[cheeselampost]

Hi all,

Could anyone explain to me how many peaks you should expect to see in the C13 NMR spectrum of phenylethanamide please?

I would have predicted 4 peaks, but my spectra is showing 6?

Thanks in advance

[sjb]

Here's a picture of your compound



Now, how many environments are there for the carbons?

[cheeselampost]

I've got an NH2  at the end of the chain, rather than an NH in the middle? SO the chain reads: CH2, C=O, NH2

I only see 4 chemically different carbon environments, the C=O, CH2, C on the benzene ring attached to CH2 and one of the other carbons on the ring. Is this right, or wrong? Im doing an analytical chemistry problem question. We are provided with the mass spec, 1H nmr, C13 nmr and IR, and an elemental analysis, and have to deduce a structure.

Phenylethanamide fits all the other spectra, but I cant make it fit the C13 NMR which displays 6 peaks.

[macman104]

Can you provide your 1H NMR information as well?

[sjb]

I've got an NH2  at the end of the chain, rather than an NH in the middle? SO the chain reads: CH2, C=O, NH2

I only see 4 chemically different carbon environments, the C=O, CH2, C on the benzene ring attached to CH2 and one of the other carbons on the ring. Is this right, or wrong? Im doing an analytical chemistry problem question. We are provided with the mass spec, 1H nmr, C13 nmr and IR, and an elemental analysis, and have to deduce a structure.

Phenylethanamide fits all the other spectra, but I cant make it fit the C13 NMR which displays 6 peaks.

Possibly my mistake then. I've put up N-phenylethanamide, but it sounds like you have 2-phenylethanamide - like this?

[cheeselampost]

yep thats what I have. 2 phenyl ethanamide fits my mass spec and 1H NMR.

So... do you think 4 or 6 C13 NMR peaks?  Or a different number, I'm confused!

Thanks



p.s 1H NMR data: 3 single peaks. At 7ppm a 5H peak, at 5.5ppm a 2H peak (v small and broad, shifted with solvent) and at 3.5ppm a 2 H peak.

[macman104]

There are still 6 different carbon environments.  However, some of your carbons are going to be very close in PPM (mainly the aromatic ones) and it's possible you are not getting enough resolution to see each single one.

[cheeselampost]

Great, thanks!

I get the following 6 environments.



I don't really understand though why carbons 3,4, and 5 are chemically different environments. I can see why carbons 1,6 and 2 would give a C13 peak, but not the rest?

(this picture was taken from the SDBS website, sorry i can't resize it better...)

[macman104]

Why do you think 3,4,5 are not chemically different?  They are very similar to be sure, but they are not in the exact same chemical environment.  They are all progressively closer/farther from the side chain, which affects the shifts.

Notice on the spectral database how close the shifts for 3,4,5 are.

[cheeselampost]

ah, i see. I was just looking at the neighbours of 3,4,5, thinking that they are bonded to similar groups. I forgot to think about the chemical shift.

Thanks very much for the help. This is a very useful site.

[sjb]

Looks like we have it sorted.

Although carbons 3,4 and 5 may be bonded identically if you simply look 1 atom away, if you look further, then they are not. Even the aromatic protons are different, it's just there may not be enough resolution to see this.

This results in a different shift, not really the other way around.

A handy tip to consider when you're looking at these problems is to imagine that each atom in question is replaced by something else and see if the new compound is the same, or different as before.

S