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AAMC FL 2 C/P #9

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djsbaseball2014

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So I am a little confused on this question. I remember from o-chem that sometimes UV light was used to create free radicals that could react, thus bonds would be breaking. I understand electrons get excited upon absorption of light but why can't i assume the bond will break? UV light is high in energy correct? Since it has a low wavelength, it has a high frequency and thus a high energy, enough to break a bond i thought.... If anybody can help clear this up for me it would be much appreciated
 

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aldol16

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The key word here is "always." UV light causes some molecules to break bonds. For instance, you can get peroxo species to cleave to form organic radicals by shining UV light on them. But this doesn't always happen - otherwise, UV spectroscopy would be almost pointless. Strong bonds don't break with UV light. For instance, you could saturate a solution with methane, take a UV spectrum of it, and you would not be cleaving any methane C-H bonds. You'd just excite the electrons. The excitation of electrons and subsequent relaxation is what gives rise to a UV spectrum. So UV always excites electrons - sometimes enough to break bonds but most of the time not enough to do that.
 
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