F324 nmr spectroscopy

im having trouble suggesting suitable structures from nmr. i dont understand how their can be several c-c peaks but they represent ch3 or ch2. how is everyone revising spectroscopy.

Reply 1

indifferencepersonified

Are you talking about proton nmr? If so, a CH3 will split the neighbouring protons in a quartet on the spectrum, and a CH2 group will split any neighbouring protons into a triplet. So, if you have a section of a spectrum with 2 peaks, one peak is split into a quartet, and the other peak is split into a triplet; it is likely you will have an ethyl group in the compund (obviously, remember to look at the value of the integration trace as well!).

Reply 2

Charzhino

give us some examples so when can see clearly

Reply 3

gingerbreadman85

Original post by Rudeboy999

im having trouble suggesting suitable structures from nmr. i dont understand how their can be several c-c peaks but they represent ch3 or ch2. how is everyone revising spectroscopy.

You'll never get a C-C peak on an NMR specta. C¹³ is spin active but has a very low relative abundance... C¹² is not spin active so the chance of having 2 adjacent spin active carbons is negligible, therefore it's not a signal that will be visible in your specta. For the same reason, H¹ NMR doesnt pick up couplings between H and C.

The signals in your spectra represent the interaction of an excited H atom with any other spin active nucleus within 3 bonds of it. For A-level you only really have to worry about H-H coupling.

The rule is you will get a signal for each H in unique environments (i.e. not equivilant to each other), each signal split into n+1 peaks, where n is the number of nearby H.

Thus something like CH₃CH₂CH₃ will have 2 signals, one for the H in the CH₃ (which are equivilant to each other) and one for the CH₂, the integral of these peaks will give a ratio 3:1. The CH₃ peaks will be split by the 2 CH₂ H atoms, giving a triplet (2+1=3) and the CH₂ will be split by the 6 CH₃ H atoms, giving a multiplet.

It gets more complicated when a H atom has 2 or more different H environments to couple with, however i dont think you need to know in that level of detail for A-level.

EDIT - Double post..... oops, not sure what happened there!

(edited 13 years ago)

Reply 4

Rudeboy999

how do we know which of the peaks corresponds with the ch3 and one with the the ch2

Reply 5

gingerbreadman85

The integration and the multiplicity

Original post by gingerbreadman85

The rule is you will get a signal for each H in unique environments (i.e. not equivilant to each other), each signal split into n+1 peaks, where n is the number of nearby H.

Thus something like CH₃CH₂CH₃ will have 2 signals, one for the H in the CH₃ (which are equivilant to each other) and one for the CH₂, the integral of these peaks will give a ratio 3:1. The CH₃ peaks will be split by the 2 CH₂ H atoms, giving a triplet (2+1=3) and the CH₂ will be split by the 6 CH₃ H atoms, giving a multiplet.

Integration should give you a peak ratio that corresponds to the ratio of H atoms in that environment.

Reply 6

Vxckyy_

In carbon NMR if there's 2 C-C peaks does that automatically mean there's 2Carbon bonds in the structure?

Reply 7

charco

Study Forum Helper

Original post by Vxckyy_

In carbon NMR if there's 2 C-C peaks does that automatically mean there's 2Carbon bonds in the structure?

You are not detecting bonds you are detecting the presence of atoms in the molecule. If you have two peaks in carbon-13 NMR then you have two carbon atoms in different environments.

Reply 8

Vxckyy_

Original post by charco

You are not detecting bonds you are detecting the presence of atoms in the molecule. If you have two peaks in carbon-13 NMR then you have two carbon atoms in different environments.

I mean that is on the CNmr graph of there are 2 peaks that are accountable for the C-C bond does that mean there are 2 carbons attached?? I'm confused.

Reply 9

charco

Study Forum Helper

Original post by Vxckyy_

I mean that is on the CNmr graph of there are 2 peaks that are accountable for the C-C bond does that mean there are 2 carbons attached?? I'm confused.