[harbourfox]

I have a question which asks which of these pairs of molecules absorbs uv/visible radiation at a longer wavelength? Briefly state a reason for your choice.
a)cyclohexane or cyclohexene
b)cyclohexanone or cyclohexen-2-one
c)dimethylether or propane
d)hept-1,3,6-triene or hept-1,3,5-triene

My thoughts are that the more conjugated molecule ie with more double bonds, always absorbed uv/vis radiation at longer wavelength but not sure if this is right? Any thoughts greatly appreciated.

[Corribus]

This is generally true, but it doesn't help you with c (no double bonds) or d (same number of double bonds).

[harbourfox]

This is generally true, but it doesn't help you with c (no double bonds) or d (same number of double bonds).

Well with d I think because 1,3,5 is conjugated, that is there is a single bond between every double bond, it absorbs uv vis radiation better wherea, 1,3,6  is not conjugated.

As you say c is still a mystery to me. Though someone has suggested to me that dimethylether may be correct because of the electron lone pairs on it compared to propane which has no unpaired elecectrons. Sounds plausible enough, but the person was just guessing.

[Babcock_Hall]

@OP, Do you know what n  pi* transitions are?

[harbourfox]

No idea whatsoever. Do tell please?

[Corribus]

n π* isn't relevant here, but n σ* is.

You are familiar with π and σ bonds, and their antibonding counterparts, yes? So you can guess what a π π* and σ σ* transition is, I wager, and which one is likely to be higher energy (lower wavelength) and which is likely to be lower energy (higher wavelength)?

Now, n represents an orbital with a non-bonding electron.

So what is likely to be higher energy, π σ* or σ σ* ? And which of these molecules has which?

Maybe this is a little helpful?

http://en.wikipedia.org/wiki/Molecular_electronic_transition

[Babcock_Hall]

@OP, my first post should have said n :rarrow:σ*.