The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Carbon-13 nmr problem

http://store.aqa.org.uk/qual/gce/pdf/AQA-CHEM4-W-QP-JAN10.PDF

Question 8 a) i) I just don't understand. I initially thought all three compounds had 2 peaks in their 13C spectrum, but the mark scheme says W has 3, X has 4 and Y has 2. How does W and X have any peaks other than d 110-160 for benzene and d 25-60 for C-N (single bond)?
X only has 3 environments and hence 3 peaks in the carbon spectrum! Maybe it's a mark scheme error?
The others only have 2 environments.

You do use decoupled spectra for carbon? ie, there is never a splitting pattern for them?

I'll run a simulation on the chem software i have just to check everything
Reply 2
Avatar for shawn_o1
shawn_o1
OP
21
Original post by JMaydom
X only has 3 environments and hence 3 peaks in the carbon spectrum! Maybe it's a mark scheme error?
The others only have 2 environments.

You do use decoupled spectra for carbon? ie, there is never a splitting pattern for them?

I'll run a simulation on the chem software i have just to check everything


No, 13C peaks are never split.
Determining the number of peaks is all about symmetry. The chemical environments are similar but not exactly the same - lets take W for example...

(It's a bit annoying that it's a ring, it make it harder to explain in text.) It has a plane of symmetry between the two NO2 groups, cutting the molecule in half. You have three carbons on each side, one carbon is directly attached to the NO2 this is one environment, the next carbon along is further away from the nitro group (environment number 2) and the last is even further away (environment 3). So you predict it to have 3 peaks.
Reply 4
Avatar for TiTo20
TiTo20
7
Original post by shawn_o1
http://store.aqa.org.uk/qual/gce/pdf/AQA-CHEM4-W-QP-JAN10.PDF

Question 8 a) i) I just don't understand. I initially thought all three compounds had 2 peaks in their 13C spectrum, but the mark scheme says W has 3, X has 4 and Y has 2. How does W and X have any peaks other than d 110-160 for benzene and d 25-60 for C-N (single bond)?


The markscheme is correct, the only trouble I have proving it is that I'm on my iPhone and I don't have my white board to help me prove. If no one else answers this before I'm done, I'll quickly take a picture when I'm finished with work =]

Only tip I can give you without my stuff is look for symmetry, and that the distance between one carbon and a group, and another carbon and a group will give rise to different peaks.

W has three peaks because of...

C -- C -- C -- NO2 : 1 peak for the C furthest away from the NO2 group, and then +1 until then C next to the NO2 group.

Hope this sort of helped!


This was posted from The Student Room's iPhone/iPad App
Sorry was in proton nmr mode when I wrote that: 3,4,2 makes sense

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you