver (I) oxide in ammonia solution to form [Ag(NH3)2]+. When Tollens' reagent reacts with an aldehyde, silver (I) ions are reduced to Ag, leading to a silver layer forming on inner surface of test tube
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Which metal hydride is used to reduce aldehydes to primary alcohols and ketones to secondary alcohols?
Sodium tetrahydridoborate (III) - NaBH4
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What is cyanohydrin?
A functional group in which a hydroxyl group and a nitrile group (CN) are attached to the same carbon atom
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What is a nitrile?
A functional group in which a carbon atom is bonded by a triple bond to a nitrogen
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What type of reaction occurs when an aldehyde or a ketone reacts with a cyanide ion?
Nucleophilic addition
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What are polyfunctional molecules?
Compounds with several functional groups
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What is the chemical test for an alkene?
Bromine water - turns from orange to colourless
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What is the chemical test for a haloalkane?
Warm with sodium hydroxide solution, then acidify and add silver nitrate - precipitate forms (Cl-white, Br-cream, I-yellow)
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What is the chemical test for an alcohol?
Warm with acidified potassium dichromate (VI) - turns from orange to green for primary/secondary alcohols (no reaction for tertiary alcohols)
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Why does a tertiary alcohol not react with acidified potassium dichromate (VI) solution (remains orange)?
In the structure of a tertiary alcohol, there is no hydrogen attached to the central carbon atom for the oxygen to remove (You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond)
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What is the chemical test for a phenol?
Iron (III) chloride - turns from yellow (neutral) to purple
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What is the chemical test for a carboxylic acid?
Add sodium carbonate solution - bubbles of gas are formed (carbon dioxide)
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In which two processes is there carbon bond formation?
Conversion of aldehydes and ketones into nitriles, acylation and alkylation of benzene rings using Friedel-Crafts reaction
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What is an addition reaction?
Two molecules react together to form a single product
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What is a elimination reaction?
A small molecule (such as water or HCl) is removed from a larger one, leaving an unsaturated molecule
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What is a condensation reaction?
Two molecules react together to form a larger molecule, and a small molecule (water or HCl) is removed
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What is a substitution reaction?
One group of atoms takes the place of another
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What is an oxidation reaction?
Oxygen atoms are gained and hydrogen atoms are lost
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What is a reduction reaction?
Oxygen atoms are lose and hydrogen atoms are gained
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What is a hydrolysis reaction?
Bonds are broken by the action of water (OH- may appear in base catalysed reactions)
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How do you calculate the overall yield, given the yields to each step?
Convert the yields into fractions, multiply the fractions together and times by 100 to convert into a percentage
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Give examples of three synthetic routes
Formation of 3-aminobenzoic acid from benzoic acid, synthesis of mandelic acid and formation of methyl phenylethanoate
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Other cards in this set
Card 2
Front
What are biological pigments?
Back
Coloured organic molecules produced naturally by specific organisms (e.g. orange-red beta carotene molecule in plants/algae and provides vivid pink feathers of flamingos)
Card 3
Front
What is a conjugated system?
Back
Card 4
Front
What does the term 'delocalised' mean?
Back
Card 5
Front
How does the number of delocalised electrons affect the energy gap between electron energy levels in a molecule?
Back
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