ma7cus
Badges: 6
Rep:
?
#1
Report Thread starter 2 years ago
#1
Can someone explain what constitutes an 'environment' in proton/carbon 13 NMR? Do you just consider the adjacent carbon to the carbon/hydrogens you're talking about or does the whole molecule matter? Is it the same rules for both cases or is hydrogen more sensitive to changes given you get peak splitting and stuff?
Any clarity at all would be helpful, this topic confuses me a lot, thanks
0
reply
Claisen
Badges: 14
Rep:
?
#2
Report 2 years ago
#2
Look for symmetry and is the carbon bonded to the same atoms as another.

CH3CH2OH

CH3 protons are split into a triplet by the adjacent CH2. CH2 protons are split into a quartet by the adjacent CH3. OH is split into a singlet.

Google spin-spin coupling, this might help your understanding. At A-level, you can learn some fairly simple rules to make the process quicker, they are correct most of the time.

t,t = CH2CH2
q,t = CH3CH2
s,9 (CH3)3-X
0
reply
ma7cus
Badges: 6
Rep:
?
#3
Report Thread starter 2 years ago
#3
Alright, what's the stuff at the bottom supposed to represent?
(Original post by Acetyl)
Look for symmetry and is the carbon bonded to the same atoms as another.

CH3CH2OH

CH3 protons are split into a triplet by the adjacent CH2. CH2 protons are split into a quartet by the adjacent CH3. OH is split into a singlet.

Google spin-spin coupling, this might help your understanding. At A-level, you can learn some fairly simple rules to make the process quicker, they are correct most of the time.

t,t = CH2CH2
q,t = CH3CH2
s,9 (CH3)3-X
0
reply
Kian Stevens
Badges: 16
Rep:
?
#4
Report 2 years ago
#4
An 'environment' is just a magnetically, and so chemically, different part of a molecule
An environment is dependent on its surroundings, and so different surroundings give rise to different environments... As a rule of thumb, each new carbon atom or proton is a new environment, on the condition that symmetry isn't exhibited (more later)
'Surroundings' in this case is which carbons and protons are adjacent to particular protons or carbon atom...
Hence, if two parts of a molecule have the same 'surroundings' i.e. they both have the same groups adjacent to them, they're said to be of the same environment as they're chemically equivalent... This particularly arises when a molecule exhibits symmetry

For example, in both proton and carbon NMR, methane has one environment: there's just one carbon atom (hence one carbon NMR environment) and the four hydrogen atoms are adjacent to one equivalent carbon atom (hence one proton NMR environment)

The same can be said about ethane: this is a symmetrical molecule, so one CH3 group is adjacent to an equivalent CH3 group... Hence, only one environment is observed overall for both NMR methods

However, what about something like ethanol? This isn't symmetrical anymore, unlike ethane, as there's an OH group involved...
For carbon NMR, two environments will be observed: the CH3 group is adjacent to a CH2 group (hence a first carbon NMR environment), and the CH2 group is adjacent to both a CH3 group and an OH group (hence a second carbon NMR environment)
For proton NMR, three environments will be observed: the three CH3 protons are adjacent to two CH2 protons (hence a first proton NMR environment), the two CH2 protons are adjacent to three CH3 protons and an OH group (hence a second proton NMR environment) and the OH proton is just its own environment (hence a third proton NMR environment)

For practice you can search the NMR spectra for simple alkanes, predict what how many environments they may have, then have a look at the spectra
It all takes practice, and the problem with NMR is that at first it is difficult to understand because it's a very applied field of Chemistry...
However, after some practice (it won't take much!) you'll start to be able to look at molecules and read off their environments very easily
Last edited by Kian Stevens; 2 years ago
0
reply
Claisen
Badges: 14
Rep:
?
#5
Report 2 years ago
#5
(Original post by ma7cus)
Alright, what's the stuff at the bottom supposed to represent?
They are just some common splitting patterns that you will encounter. If you know them, you can deduce compounds from the spectra much quicker.
0
reply
ma7cus
Badges: 6
Rep:
?
#6
Report Thread starter 2 years ago
#6
Well that's about as comprehensive answer as I could ever have expected to get, thanks, that's really helpful
(Original post by Kian Stevens)
An 'environment' is just a magnetically, and so chemically, different part of a molecule
An environment is dependent on its surroundings, and so different surroundings give rise to different environments... As a rule of thumb, each new carbon atom or proton is a new environment, on the condition that symmetry isn't exhibited (more later)
'Surroundings' in this case is which carbons and protons are adjacent to particular protons or carbon atom...
Hence, if two parts of a molecule have the same 'surroundings' i.e. they both have the same carbon/proton groups adjacent to them, they're said to be of the same environment as they're chemically equivalent... This particularly arises when a molecule exhibits symmetry

For example, in both proton and carbon NMR, methane has one environment: there's just one carbon atom (hence one carbon NMR environment) and the four hydrogen atoms are adjacent to one equivalent carbon atom (hence one proton NMR environment)

The same can be said about ethane: this is a symmetrical molecule, so one CH3 group is adjacent to an equivalent CH3 group... Hence, only one environment is observed overall for both NMR methods

However, what about something like ethanol? This isn't symmetrical anymore, unlike ethane, as there's an OH group involved...
For carbon NMR, two environments will be observed: the CH3 group is adjacent to a CH2 group (hence a first carbon NMR environment), and the CH2 group is adjacent to both a CH3 group and an OH group (hence a second carbon NMR environment)
For proton NMR, three environments will be observed: the three CH3 protons are adjacent to two CH2 protons (hence a first proton NMR environment), the two CH2 protons are adjacent to a CH3 group and an OH group (hence a second proton NMR environment) and the OH proton is just its own environment (hence a third proton NMR environment)

For practice you can search the NMR spectra for simple alkanes, predict what how many environments they may have, then have a look at the spectra
It all takes practice, and the problem with NMR is that at first it is difficult to understand because it's a very applied field of Chemistry...
However, after some practice (it won't take much!) you'll start to be able to look at molecules and read off their environments very easily
0
reply
ma7cus
Badges: 6
Rep:
?
#7
Report Thread starter 2 years ago
#7
Alright, cool, I'll have a look at them then, thanks for the help
(Original post by Acetyl)
They are just some common splitting patterns that you will encounter. If you know them, you can deduce compounds from the spectra much quicker.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Should the school day be extended to help students catch up?

Yes (21)
30.43%
No (48)
69.57%

Watched Threads

View All