CD.1 Flashcards
The origin of color in organic molecules
Why do white opaque objects appear white?
None of the visible light wavelength is absorbed by the surface of the object. All visible light wavelengths are reflected and we perceive the object as white.
Why do colourless transparent substances appear colourless?
None of the visible light wavelength is absorbed. All visible light wavelengths are transmitted and therefore it looks colourless.
Why are some objects coloured?
Because they absorb certain frequencies of the visible region, corresponding to different colours.
When an opaque object absorbs wavelength corresponding to a colour, what colour would it appear to be?
The complementary colour, which corresponds to the colour of the wavelengths that has been reflected.
When a transparent substance absorbs wavelength corresponding to a colour, what colour would it appear to be?
The complementary colour, which corresponds to the colour of the wavelengths that has been transmitted.
What is the complementary colour of red?
Green
What is the complementary colour of yellow?
Violet
What is the complementary colour of blue?
Orange
The lower frequency/longer wavelength end of the visible light spectrum corresponds to which colour?
Red
The higher frequency/shorter wavelength end of the visible light spectrum corresponds to which colour?
Violet
What kind of electronic energy change happens when molecules absorb visible light radiation?
Excitation.
What happens when molecules absorb visible light radiation?
Electrons are excited from their ground state to an excited state.
What happens when molecules require greater energy that visible light to cause excitational energy change?
They would absorb UV radiation rather than visible light.
What colour would molecules be if they require UV radiation to raise electron to their excited state?
They would be white or colourless.
What is a conjugated system?
In an organic molecule, where it consists a system of alternating double and single bonds.
How are electrons arranged in a conjugated system?
The electrons in the p-orbitals that make up the π-bonds of the double bonds are able to spread out over all atoms of the system. The electrons are said to be delocalised in a conjugated system.
As well as the normal C=C double bonds, what other bonds or atoms can contribute to the delocalisation of electrons in a conjugated system?
C≡C, C=O, N=N and other multiple bonds. Lone pairs on oxygen and nitrogen atoms can also be part of the conjugated system if they are aligned in the correct direction to overlap with other delocalised electrons in the system.
What is a very important example of a conjugated system?
Benzene.
In terms of molecular structure, what explains the colours in organic molecules?
The presence of conjugated systems.
What effect does a conjugated system have on the energy required for a molecule to undergo excitation electronic energy change?
It decreases the energy needed to raise electrons from their ground state to the excited state.
Explain in three steps why a large conjugated system means a molecules is more likely to be coloured?
- The more electrons that are delocalised in the conjugated system, the smaller the energy gap between the ground stated and the excited state.
- A smaller energy gap results in the absorption of longer wavelength/lower frequency radiation.
- This means the larger the conjugated system, the more likely the molecule will absorb in the visible light region rather than UV region and be coloured.
What does the basic rule of thumb state about the least number of π bonds a conjugated system should have for it to likely absorb in the visible light region and be coloured?
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