Why are transition metals coloured?
Electrons in their d-orbitals can be excited.
How do ligands affect the d-orbitals of transition metals?
Ligands cause the d-orbital electrons of the metal ion to split into two different energy levels. Orbitals close to the ligands are pushed to slightly higher energy levels than those further away.
How does E=hv apply for complexes?
When light is passed through a solution of a complex, a photon of light is absorbed, causing electrons to be promoted to a higher energy level. The energy of the photon corresponds to the excitation of an electron from a low energy level to a high energy level. This affects the frequency of light absorbed due to the formula E=hv.
What four factors affect the excitation energy?
- The type of ligand
- The shape of the complex
- The charge of the central transition metal
- The coordination number
Which four methods are there of identifying the materials in a painting?
- Atomic emission spectrum
- Ultraviolet and visible spectrometer
- Reflectance spectra
- Gas-liquid chromatography
How does a UV and visible spectrometer work?
Source of UV and visible light split into two beams. One passes through a sample and one through the pure solvent. The two beams recombine to produce an absorption spectrum.
How can you interpret the spectrum of a UV and visible spectrometer?
The wavelength of the maximum absorption should be found. The colour of the solution has the colour of the complementary wavelength to this.
When is a reflectance spectra used, opposed to a UV and visible spectra?
If a substance cannot be made into a solution reflectance can be used.
How does a reflectance spectra work?
UV and visible light is shone onto the surface and the reflected light is collected and analysed.
When is gas-liquid chromatography used?
Chromatography is a method of separating and identifying the components of a mixture e.g. the components that make up the oils used in a painting.
What is the mobile phase in GLC?
An inert/unreactive carrier gas e.g. nitrogen.
What is the stationary phase in GLC?
A small amount of high boiling liquid held on a finely divided inert solid support and packed into a long thin tube called a column.
What are the steps of GLC?
- A sample is injected into an inert carrier gas stream just before it enters the column.
- The components of the mixture are carried through the column in the stream of gas.
- Each component has a different affinity for the stationary phase compared to the mobile phase and distributes itself to different extents between the two phases.
- Each component emerges from the column at different times. Those which favour the mobile phase emerge first.
- A detector monitors the compounds coming out of the column.
- Signals from the detector are plotted out by a recorder as a chromatogram.
What is the retention time in GLC?
The time that a compound is held on a column under given conditions and takes to emerge.
Which three factors affect the retention time of a compound?
- Length and packing of the column
- Nature and flow rate of the carrier gas
- Temperature of the column
How does electron delocalisation occur in benzene?
Carbon atoms have one outer electron which is not involved in bonding. These electrons are spread out evenly and shared by all the carbon atoms in the ring.
What are arenes?
Hydrocarbons (like benzene), which contain rings stabilized by electron delocalisation.
What is a three ring carbon fused system called?
Anthracene.
Why is the benzene ring stable due to the delocalisation of electrons?
The electrons are far apart and there is therefore, minimum repulsion.
What are the two products of sulfonation?
Benzenesulfonic acid and water.
What is the electrophile of sulfonation?
SO3
What is the electrophile of Friedel-Crafts alkylation?
R+
What is the electrophile of Friedel-Crafts acylation?
RCO+
What is a chromophore?
The part of an organic compound responsible for its colour. An extended delocalised system of electrons containing unsaturated groups, for instance, C=C, C=O and -N=N-.
