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Organic Chemistry
Justine_5420
#1 Posted : Saturday, June 06, 2020 12:08:58 AM
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I have just completed the altius 1/2 practice exam and am confused about a question re: organic chemistry. The question (question 5 in the chemical and physical section) says that a chromophore absorbs light over a wide range of wavelengths because it contains what?

The answer to this question is because it contains an extended pi system coupled to electron donating amine groups and electron withdrawing carbonyl groups. I understand that many aromatic compounds could create a saturated pi system, but the EK study guide never really spoke to how conjugated pi systems absorb lights with a range of differing wavelengths and donating/accepting functional groups. Could you please explain this in more detail? Is this all the detail we need to know for the MCAT re: light absorption and pi bonds? Do pi bonds absorb wavelengths better than sigma bonds?
INSTR_Katerina_102
#2 Posted : Sunday, June 07, 2020 4:43:50 PM
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Hi,

An extended pi system or conjugated system (ie 3 or more conjugated bonds) absorbs visible light. This can be the case for a compound like beta carotene which makes carrots red, or something that contains aromatic rings like melanin that makes your skin tan.

For the case of the MCAT, just remember that if you have enough conjugated double bonds, you start absorbing in the visible light spectrum or UV spectrum (depending on the number of conjugated double bonds). With a compound that does not have conjugated double bonds, you do not observe this absorption (unless it is a metal complex).

The reasoning behind this is that the more conjugated double bonds a molecule possess, the lower the energy difference is between its ground state and its first excited state.

For a singly bonded compound, we often observe it as white in colour because the energy difference between its orbitals is very large. However, when you start getting lots of pi bonding, the energy difference becomes smaller. As E= hc/lambda, you can start absorbing wavelengths of visible light if this energy difference becomes small enough (with enough conjugation). You can for example absorb blue light, yielding the complementary red colour.

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