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CONJUGATED SYSTEMS

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medbunny56

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I'm having trouble understanding conjugated systems and their absorbance, could someone please clarify for me?

This is what I have in my notes:

extensive conjugation allows the ion to absorb in the visible region
conjugation results in absorbance at higher wavelengths (so it appears red)

but somewhere else I wrote conjugated systems strongly absorb UV light (around 400nm and lower)

which one is correct?
 

BerkReviewTeach

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Pi-bonds between carbons absorb around 180 to 190 nm. With conjugation, the energy gap between π and π* gets smaller, so the lambda-max goes up about 45 nm per additional π-bond in the network. If there is enough conjugation, then that absorbance can be greater than 400 nm, which makes it in the visible range.

So R2C=CR2 should be maybe 185 nm, R2C=C-C=CR2 should be maybe 230 nm, R2C=C-C=C-C=CR2 should be maybe 275 nm, and so on.

Pi-bonds between a carbon and an oxygen absorb around 270 to 200 nm. With conjugation, the energy gap between nb and π* gets smaller, so the lambda-max also goes up about 45 nm per additional π-bond in the network.

So R2C=O should be maybe 275 nm, R2C=C-C=O should be maybe 320 nm, R2C=C-C=C-C=O should be maybe 365 nm, R2C=C-C=C-C=C-C=O should be maybe 410 nm, and so on.

So both statements you wrote are correct, but neither tells the entire story. Page 180 of Book 1 shows a good summary of UV spectroscopy and conjugation effects.
 
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