F335 Colour by design

F335 – Colour by Design

Oils and Fats:

Important molecules for storing chemical energy in living systems. Oils and fats have the same basic structure. Fats are solid at room temperature. Oils are liquid at room temperature. Both oils and fats are esters of glycerol with long chain carboxylic acids. Below is the

structure of glycerol as well as an example of a long chain carboxylic acid.

Glycerol can react with three palmitic acid molecules to form a triester

The triesters which are found in natural oils are usually mixed triester. This means that three fatty acid groups are different.

Fats and Fatty Acids:

The carboxylic acids in fats and oils are unbranched and usually contain an even number of carbon atoms ranging for C4 to C22.

The fatty acids can be saturated meaning they contain only carbon-carbon single bonds.

The fatty acids can be unsaturated meaning they contain one or more carbon-carbon double bond.

Below are some examples of some common fatty acids:

Solid or Liquid:

The following structure shows a saturated triglyceride:

As the chains are unbranched, the triglycerides can therefore pack closer together meaning that the intermolecular forces are stronger and hence more energy is needed to pull them apart, therefore a solid at room temperature.

The following structure shows an unsaturated triglyceride:

The unsaturated triglyceride molecules cannot pack as closely together due to the kink in the chain caused by the cis double bond.

Therefore the intermolecular forces are weaker and so less energy is needed to separate the molecules and therefore a lower melting point and liquid at room temperature.

Hydrogenation:

Oils can be converted into fats via a hydrogenation reaction; this is how margarine is made.

The addition of hydrogen to unsaturated molecules reduces the number of double bonds; the triglycerides become more saturated.

The conditions for hydrogenation are a Ni catalyst, 150oC and 5 atmospheres. This is an example of an addition reaction.

In reality only partial hydrogenation occurs since total hydrogenation would give a fat which is to hard.

Hydrolysis of fats:

The ester linkage holding the molecule together can be hydrolysed. When this is done with concentrated sodium hydroxide solution soap is made. Hence the name saponification. Soaps are the sodium or potassium salts of fatty acids. They are made by heating the oils or fats with sodium or potassium hydroxide. The free fatty acids can be obtained from their salts by adding an acid such as HCl.

Ultraviolet and Visible Spectroscopy:

Coloured Compounds:

Substances appear coloured when they absorb radiation from the visible region of the electromagnetic spectrum (400-700nm)

The light that is transmitted or reflected will be lacking in certain frequencies of visible light and so will appear coloured.

Carrots contain the pigment carotene:

Carotene absorbs blue light strongly, so the blue light is lacking when the light reaches our eyes.

Therefore, carrots appear orange-red. A spectrometer can be used to measure the quantity of light absorbed by the pigment

at each wavelength. The recorder plots out the intensity of absorption against wavelength. Below is the absorption spectra for carotene in hexane:

Absorption spectrum:

A UV and visible spectrometer works on the same principle as an I.R. spectrometer, however in this case UV and visible radiation is used.

Light is split into two beams, one passes through the solution and the other through pure solvent.

The intensity of the two beams is compared to give an absorption spectrum. Absorption of UV and visible radiation causes changes in the electronic energy of

molecules and therefore can be called electronic spectra. In the spectra the peaks rise from the baseline because the trace shows the

intensity of radiation absorbed. The scale on the horizontal axis is wavelength. The absorption of UV has no effect on colour. For example benzene absorbs only UV and is therefore colourless. All the visible

radiation is transmitted.

Using UV and Visible Spectra:

We often get a broad band which is characteristic of general structural features rather than specific functional groups.

A colorimeter works only over a narrow range of frequencies; it is often used to find the concentration of a coloured compound.

Interpreting the Spectra:

Attention should be paid to:1. The wavelength of radiation absorbed.2. Intensity of absorption3. Shape of the absorption band.

The wavelength of the maximum absorption, λmax, is often given.

For organic molecules with delocalised electron systems, the longer the conjugated

chain, the more intense the absorption and the longer the wavelength of λmax. The intensity of the absorption also depends on the concentration of the solution and

the distance the light travels though the solution. It is important as it determines the amount of pigment or dye needed to produce a

good colour. The shape and width of the absorption band governs the shade and purity of the

colour seen.

Reflectance Spectra:

In some cases it is difficult to make a solution of a coloured substance; in cases like this chemists can use reflectance spectroscopy.

A light source is shone onto a sample and the composition of the reflected light is examined.

This is the part of light that has not been absorbed by the pigments. It is therefore like the negative of the absorption spectrum.

Electronic Changes:

When visible light falls on a coloured compound, certain frequencies of it are used to promote electrons to a higher energy level.

The electron is unstable in the higher energy level and so falls back down to a lower energy level, and so the energy absorbed is re-emitted.

It is not necessarily re-emitted all at the same time; the molecule may emit a smaller quantum of energy and only fall back to an intermediate energy level; the remaining energy can then be converted to kinetic energy for the molecules, which leads to them moving around faster and becoming warmer.

Many unsaturated molecules and those with conjugated systems absorb UV and visible light.

The delocalised electrons in these systems require slightly less energy to become excited compared with electrons in single bonds.

Gas-Liquid Chromatography:

Chromatography is a method of separating and identifying the components of a mixture.

All types of chromatography depend on the equilibrium set up when the components of a mixture distribute themselves between the stationary phase and the mobile phase.

Components with a higher affinity for the stationary phase move more slowly than those with a lower affinity.

The Instrument:

The stationary phase is a non-volatile liquid coated on the surface of finely divided solid particles.

This material is packed inside a long this column which is coiled inside an oven. An unreactive carrier gas acts as the mobile phase and carries the mixture through

the column.

Nitrogen and noble gases are used as inert carrier gases.

As each component emerges from the column, a peak is recorded on the chromatogram.

The area under each peak is proportional to the amount of that component in the mixture.

The time a compound is held on a column under given conditions is characteristic of each compound and is referred to as its retention time; this can be affected by many factors, such as:-The length and packing of the column.-The nature and flow rate of the carrier gas.-The temperature of the column.

The retention time can be used to identify the different compounds. The instrument is calibrated with known compounds, so that the conditions are kept

constant throughout the analysis.

The arenes

The Structure of Benzene

Benzene is a colourless liquid with the molecular formula C6H6. As it has an equal number of carbon atoms to hydrogen atoms, so it must be very

unsaturated; however it does not behave like other unsaturated molecules; it is very unreactive and has its own characteristic properties.

The reason for this is related to the cyclic structure of benzene:

The benzene ring is a flat hexagon, all of the bond angles are 120o and the carbon-carbon bonds are all the same length (0.139 nm)- the length is less than a carbon carbon double bond (0.154 nm), but more than a carbon carbon single bond (0.134).

The structure of benzene puzzled scientists in the nineteenth century; they knew that its molecular formula was C6H6, but they could not work out how this could be.

In 1856, August Kekulé proposed that benzene’s structure was cyclic and comprised of alternating single and double bonds:

However, this was disproved for two reasons; firstly all of the carbon carbon double bonds were found to be the same length; this can not be the case for double and single bonds. Secondly, the benzene ring is much more stable than the Kekulé structure suggests it should be.

Rather than the alternating single and double bonds, the structure contains a delocalised electron system.

Each carbon atom has four outer electrons which can be used to form bonds; three of these are used to form sigma bonds with the two adjacent carbon atoms and a hydrogen atom.

The extra electrons, instead of overlapping in pairs to form pi bonds, are spread out evenly and are shared by all 6 carbons in the benzene ring (electron delocalisation).

Additional evidence for the delocalised structure came from electron density maps which can be produced from x-ray diffraction studies.

Stability of Benzene

The delocalised electrons in the benzene ring result in it being much more stable than it would be expected to be if it had the Kekulé type structure.

The delocalisation allows the electrons in the molecule to be further apart. As electrons repel one another, the further apart they are, the lower the repulsion and therefore the more stable the structure.

The stability of the benzene ring affects the reactions it can undergo; it will tend to only undergo reactions in which the stable ring is preserved.

Those reactions which disrupt the delocalised electron system are less favourable and so require higher temperatures and more vigorous conditions.

Thermodynamic experiments proved that the benzene ring did not have the proposed Kekulé structure; the enthalpy change when benzene reacts with hydrogen to form cyclohexane can be used to determine its stability:

A reasonable estimate can be determined for the reaction if benzene had the Kekulé structure.

The enthalpy change for the reaction of cylcohexene with hydrogen to form cyclohexane is -120 kJ mol-1:

Therefore, the enthalpy change for the Kekulé structure would be:

This means that less energy is given out than would be expected if the structure had the Kekulé structure. As less energy has been given out, more energy has been used to break the bonds in the structure in order to put hydrogen across them, therefore the structure must be more stable.

The Arenes

Hydrocarbons that contain rings stabilised by delocalised electrons, like benzene, are called arenes. The suffix –ene, refers to them being unsaturated, like the alkenes, and the prefix ar- comes from many of them having an aromatic fragrance.

There are many arenes, for example:

Benzene rings can also join together to give fused ring systems, such as naphthalene and anthracene:

The delocalised electrons are spread over all of the rings.

Arene Derivatives

There are many derivatives of the benzene ring which are formed by replacing a hydrogen atom with different functional groups, for example:

The C6H5 group derived from benzene is referred to as the phenyl group. Two important aromatic compounds based on this group are phenol and

phenylamine:

Reactions of The Arenes The six electrons in benzene's delocalised system do not belong to any one carbon

and are free to move around the ring. They provide benzene with a high electron density. Regions of high density tend to attract positive ions, or atoms with a partial positive

charge; benzene, like the alkenes reacts with electrophiles. The reactions are much slower with benzene, due to the high energy required to

disrupt the delocalised electron system. Where alkenes undergo electrophilic addition reactions with electrophiles, benzene

undergoes electrophilic substitution reactions; this is to maintain the stable benzene ring system in the product.

There are many electrophilic substitution reactions involving benzene; however the mechanism is similar for each one and can be summarised as follows (X represents an electrophile):

The electrophile is attracted to delocalised electrons and the benzene ring donates a pair of electrons to it forming a covalent bond.

The ring is now only partially delocalised; it has two less electrons and so has a positive charge. The hydrogen gives up its electrons to the benzene ring, reforming the delocalisation.

What makes the electrophilic substitution reactions different is how the electrophiles are first formed.

Bromination of Benzene In the presence of iron filings or iron(III) bromide, the bromine is decolourised and

fumes of hydrogen bromide (HBr) are given off. The reaction which takes place between bromine and benzene is a substitution reaction:

The mechanism works as follows: The benzene ring induces a dipole in the Br2 molecule. The slightly positively

charged end of the bromine molecule is now slightly electrophilic. The iron filings help speed up the reaction by reacting with bromine to form iron(iii) bromide; this then accepts a pair of electrons from the slightly negative end of the polarised bromine molecule, forming a more reactive electrophile:

A H+ ion is lost from the ring; this reacts with FeBr4- to produce HBr, regenerating the

catalyst:

Nitration of Benzene An NO2 group won’t react directly with benzene; it must have a positive charge to do

so. Therefore nitric acid is reacted with sulphuric acid to form the nitronium ion, NO2

+.

HNO3(aq) + 2H2SO4(aq)   NO2+

(aq) + HSO4- (aq) + H3O+

 (aq)

If the reaction mixture is kept below 55oC, then the product is nitrobenzene; at higher temperatures, further substitution of the ring can occur forming di- and tri- substituted compounds.

The mechanism for the nitration of benzene works as follows: NO2

+ is attracted to the delocalised electrons in benzene. The benzene ring donates a pair of electrons to it forming a covalent bond:

The ring is now only partially delocalised; it has two less electrons and so has a positive charge. This positive charge attracts the hydrogen sulphate molecule which donates a pair of electrons to the hydrogen atom. The hydrogen gives up its electrons to the benzene ring, reforming the delocalisation. This reforms the sulphuric acid which has acted as a catalyst:

Sulphonation of Benzene This is useful for making benzene soluble in water. The mechanism takes place

when heated under reflux with concentrated sulphuric acid. There is a reversible reaction for the dissociation of H2SO4:

H2SO4(aq)   H2O (l) + SO3(aq)

The SO3 can undergo an electrophilic substitution reaction with benzene:

The product is a strong acid which forms salts in an alkaline solution:

Most solid detergents contain salts of this kind; a long alkyl group attached to a benzene ring.

The hydrocarbon part of the molecule mixes with fats and the ionic part mixes with water.

Chlorination of Benzene It is possible for a chlorine atom to be substituted onto a benzene ring in much the

same way as a bromine atom can be; however, the catalyst for the chlorination of benzene is usually aluminium chloride (AlCl3).

Aluminium chloride helps to polarise the chlorine molecule. The reaction must be carried out under anhydrous conditions (without water) as

aluminium chloride reacts vigorously with water. The mechanism for the reaction is as follows: The benzene ring induces a dipole in the Cl2 molecule. The slightly positively charged

chlorine atom donates a pair of electrons to the AlCl3 molecule forming a covalent bond. The benzene ring then donates a pair of electrons to the positive chlorine atom forming a covalent bond:

The AlCl4- then donates a pair of electrons to the hydrogen atom, which donates a pair of electrons to the benzene ring, reforming the delocalisation. AlCl3 is also reformed and HCl is produced:

Friedel-Crafts reactions Aluminium chloride can also act as a catalyst to help polarise halogen-containing

organic molecules and cause them to substitute in a benzene ring. Once again, this reaction needs to be carried out under anhydrous conditions, as

aluminium chloride reacts vigorously with water. Aluminium chloride reacts with either the acyl chloride or halegenoalkane:

AlCl3(aq) + CH3COCl (aq)   AlCl4-(aq) + CH3CO+

The mechanism then goes as follows:

Because an alkyl group has been introduced into the ring, the reaction is sometimes

called alkylation. A similar reaction also takes place when benzene is reacted with a halogenoalkane

or an acid anhydride:

Summary Benzene is very important for the synthesis of dyes, pharmaceuticals and perfumes

etc. Electrophilic substitution reactions provide us with ways of introducing different

groups into the benzene ring.

These groups can then be modified further and more complex molecules can be synthesised.

The conditions for the important reactions are summarised in the diagram below:

Azo Compounds Azo compounds contain the –N=N– group:

In aromatic azo compounds, the R groups are arene rings; the structures of these are

more stable than if the R groups are alkyl groups. This is because the –N=N– group becomes part of an extended delocalised system

involving the arene groups. The aromatic azo groups are highly coloured and are often used as dyes. Aromatic azo compounds are formed by a coupling reaction between a diazonium

salt and a coupling agent.Diazonium Salts

The diazonium salts are very unstable; the only relatively stable diazonium salts are the aromatic ones, and these are not particularly stable.

This is because the presence of the benzene ring with its high electron density stabilises the –+  group.

Benzenediazonium chloride is an example of a diazonium salt:

In aqueous solution, Benzenediazonium chloride decomposes above temperatures of 5oC and the solid compound is explosive; for this reason, diazonium salts are prepared in ice-cold solutions and are used immediately.

They are synthesised in a diazotisation reaction- a cold solution of sodium nitrate is added to a solution of arylamine in concentrated acid (below 5oC).

The acid firstly reacts with the sodium nitrate to form an unstable nitrous acid (nitric(iii) acid):

NaNO2(aq) + HCl(aq)   HNO2(aq) + NaCl(aq)

The nitrous acid then reacts with the arylamine:

Diazo coupling reactions In a diazo coupling reaction, the doazonium salt reacts with another arene (the coupling

agent). The diazonium salt acts as an electrophile, reacting with the benzene ring of the coupling

agent. When the ice-cold solution of the diazonium salt is added to a solution containing the

coupling agent, a coloured precipitate of an azo compound is formed; many of these compounds are dyes.

The coupling agent always reacts in the two or four position of the benzene ring (where one position is the functional group).

The colour of the compound formed depends on the coupling agent that is being reacted with diazonium salt.

Coupling with phenols When the diazonium salt is reacted with a phenol, a yellow/orange azo compound is

formed:

With an alkaline solution of napthalein-2-ol, a red azo compound is formed:

Coupling with amines A yellow dye is often formed when a diazonium salt is reacted with arylamines:

Many different azo compounds can be formed by coupling different diazonium salts with a range of coupling agents.

The azo compounds are stable and so their colours do not fade.

The Chemistry of Colour Substances are coloured if they absorb energy that is in the visible frequency of the

electromagnetic spectrum. The energy absorb causes electronic changes; electrons are promoted from their ground

state to a higher energy level (they become excited). The difference in energy between the ground state and the excited state is equal to the

quantum of energy absorbed and so determines the wavelength of light absorbed. It is the outermost electrons that are excited; these are the electrons involved in bonding

or in lone pairs. The inner electrons are held tightly by the positively charged nucleus; the energy needed

to promote these is very large. Not all electronic transitions are brought about by visible light; some transitions require

more energy and are brought about by absorbing light in the ultraviolet region of the spectrum.

If only light from the ultraviolet region of the spectrum was absorbed, the compound would appear colourless.

The energy needed to excite an electron in a coloured compound is referred to as the excitation energy:

Coloured Organic Compounds These often contain unsaturated groups such as C=C, C=O or -N=N-. These can form part of an extended delocalised electron system called a chromophore. Electrons in double bonds are more spread out than those in single bonds, and therefore

require less energy to become excited; this means that the energy absorbed is within the visible region rather than in the ultraviolet region of the electromagnetic spectrum.

Functional groups, such as –O-H, –NH2, or –NR2 are often attached to the chromophores to modify the colour of the molecule; these groups all contain lone pairs of electrons which become involved in the delocalised electron system.

Small changes to the delocalised system can change the energy of light absorbed by the molecule, thus changing its colour.

Dye molecules, such as methyl orange, can have a different colour in acidic or alkaline conditions; compounds like this are often used as acid-base indicators:

Coloured inorganic compounds Transition metals are often present in coloured in organic compounds. Ligands datively bonded to the transition metal cause the splitting of transition metal’s d-

sub shell into two energy levels. The d-orbitals are partially filled. Electrons are promoted from the lower energy level to the higher one, absorbing the

frequency of light corresponding to the difference in energies between the two energy levels.

The frequency of light absorbed corresponds to visible region of electromagnetic spectrum.

Therefore this light is absorbed, so solution appears coloured. Sometimes the absorption of visible light can cause the transfer of an electron from the

ground state of one atom, to the excited state of an adjacent atoms; this is referred to as electron transfer.

Electron transfer is responsible for many bright pigments, such as Chrome Yellow and Prussian Blue.

How dyes attach themselves to fabric There are both natural fibres and synthetic fibres; due to the different functional groups

on the fibres, different dyes are required to colour them. There are a number of different types of dye:

Acidic Dyes These contain acid groups, such as –COOH and –SO3H which form attractions to the

slightly basic –NH groups in the amide links of wool, silk and nylon:

Direct Dyes

These bond to fabrics by hydrogen bonding and so are particularly attracted to cellulose fibres, such as cotton and rayon, which have many –OH groups.

Hydrogen bonds are weak compared to covalent bonds and so the dyes are only fast if the molecules are long and straight; they must be able to line up with the cellulose fibres and form several hydrogen bonds.

Disperse/vat dyes These do not dissolve in water, but instead are oxidised in the solution and physically

held in place within the fibres. Many azo compounds are examples of vat dyes. Fabric reactive dyes Fastness is a measure of how strongly a dye is attached to a fabric and is an important

indication as to whether the dye will move into water when the material is washed. For many years, chemists dreamed of developing fast dyes that would covalently bond to

fabrics rather than only joining to the fabric by weak intermolecular forces. During the 1950’s, a group of chemists working for ICI embarked on their search for a

better dye for wool. William Stephen, a member of that group, decided to modify the structure of azo dyes by

adding reactive groups in the hope that they would combine with the amino groups of proteins in wool.

One of his ideas was to modify an azo dye containing an amino group by reacting it with trichlorotriazane:

It was hoped that the new dye would react with wool:

However, the results were very poor and so more work needed to be done on the dyes. Stephen realised that the reaction would be more likely to happen in alkaline conditions;

however, this caused a problem, as alkaline conditions would damage the wool. Instead, they used the dyes with cotton, which would not be damaged by the alkaline

conditions. This was a success; the dye molecules reacted with both the amine and hydroxyl groups

on the cotton fibres. The first fibre reactive dyes had been produced.Synthetic DyesAlizarin

Mauve was the first synthetic dye ever produced; it was synthesised by William Perkin in 1856.

Up until Perkin had produced the first synthetic dye, plants were the main source for colouring material.

For example, the indigo plant was used to make blue dyes and the madder root was used to make red dyes.

The colouring matter from the madder root is a substance known as alizarin; alizarin only sticks to material that has been impregnated with a metal compound such as aluminium sulphate.

This process is known as Mordanting, and the colour of the material depends upon the metal used; with an aluminium mordant the dye is red, with tin(ii) the cloth is pink, and iron(ii) gives a brown colour.

Mordanting takes place under alkaline conditions so that a metal hydroxide is precipitated in the fibres; the metal ions firmly attach themselves to the cloth and then bind to the dye molecules by forming chelate rings (complex where ligands are datively bonded to a central metal ion):

Even though alizarin had been used for thousands of years, its structure was not discovered until 1868, when Carl Graebe and Adolf Bayer proved that alizarin was derived from anthracene, a minor component of coal tar.

From 1868-69, Graebe and Bayer developed a method for the production of alizarin.

In 1868, a crop of 70,000 tonnes of madder root was processed to produce around 700 tonnes of alizarin; by 1873, the madder fields had disappeared.

Azo Dyes The first azo dyes were synthesised by coupling a diazonium salt (obtained from phenyl

alanine) with one of a variety of coupling agents. Otto Witt, a Swiss trained chemist, completed research into why the aromatic azo compounds were coloured.

He put forward the theory that a dye molecule is built up from a group of atoms called a chromophore, which is responsible for its colour.

It is now known that chromophores usually contain unsaturated groups, such as C=O, -N=N- which are often part of an extended delocalised electron system involving arene ring systems.

For example, Chrysoidine, an orange dye, is made up of a chromophore with a delocalised system:

Attached to the chromophore in chyrsoidine are the two amine groups; these interact with

the chromophore to produce the orange colour. Other functional groups can be added to: Modify/enhance the colour of the dye. Make the dye more soluble in water. Attach the dye molecules to the fibres of the cloth. All azo dyes have the same common structure:

Chemists worked to produce as many XY combinations as possible to make a wide

range of new dyes with good colours, which would fast to fabrics and were commercially viable.

A vast range of azo dyes are now available by coupling one of fifty diazonium salts with one of fifty two coupling agents. Most of the colours formed are yellow, orange or red.