Proton environments for NMR

Hi,

Was hoping someone could help me identify what is meant by different proton environments as that is vital to be able to interpret NMR. For example, how come pentan-2-one has 4, pentan-3-one has 2 and methylbutanone has 3. I can't understand what is meant by different proton environment.

Thanks in advance :wink:

Reply 1

malolis

Look at what is bonded to the H. If it is different, then it's a different environment.

Reply 2

tazarooni89

It's difficult to define, but let me put it this way:

Build the physical structure of the atom using moly-mods, and then choose one hydrogen atom and keep it in your mind. Give me the molecule, close your eyes, and I'll rotate the molecule and the bonds around a bit, but I won't break any of the bonds, or make any new ones. If it is possible to point to a particular hydrogen atom, and say for certain that it is NOT the one you chose in the beginning (because it is bonded to something different), then it is in a different proton environment.

That's the best way I can think of putting it at the moment :s-smilie:

Reply 3

T kay

Methylbutanone: CH3, CHO, CH2, that's three different peaks.

Pentan-3-one: CH3, CH2. Two peaks.

If you draw the structures out you'll see the different non-equivalent hydrogen peaks.

Reply 4

JC-2

think about changing the protons for, say a chlorine atom. if you change two protons for two Cl atoms (one at a time), and call both molcules exactly the same thing, the protons are in the same environment. if you need to call the molecules different things, they are in different environments.

the slight exception comes when you have diastereotopic protons, but I doubt you need to worry about those yet!

Reply 5

a-zip

Original post by T kay

Methylbutanone: CH3, CHO, CH2, that's three different peaks.

Pentan-3-one: CH3, CH2. Two peaks.

If you draw the structures out you'll see the different non-equivalent hydrogen peaks.

your example really helped me thank you sooooo much! :smile:

Reply 6

Cyclohexane

Original post by a-zip

your example really helped me thank you sooooo much! :smile:

Yes, branching often (but not always) implies equivalence if the branch is on carbon two.

Pentan-3-one only has two because of its symmetry.
For non-symmetrical molecules such as pentan-2-one, this often means that all of the protons environments are different (assuming the compound isn't branched).

Reply 7

Pswl

Actually, in proton NMR, methylbutanone has 3 peaks, but those peaks are CH3, CH and CH3. One of the CH3 is connected to the CH group while the other CH3 is attached to the CO group. Hence, the two CH3 do not have the same chemical environments. Btw, since this is methyl butanone, there are two groups having the CH3-CH environment.