Why is benzene colourless?

I know it's 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!

Reply 1

icapturethecastle

16

i have no idea...nor have i started working for the synoptic :eek:

Q: OP, what board are u for chem?

ps. sorry for not answering the q

Reply 2

Icy_Mikki

13

I *think* the electrons have to go up energy levels if they absorb photons, and they do this on transition metals by going up to different subshells (hence why transition metals with full subshells don't absorb photons; there aren't any subshells that electrons can 'jump' up to!).

In Benzene, the electrons are delocalised (as you say) so I'd imagine there aren't 'subshells' they can jump up to/down from, meaning they don't absorb any photons.

Reply 3

Joe555

OP

13

I'm OCR Salters if this helps....hmmm

Reply 4

vaseand

11

Benzene absorbs light mostly in the infrared range so visible light is not affected

Benzene absorbs light in the ultraviolet region.
Increase the conjugation and the energy needed for electron transitions decreases until it eventually absorbs in the visible region (less energy)
That's why compounds such as azo dyes are highly coloured.

Reply 6

Joe555

OP

13

charco

Benzene absorbs light in the ultraviolet region.
Increase the conjugation and the energy needed for electron transitions decreases until it eventually absorbs in the visible region (less energy)
That's why compounds such as azo dyes are highly coloured.

Cool cheers, but surely benzene is conjugated (definition of conjugation is delocalised electrons across a molecule etc.). So why isn't benzene highly colourful like azo compounds?

Reply 7

-Kav-

12

Joe555

Cool cheers, but surely benzene is conjugated (definition of conjugation is delocalised electrons across a molecule etc.). So why isn't benzene highly colourful like azo compounds?

I can only assume that it's not conjugated enough. The conjugation in avo dyes is massive, encompassing two aryl rings, your azo nitrogens, and whatever substituents are present. The degree of delocalisation is important.

Reply 8

beccy.

11

-Kav-

I can only assume that it's not conjugated enough. The conjugation in avo dyes is massive, encompassing two aryl rings, your azo nitrogens, and whatever substituents are present. The degree of delocalisation is important.

thats what i think too. it was the only explanation i could think of!

Woodward's rules for UV absorption gives a list of non-empirical wavelength rules governing conjugated systems (amongst other substituents)....

Check out
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm

Original post by HoldtheDoor

Can someone please do Jan 2013 question 5 get please, on NMR, quite difficult.

Bizarre ...

Would it not be better to start a new thread rather than unearthing one from 8 years ago, which is not on the same topic!

.. and besides, which exam board are you referring to?

Reply 11

HoldtheDoor

2

Original post by charco

Bizarre ...

Would it not be better to start a new thread rather than unearthing one from 8 years ago, which is not on the same topic!

.. and besides, which exam board are you referring to?

I accidentally posted this question on this thread, I had three tabs open with three different threads, was suppose to put this on the ocr salters f335 chemistry 2016 thread, sorry. :colondollar:

Why is benzene colourless?

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