Nmr help!

abcdefg1990

Hi, I'm having trouble getting my head around nmr.

I know the adjacent carbon splits the neighboring with the n + 1 rule, but which adjacent carbon does the splitting if a carbon is attached to more that one other carbon? Also if a carbon is attached to two other carbons both with the same proton environment e.g the CH2 in CH3CH2CH3, how many splits are there?

thanks

Reply 1

EierVonSatan

21

Propane will give you two signals:

The CH3's (both identical) see the CH2 => triplet
the CH2 see both identical CH3's => septet

Reply 2

abcdefg1990

OP

2

EierVonSatan

Propane will give you two signals:

The CH3's (both identical) see the CH2 => triplet
the CH2 see both identical CH3's => septet

oh, so if a carbon is attatched to two identical groups, it's peak is split into the total number of protons of those groups plus one.But what if the molecule is CH3CH2CH2CH3. would it have two peaks with the CH2 proton peak split into quartet and the CH3 proton peak split into triplet?

Reply 3

EierVonSatan

21

abcdefg1990

oh, so if a carbon is attatched to two identical groups, it's peak is split into the total number of protons of those groups plus one.But what if the molecule is CH3CH2CH2CH3. would it hav two peaks with the CH2 proton peak split into quartet and the CH3 proton peak split into triplet?

Yes, exactly :smile:

Reply 4

abcdefg1990

OP

2

EierVonSatan

Yes, exactly :smile:

cool. thanks :biggrin:

One last question, if you have for example CH3CH2CHO, im guessing the CH2 proton peak splits into quartet because the adjacent carbon has 3 protons, but why doesn't it split into doublet, because the adjacent carbon on the other side only has one proton?

Reply 5

EierVonSatan

21

abcdefg1990

cool. thanks :biggrin:

One last question, if you have for example CH3CH2CHO, im guessing the CH2 proton peak splits into quartet because the adjacent carbon has 3 protons, but why doesn't it split into doublet, because the adjacent carbon on the other side only has one proton?

Yeah, this is a slightly more complicated case - I'd assign them as follows:

CH3 => triplet
CH2 => quintet (5 peaks)
CHO => triplet

The problem with this is that the CHO and CH3 are quite different in terms of environment, so what would actually happen is that you would get something called a 'doublet of triplets' but this is beyond the scope of A-level :smile:

Reply 6

abcdefg1990

OP

2

EierVonSatan

Yeah, this is a slightly more complicated case - I'd assign them as follows:

CH3 => triplet
CH2 => quintet (5 peaks)
CHO => triplet

The problem with this is that the CHO and CH3 are quite different in terms of environment, so what would actually happen is that you would get something called a 'doublet of triplets' but this is beyond the scope of A-level :smile:

So CH2 splits into 5 peaks :confused:

. Does that mean when the adjacent carbons have different proton environments you just add up the number of protons on all adjacent carbons and plus one, when working out the number of splits?

Reply 7

abcdefg1990

OP

2

anyone...

Reply 8

abcdefg1990

OP

2

so when working out the splitting pattern on a peak you count the number of protons on all adjacent carbons and the add one.

so in the case of butanoic acid CH3CH2CH2COOH, the peak from the CH3 will be split into triplet, the CH2 group attatched to the CH3 group peak will be split into quartet and the CH2 group attatched to COOH will be split into triplet?

there will also be a singlet peak from the labile proton.

Reply 9

abcdefg1990

OP

2

please correct me if I'm wrong.

Reply 10

EierVonSatan

21

abcdefg1990

so when working out the splitting pattern on a peak you count the number of protons on all adjacent carbons and the add one.

Yeah, thats the n+1 rule

so in the case of butanoic acid CH3CH2CH2COOH, the peak from the CH3 will be split into triplet, the CH2 group attatched to the CH3 group peak will be split into quartet and the CH2 group attatched to COOH will be split into triplet?

there will also be a singlet peak from the labile proton.

CH3 will be split into a triplet as you said (CH2 on the right 2+1 = 3)
CH2 will be a sextet (CH3 on the left and CH2 on the right 5+1 = 6)
CH2 will be a a triplet (CH2 on the right, nothing on the left 2+1 = 3)
The COOH will have a singlet

:smile:

Reply 11

shungun

1

but its different for low nmr, in low nmr it is the relative area under the graph...right?

Reply 12

EierVonSatan

21

shungun

but its different for low nmr, in low nmr it is the relative area under the graph...right?

Low res. NMR won't give you the spliting at all, both low and high res. can get you the integration.

Aside: Low res. NMR basically doesn't exist anymore (the waekest NMR machine I've seen is a 250MHz), I'm lost as to why its mentioned at A-level :shifty:

Reply 13

shungun

1

EierVonSatan

Low res. NMR won't give you the spliting at all, both low and high res. can get you the integration.

Aside: Low res. NMR basically doesn't exist anymore (the waekest NMR machine I've seen is a 250MHz), I'm lost as to why its mentioned at A-level :shifty:

yes thats what i meant, it wont give you splitting but it is related to the area under the graph...true low NMR is pretty useless today but i guess it is tested because it is easier to grasp than the high NMR?
well consider this, you can use kekule structure for benzene in the exams :eek:

Reply 14

EierVonSatan

21

shungun

yes thats what i meant, it wont give you splitting but it is related to the area under the graph...true low NMR is pretty useless today but i guess it is tested because it is easier to grasp than the high NMR?

Yeah, I guess that makes sense...

well consider this, you can use kekule structure for benzene in the exams :eek:

We do away with the delocalised version at university anyway (though we realise that it is delocalised) :p:

Reply 15

shungun

1

LOL...even at university :eek:

? haha so do you guys put like alternating double bonds? i personally find it awkward to put 3 double bonds...prefer the circle...and that seems contradicting...

Reply 16

abcdefg1990

OP

2

EierVonSatan

Yeah, thats the n+1 rule

CH3 will be split into a triplet as you said (CH2 on the right 2+1 = 3)
CH2 will be a sextet (CH3 on the left and CH2 on the right 5+1 = 6)
CH2 will be a a triplet (CH2 on the right, nothing on the left 2+1 = 3)
The COOH will have a singlet

:smile:

ok. but with butane CH3CH2CH2CH3 doesn't that mean that there will be two peaks the CH3 peak split into triplet and the CH2 split into sextet? :confused:

Reply 17

EierVonSatan

21

shungun

LOL...even at university :eek:

? haha so do you guys put like alternating double bonds? i personally find it awkward to put 3 double bonds...prefer the circle...and that seems contradicting...

Yeah, it makes more sense when trying to draw mechanisms

abcdefg1990

ok. but with butane CH3CH2CH2CH3 doesn't that mean that there will be two peaks the CH3 peak split into triplet and the CH2 split into sextet? :confused: