Colour in organic compounds Flashcards
What can molecular orbital theory be used to explain?
why organic molecules are colourless or coloured.
HOMO and LUMO
-Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.
-The highest bonding molecular orbitals containing electrons is called the Highest Occupied Molecular Orbital (HOMO).
-The lowest antibonding molecular obital id called the Lowest Unoccupied Molecular Orbital (LUMO)
What does absorption of electromagnetic energy cause?
electrons to be promoted from HOMO to LUMO
Why do most organic molecules appear colourless?
because the energy difference between HOMO and LUMO is relatively large. This results in absorption of light from the UV region of the spectrum.
What is a chromofore?
-some organic molecules contain chromofores
-A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum. Light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO
(section of a molecule responsible for colour of molecule)
(stop at double bonds when highlighting)
Where do chromophores exist?
in molecules containing a conjugated system
What is a conjugated system?
a system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms .
Electrons within this conjugated system are delocalised
(feature/property of molecule allowing light to be absorbed)
Which molecules contain a conjugated system?
molecules with alternating sungle and double bonds and aromatic molecules
What makes the energy gap between HOMO and LUMO smaller? what does this mean?
more atoms in the conjugated system
this means that a lower frequency of light (longer wavelength, lower energy) is absorbed by the compound.
What happens when the wavelength of light absorbed is in the visible region?
the compound will exhibit the complementary colour
