[meta1033]

C6H5-CH2-CHOH-CH2COOH

1. Which is the most deshielded proton?
2. Which proton(s) will give a singlet signal?
3. Which proton(s) signal will disappear if the compound is dissolved on D2O before the PMR spectrum is run?
4. Which proton(s) will give a quintet signal?

Homework from a class I missed out on, any tips for me?
Any help is greatly appreciated.
Thank you.

[Oxygen]

C6H5-CH2-CHOH-CH2COOH

1. Which is the most deshielded proton?
2. Which proton(s) will give a singlet signal?
3. Which proton(s) signal will disappear if the compound is dissolved on D2O before the PMR spectrum is run?
4. Which proton(s) will give a quintet signal?

Homework from a class I missed out on, any tips for me?
Any help is greatly appreciated.
Thank you.

1. Deshielding occurs when a proton is near an electronegative atom(s).  The electronegative atom(s) will pull electrons away from the proton and the signal will occur further downfield because electrons shield the proton from the Rf pulse...without the electrons it is considered "deshielded."

2. A singlet peak will occur when a hydrogen has no neighboring hydrogens on neighboring carbons.

3. Which proton is acidic?

4. A quintet signal occurs when a hydrogen (or group of hydrogens) have four neighboring hydrogens on neighboring carbons.

[meta1033]

Ah thanks for the tips! I just tried it out and well here are my answers...hopefully I got the idea down.

1.  So then the most deshielded proton would be the C6H5 since it is most furthest away.

2.  Seems like there is no proton that will give a singlet signal in this molecule.

3.  The COOH

4.  CHOH proton will give a quintet signal.

Can anybody clarify my answers for me? Thank you.

[Oxygen]

I would suggest reviewing your chemical shift tables....

1. C6H5 is a phenyl...  Yes, the aromatic hydrogens will be deshielded but not because they are furthest away from an electronegative atom, the aromatic pi system is deshielding in itself.  I think you may have "shielding" and "deshielding" backwards....

None the less, there's definitely a hydrogen that would be even more deshielded than the aromatic hydrogens.  If it helps, you can think back to acids and bases from gen chem.  The most acidic hydrogen is the one that has electron density being pulled away from it (lowest pKa).  It works the same with deshielding.  If a hydrogen has all of its electrons being pulled away from it towards an electronegative atom like oxygen, it's going to show up way "downfield" (aka to the left of the spectra).  This is due to the electron not being able to "protect" the proton from the microwave pulse. 

2. A singlet occurs when a hydrogen has no "neighboring" hydrogens on adjacent atoms.  A hydrogen on an -OH will always be a singlet even if the carbon the oxygen is connected to has hydrogens on it. 

3. Yes, and there are times where you wont even get a peak for the H on an alcohol (frustrating!).

4. The hydrogen on the -CH only has two neighboring hydrogens (remember, hydrogens on an oxygen don't split others and are not split themselves).  It's likely that with the 5 hydrogens on the benzene ring you could see either one singlet integrating for 5H, or you might see some stranger splitting patterns (doublet of quartets, doublet of triplets or something along those lines).  It's hard to tell without a spectra, but no other hydrogens would give you a quintet signal....

I would suggest reviewing splitting patterns (recall the n+1 rule), the patterns of shielding/deshielding, aromatic hydrogen patterns, and of course keep your tables at hand!

Good luck, hope this helps.  Let me know if you have more questions.

[meta1033]

Helps a bunch really thanks a lot this is a lot of good information.  I remember reading about the n+1 rule and yeah that definitely helps a bunch.  Thanks for the help, and enjoy the mole snack.

[harshadaa22]

1)The carboxylic acid proton is the most deshilded proton because the carbonyl group draws the electrons decreasing the electron density at the proton and gets deshilded.
2)The protons attached to hydroxyl or carboxyl group will give singlets if there is no exchange is taking place.
3)The hydroxylic or carboxylic acid proton will get exchanged with Deuterium when dissolved in D2O and the signels will get disappeared.
4)The methine proton will give a quintet as there are two adjacent methylene groups are present.total 4Hs
    4+1=5  Quintet.