Benzene color

  • Thread starter gracy
  • Start date
2,486
82
know benzene is colourless....but i can't seem to get my head around it at the moment! As benzene has delocalised electrons, it should require less energy to excite the electrons. And Visible light is a lower frequency to UV? So according to the equation: E=hv Benzene should be coloured as it absorbs visible light? I know this is wrong and it is colourless, but i can't see how it works? Thanks for any help in advance!
 

Borek

Mentor
28,036
2,546

DrDu

Science Advisor
6,003
743
There is not just one UV but UV is a very broad range covering many octaves (in contrast to visible light which covers only about one octave). So e.g. a saturated hydrocarbon has some electronic transitions in the far UV (approx. 130 nm wavelength), while an isolated CC double bond absorbs at about 165 nm and the conjugated pi bond in benzene give rise to absorption at about 245 nm. For comparison, the visible range (violet) starts at about 400 nm. See any introductory text about UV- spectroscopy.
 
Less than what?
This is pretty much it.

For a molecule to absorb light of a given wavelenght, there must be two molecular orbitals (one filled and one with room for an electron) between which an electronic transition is allowed. The energy difference between these two orbitals, equals the energy of the light that will be absorbed in that transition or excitation process.

Pi conjugation usually makes possible transitions between pi and n molecular orbitals closer in energy than in a situation where this conjugation is missing. Briefly speaking, if the conjugation is enough to make these orbitals close enough to absorb light from the visible spectrum, the compound will have color. Otherwise, like in the case of benzene, the conjugation is not enough to absorb visible light, but UV light will indeed be absorbed.
 

Related Threads for: Benzene color

  • Posted
Replies
5
Views
5K
  • Posted
Replies
1
Views
6K
  • Posted
Replies
2
Views
5K
  • Posted
Replies
4
Views
3K
  • Posted
Replies
7
Views
3K
  • Posted
Replies
4
Views
5K
  • Posted
Replies
1
Views
2K
Replies
6
Views
6K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top