[BrookElise]

I am currently working on a formal lab write up for the Pinacol Rearrangement experiment and I am comparing the UV spectra of benzopinacol and benzopinacolone.  I have checked a few resources, the internet, my book, lab slides, and my notes, but haven't been able to find any explanation as to why I would see an increase in absorbency, but not an increase in wavelength.  Both UV spectra shows two peaks around 217nm and 253nm, but the absorbency shows an increase for the benzopinacolone. 
I know that these increases are due to an increase in conjugation, but why doesn't that show an increase in wavelength as well?

[Corribus]

Are you sure you've made what you think you've made?

254 nm is benzene.  If you have a conjugated carbonyl, as in benzaldehyde, the UV-vis absorption will red-shift (due to increased conjugation) and intensify (relaxation of symmetry selection rule) by a fair margin.  See, e.g., http://webbook.nist.gov/cgi/cbook.cgi?Spec=C100527&Index=0&Type=UVVis&Large=on

Can you confirm the structures of your reactant and product? 

[BrookElise]

I have been able to confirm my products based on melting point and IR. 

I am not comparing my products to benzene, but to each other.  The only difference between them being the removal of 2 OH groups and the addition of a carbonyl.

[Corribus]

Benzopinacol is 1,1,2,2-tetraphenyl-ethan-1,2-diol, correct?

Reason I compare to benzene is because an electronically isolated phenyl functional group would give roughly the same absorption signature as benzene.  Your reported UV-Vis peaks are suggestive of electronically isolated phenyl functional groups.  This would not be the case if you increase the conjugation in your product, which is why I ask whether you are sure you made what you think you made.  IR and melting point identification isn't really that great.  NMR would be better. 

There are a number of reasons why the intensity might increase, and one of them is simple human error.  What kind of increase are we talking about - 10%, 20%? Is it the same over the entire spectrum?  Is this plotted on an extinction coefficient scale?  You just haven't given a whole lot of information needed to answer this question.  I'm not even sure what the structures are that we're dealing with.  Using IUPAC nomenclature or putting structures in your post with SMILES would help immensely, and so would seeing your data.

[BrookElise]

I have attached IR and UV spectra for both compounds.  The IR spectra contain the structures.

[Babcock_Hall]

Check the literature melting ranges for benzopinacol and benzopinacolone.  They are not very different IIRC.  The hump in your second UV spectrum around 330 nm is significant, IMO.  I would also look into finding other names for these two compounds.  http://webbook.nist.gov/cgi/cbook.cgi?ID=C466375&Mask=400

[Corribus]

Ok, the appropriate way to do a comparison would be to plot one spectrum right on top of another.  This would let you see any difference in wavelength of the transitions. Even then, unless these spectra are taken for equivalent concentrations, the relative intensity (at least, between the two spectra) means nothing.

Why are there three UV-Vis spectra for each sample?

[BrookElise]

Professor requires us to run the UV at three different concentrations for the same compound.  We typically compare the middle curve of each.

We have never put two compounds on the same spectra...  It's just not the way our school does things...  I do see how that would make things so much easier!

[Corribus]

You don't have access to the raw data?

Anyway, it's real hard to make any kind of strict comparison.  As I said, you shouldn't read much into the intensity differences between the two compounds.  Beer's Law tells you that the intensity is linearly related to the concentration, so if one is 20% more concentrated than the other, it'll be 20% more intense (all other things equal). 

Your intuition is correct that the compound with the carbonyl should have a red-shifted absorption due to extension of conjugation.  I want to point out that the middle spectrum of your product does not appear to be the same as the low concentration spectrum of this compound.  See how the middle-concentration spectrum has a pretty sharp peak at 253 nm, but the low concentration spectrum features a broad peak that tails significantly into the red? (It even reads the average peak maximum as 267 nm).  Now, some compounds do have concentration dependent changes in peak position.  THis doesn't look to me like it'd be one of them.  Are you sure you used the right compound for that middle-concentration spectrum?

Also bear in mind that 3/4 of the benzene functional groups in your product will be spectroscopically identical to the 4 benzene functional groups in your starting material.  Therefore it shouldn't surprise you that the product spectrum also features some 254 nm peak character.  Were I you, I'd focus on the low-concentration spectra when making your comparisons.  THis is a good practice anyway.  Your middle and high concentration spectra are approaching the saturation point (most light absorbed) and the spectrophotometers are less accurate in this range.  In the low concentration spectra there are some definite differences.  The product spectra has definite intensity in the 270-280 nm region whereas your reactant spectra do not.

[BrookElise]

Okay, thanks for the insight.  I don't really feel like I got enough instruction on how to read these and as you can tell we don't do them in the most efficient manner.

[Corribus]

Also I didn't even look at the high concentration spectra before, and I agree with Babcock hall - the lower energy region (>300 nm) region of the concentrated spectrum for the product is very significant.