[dead actor]

Hi all,

I was struggling in solving this question which is a part of a practice exam, and I need you to be patient with me and teach me step by step how to figure out the structure and how to read and analyze the spectra.

I attached the two 1H NMR & IR spectra so you will be able to know what structure I'm talking about.

I started with finding the saturation number which is 5 so I know that at least I have aromatic ring and double bond(s)
I predicted the compound to have an ester but I'm not sur.

please help me to know what is this structure.
The molecular formula is C8H8O2

and all the peaks and ppm readings are in the attached figures.



thanks all.

[orgopete]

I have been working on a tutorial for interpreting NMR spectra. In it, I advocate using the method I use myself in interpreting the spectra. Imagine that you have molecular models and you have eight carbons, eight hydrogens and two oxygen atoms. How can you put them together?

Just as a hint, check a chemical shift table and see if they give you any information about six of the carbons. After that, how many ways can you use two more carbons? If you do this, I cannot imagine that you should fail to come up with a structure that does not include your unknown compound.

[dead actor]

I'm thinking of one aromatic ring and a double bond here, maybe c=O & i'm not sure if there is OH, too.
but I don't know how to arrange them! or how to make sure that I'm doing it right.
give me more details plz.

[orgopete]

If you have an aromatic ring, how many hydrogens are left? Check your integral. How many ways can you organize the remaining atoms? I have two possibilities. Oh, while checking your NMR, how many signals do you have left? One? That means whatever you have left has to fit into that signal.

[dead actor]

armoatic can take up to 6 hydrogens if it's alone, but I assume in this case it will take either 4 or 5 since the remaining locations on the ring will be bonded to something else, I would assume that it will be a bonded to - OCH3 or OCH2 from one side since there is a signal at about 3.9 ppm on NMR spectrum,  and C=O  but I don't know if this will work. do u think there will be a COOH here? since this will give us 4 hydrogens in the ring, 1 in the COOH, and 2 or three in the OCH2 ( or OCH3) .. but do I have 8 signals or seven ? 

[orgopete]

There are five aromatic hydrogens. Check your integral. Read the numbers, guessing is not allowed. They are in the ratio of 2:1:2, but it is still five hydrogens. There is one other singlet of 3 hydrogens.

[dead actor]

Ok,
so I will have the ratio as 2:1:2:3

*5 aromatic hydrogens in the ring.
*3 other hydrogens remaining
*2 other carbons remaining
* 2 oxygens remianing
* C=O from the IR
* OH from the IR
most likely the structure will be (C6H5) -CH2COOH

is that right?

[dead actor]

I'm also thinking of :

(C6H5)-COO-CH3 which is an ester.

[Arctic-Nation]

Based solely on NMR, I'd say methyl benzoate. My knowledge of IR is a bit lacking.

When thinking of carboxylic acids, the acidic proton usually is in the 12-14 ppm range and gives a very broad peak. That's a dead giveaway.

[orgopete]

If it were phenylacetic acid, the integral ratio would be 1:5:2, not 5:3.

I agree with methyl benzoate, but I had hoped the other alternate structure, phenyl acetate would have been considered and then one could simply check a chemical shift table (if needed) to realize it should be methyl benzoate.

If you knew this compound had MF C8H8O2, one should be able to identify the compound only from the NMR spectrum. 

[Arctic-Nation]

Can't disagree with that. I've seen so many NMR's I just tend to evaluate the chemical shifts subconsciously. Which is not always the best strategy.

[dead actor]

thankkkkkkkks,
that's really great,  I concluded that it should be a methyle benzoate after you helped me, and now I'm sure it's a methyle benzoate.
but why it's not a phenyl accetate? could u please give me your reasonong fo that.
I really appreaciated your help.

[orgopete]

If you check a chemical shift table, you will find a much greater shift for a CH3-group that is connected to an oxygen. With phenyl acetate, the CH3 group will be attached to a C=O carbon.

For those who have interpreted enough spectra, we don't even look at the specific values as examples are encountered frequently and the differences are large.