Colour by design

'Fatty acids' are the carboxylic acids in fats and oils.

They have an even number of carbons in their unbranched carbon chain.

These chains can be either saturated (only single bonds), or unsaturated (double bonds).

Triesters from largely saturated fatty acids are solids or fats - there is better packing of the molecules resulting in stronger intermolecular bonds.

Hydrolysis of esters

Any natural oil or fat can be broken down into the sodium salt of the fatty acid and glycerol, by heating with dilute sodium hydroxide solution. 

Hydrogenation

The addition of hydrogen to unsaturated oils, using a nicle catalyst and the correct conditions, produces a more saturated solid fat.

- If visible light falls on a coloured solution, some wavelengths are absobed and some are transmitted.

- We see the transmitted wavelengths (colours)

- many molecules also absorb ultraviolet (UV) radiation.

- As we only see in the visible region, a molecule that absorbs only UV light will appear colourless. 

Electronic transitions and unsaturated molecules 

Absorption of UV or visible light causes electronic transitions in a molecule. 

• Electrons move to higher energy levels, thus becoming 'excited'
• Many unsaturated molecules and those with a conjugated system absorb UV and visible light.
• The delocalised electrons in these systems require slightly less energy to become excited compared to electrons in single bonds. 

UV-visible spectroscopy

In UV and visible spectroscopy, the spectrometer scans a range of wavelengths of both lights. This produces a spectrum.

Features of a spectrum:

• the x-axis shows wavelength, (nanometres nm)
• the y-axis shows intensity of absorption; usually no units
• unlike and infrared spectrum, the trace is much broader. It is the overall shape of the spectrum that is important rather than individual peaks.

Remember the order of colours - Richard of York gave battle in vain

A method of analysis of pigments in old paintings is through a reflectance spectrum. 

This is the opposite of an absorption spectra. 

Chromatography is a method of separating and identifying the components of a mixture. For example, the components that make up the oils used in painting. All  chromatography depend on the equilibrium set up between components of mixtures. They are stationary phase and mobile phase. A component with a higher affinity to the stationary phase will take longer to travel and vice versa.  

• In g.l.c, the S.P. is a inert liquid coated on the surface of finely divided solid particles.
• This material is packed inside a long thin column, coiled inside an oven.
• A inert carrier gas acts as the mobile phase and carries the mixture through the column 

The simplest arene is benzene, C6H6, which has a flat hexagonal structure with a bond angle of 120 degrees. Arenes are hydrocarbons that contain benzene rings. Their names always end in  -ene. All carbon-carbon bonds are the same length, with a value between that of a single and double bond. This implies that one electron from each carbon is delocalised over the whole ring. Benzene is more stable than alternating single double bonds.  

Benzene undergoes electrophilic substitution.

Nitration 

C6H6 + HNO3 ---> C6H5NO2 + H20         (Reagents/conditions, benzene, concentrated nitric acid                                                                                      concentrated sulfuric acid <55 degrees)  

Sulfonation 

C6H6 + H2SO4 --->  C6H5SO3H + H20  (Reagents/conditions: benzene, concentrated sulfuric acid.                                                                                Heat under reflux)

Chlorination 

C6H6 + Cl2 --->  C6H5Cl + HCl             (Reagents/conditions: benzene, chlorine, anhydrous                                                                                           aluminium chloride room temperature)

Bromination 

C6H6 + Br2 --->  C6H5Br+ HBr             (Reagents/conditions: benzene, bromine, anhydrous iron                                                                                 (iii) bromide or iron filings. Room temperature)

Alkylation

C6H6 + CH3CH2Cl  --->  C6H5CH2CH3   + HCl (Reagents/conditions: benzene, chloroalkane,                                                                                                       anhydrous aluminium chloride. Reflux)

Acylation 

C6H6 + CH3COCl  --->  C6H5COCH3 + HCl     (Reagents/conditions: benzene, acyl chloride,                                                                                                       anhydrous aluminium chloride. Reflux)

Alkylation and acylation can be carried out in ionic liquids as combination solvent-catalyst systems. These reduce solvent emissions, reduce flammability, often recycle easily and allow reactions to be carried out at reduced temperature.

Friedel-Crafts reactions are useful because they add carbon side chains to the benzene ring. These can be modified further. 

Diazonium ions have the general formula R-N=~N (Triple bond) Only aromatic diazonium ions are stable, and still have to be at temperatures <5 degrees Celsius.

Reagents, nitrus acid, hydrochloric acid, bellow 5 degrees 

Diazoniums are weak electrophilic. They will attack phenols and aromatic amines (coupling agents), both of which have especially electron rich benzene rings. During the reaction, -N=N- bond is formed. The reaction is named, coupling reaction, and the compound formed is an azo compound.

The azo compounds make excellent fade-resistant dyes. By attaching different functional groups to the chromophore, the properties of the molecule are modified. 

Functional group     Property modified 

-SO3-Na+              More soluble in water, due to the ionic group - it can form ionic bonds with protein fibre 

-NH2 or NR2          The colour of the dye is modified or enhanced. 

Many transition metal ions are coloured because electrons in their d orbitals can be excited. When the transition metals are surrounded by ligands, the d orbital shells are split into two different energy levels.

Electrons in the lower of these two new energy levels can be excited to the higher level. The excitation energy required corresponds to the absorption of visible light. Factors that affect the excitation energy, and the colour, are:

• the type of ligand
• the shape of the complex - octahedral or tetrahedral
• the coordination number of the complex
• the charge on the central transition metal ion. 

The part of the molecule that is responsible for colour is the chromophore. This is an extended delocalised system of electrons containing unsaturated groups such as C=C, C-O, N=N and benzene rings. 

Electrons in the delocalised system generally need less energy to become excited, this energy is available when the molecule absorbs visible light.