NMR Spectrum for Phenol

[Hashtag]

Hello guys!

I am studying A-Level chemistry in Grade 12. Regarding the Carbon-13 NMR spectrum of Phenol, I do not seem to understand why we get 4 distinct peaks. Can someone be so kind as to explain why carbons 2 and 6, and  carbons 3,4,5 have different peaks?

[hypervalent_iodine]

The chemical shift is determined by the magnetic environment the atoms are in, which is related of course to their electrostatic environment and therefore, the electronics of the molecule. When determining how many signals you expect to see, you are looking for chemical equivalence - i.e., how many distinct chemical environments there are. For example, if we take this molecule (pentane-3-one):

 

 

I have drawn a line through the centre of the molecule to show where there is a plane of symmetry. You can see the first and last carbon see exactly the same thing as one another, as do the second and fourth. They are in the same environments, and therefore you expect them to occur at the same chemical shift in a ¹³C NMR spectrum. In this case, we would expect to see 3 peaks; one representing C-1 and C-5, one representing C-2 and C-4, and one for C-3. 

Some molecules can have multiple planes of symmetry:

In this case, all of the -CH carbons (C-2, C-3, C-5, and C-6) are all identical, and would all occur at the same chemical shift. Likewise for C-1 and C-4 (the ones bonded to the methoxy groups), as well as the methoxy carbons. In this case, we see three sets of equivalent carbons, and so we expect three peaks in a ¹³C NMR.

Have a look at phenol, and ask yourself if there is a plane of symmetry, and how many chemically equivalent carbons there are:

 

[studiot]

What a wonderfully clear explanation, HI. thank you +1

 

Hashtag, I didn't know they did NMR at A level these days, but welcome and bring on your questions.