Predicting the pKa of a molecule

Heres the question:
http://img266.imageshack.us/img266/5071/sar2.png [Broken]
I have no idea how to answer either of these questions. I know that the methyl group attached to the carboxyl groups alpha carbon will decrease the acidity due to electron donating properties. It doesn't look like the carbonyl group is conjugated with the aromatic rings so I don't think they will increase the acidity too much. Thats all I can say about it though, I have no idea how to estimate the pKa.

I'm also confused about question b. I suppose one advantage is that the salt is not acidic.
 
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chemisttree

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Start by comparing the acidic group with other known compounds. Acetic acid, pKa = 4.75; propionic acid, pKa = 4.87; 2-phenylpropionic acid, pKa = ???

For b, you are partly correct but there is something else you need to consider. Simple carboxylic acids are fairly polar but this structure has a fairly large, greasy biphenyl group attached.
 
My guess for 2-phenylpropanoic acid is pKa = 4.2 since the phenyl group is close enough to significantly delocalise the acids electrons. If I'm not mistaken, the F group should increase electron density of the benzene ring because halogens are stronger resonance electron donors than inductive electron acceptors therefore is should raise the pKa a bit. However the second benzene ring in flurbiprofen will delocalise electrons even further so I'm guessing flurbiprofen has a pKa of around 4.0.
 

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