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Aromatic proton environments

I've done a question involving aromatic protons in an nmr spectrum and a few things have worried me, so hopefully somebody can help.

Its the spectrum in the final question. Mark scheme is here if you want to see the compound that produced it...

Q http://www.thestudentroom.co.uk/attachment.php?attachmentid=189507&d=1356998788
MS http://www.thestudentroom.co.uk/attachment.php?attachmentid=189508&d=1356998788

Now the peak on the far left is for the 5 aromatic protons.

What I want to know is:

Why is there only one peak corresponding to one aromatic proton environment? When considering Carbon-13 nmr then for this molecule it would have 4 aromatic environments due to the symmetry - so why one for protons.

Why is such a splitting pattern produced?

Thankyou :smile:

(edited 11 years ago)

Reply 1

charco

Study Forum Helper

Original post by fayled

I've done a question involving aromatic protons in an nmr spectrum and a few things have worried me, so hopefully somebody can help.

Its the spectrum in the final question. Mark scheme is here if you want to see the compound that produced it...

Q http://www.thestudentroom.co.uk/attachment.php?attachmentid=189507&d=1356998788
MS http://www.thestudentroom.co.uk/attachment.php?attachmentid=189508&d=1356998788

Now the peak on the far left is for the 5 aromatic protons.

What I want to know is:

Why is there only one peak corresponding to one proton environment? When considering Carbon-13 nmr then for this molecule it would have 4 aromatic environments due to the symmetry - so why one for protons.

Why is such a splitting pattern produced?

Thankyou :smile:

There isn't only one peak! It's a complex multiplet, which would need higher resolution NMR to be clearer...

Reply 2

fayled

Original post by charco

There isn't only one peak! It's a complex multiplet, which would need higher resolution NMR to be clearer...

By complex multiplet do you mean that the 5 protons all produced different signals but they were of close chemical shift so superimposed over one another?

Would I be right in expecting there to be a doublet, triplet and triplet as the three environments, which all overlap?

Reply 3

charco

Study Forum Helper

Original post by fayled

If it's a mono-substituted benzene then there are three environments, ortho, meta and para (2,6 .. 3,5 ... and 4)

The protons at 2,6 are split into a doublet as 3 splits (couples with) 2 and 5 splits (couples with) 6 identically.

The signal of the 3,5 protons is split into a doublet by the protons at 2,6 and into another doublet by the proton at 4.... i.e. a doublet of doublets.

The proton at 4 is split into a triplet by the protons at 3 & 5.

However, to see all of this you would need a high resolution instrument.

Reply 4

fayled

Original post by charco

Ok, makes sense, thanks.

So in an aromatic molecule the number of proton environments is based upon symmettry in the same way the number of carbon environments is?

Reply 5

charco

Study Forum Helper

Original post by fayled

Ok, makes sense, thanks.

So in an aromatic molecule the number of proton environments is based upon symmettry in the same way the number of carbon environments is?