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Organic Chemistry
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UV-vis Interpretation
A UV-vis spectrum looks at the UV (200-400 nm) and visible (400-700 nm) region of the electromagnetic spectrum.
There are two pieces of information:
- y-axis is absorbance: how intensely (i.e. dark) coloured something is.
- x-axis is wavelength: how conjugated something is (more conjugated molecules are red and orange, less conjugated molecules are yellow).
Example: A UV-vis spectrum.
UV-vis Study Sheet
What makes a molecule absorb in the UV-Vis region? Two things: lone pairs and pi bonds! On the MCAT, the pi bond will be most important.
Conjugated systems have double bonds in sequence without interuption (beware the CH2!). You will remember conjugation from your lesson on aromaticity.
Example: a conjugated system (note the double bonds in sequence!)
The more conjugated a molecule is, the "more coloured" it is!
Wize Concept
In other words: more double bonds means a molecule will have a larger λmax!
Example: beta-carotene (found in carrots!) is highly coloured as it has 11 sequential double bonds!
