NMR spectroscopy

I know that the number of peaks correspond to the number of different chemical environments a hydrogen nuclei can be in - but how do you know what is a different chemical environment? Is it just influenced by the atom the Hydrogen is immediately attached to, or to the next group as well?

For example, the molecule CH₃CH₂CH₂CH=CH₂ apparently has 5 peaks - but I thought it would be only 4, the hydrogens on the end CH3, the hydrogens in the CH2 groups, the hydrogen on the double bond C and the two hydrogens on the end double bond C. Or are the two CH2 groups different?

Sorry if that was confusing!

Reply 1

oliverkeers13

I believe that the hydrogen is affected (as a rule of thumb) by anything within 3 bonds of it in any direction.

Reply 2

EierVonSatan

21

Excalibur

I know that the number of peaks correspond to the number of different chemical environments a hydrogen nuclei can be in - but how do you know what is a different chemical environment? Is it just influenced by the atom the Hydrogen is immediately attached to, or to the next group as well?

For example, the molecule CH₃CH₂CH₂CH=CH₂ apparently has 5 peaks - but I thought it would be only 4, the hydrogens on the end CH3, the hydrogens in the CH2 groups, the hydrogen on the double bond C and the two hydrogens on the end double bond C. Or are the two CH2 groups different?

Sorry if that was confusing!

Differenet environment means, any difference in the groups that it is next to directly or indirectly so for CH₃CH₂CH₂CH=CH₂ there is no symmetry in the molecule so yes the CH₂ are different (albeit similar environments)

Reply 3

Excalibur

OP

15

EierVonSatan

Differenet environment means, any difference in the groups that it is next to directly or indirectly so for CH₃CH₂CH₂CH=CH₂ there is no symmetry in the molecule so yes the CH₂ are different (albeit similar environments)

OK - so it's influenced by the groups on either side of the carbon? So for the first CH2 it's next to CH3 and CH2, whereas the second one is next to CH2 and a C=H group, hence why it's different?

Reply 4

oliverkeers13

Well, in truth. The hydrogens are only in identical environments if they're in precisely the same situation. for example, if there are two identical side chains from phenol in the 3 and 5 positions, the environments down them will be the same, but if they're in (say) 2 and 5, they won't.
My earlier response was in relation to the number of peaks inside a group, in High Res. NMR, I imagine you're talking about low res., involving area of peaks for a given environment instead of number of peaks?

Reply 5

EierVonSatan

21

Excalibur

OK - so it's influenced by the groups on either side of the carbon? So for the first CH2 it's next to CH3 and CH2, whereas the second one is next to CH2 and a C=H group, hence why it's different?

exactly

Reply 6

Excalibur

OP

15

Thanks to you both :smile:

Yes, I am talking about low res NMR. So basically you'd consider the groups either side of the carbon (which the H is bonded to), and if they're the same, they're more or less in similar chem environments. That's probably a simplification anyway, but it's A-level, so.... :p:

Reply 7

EierVonSatan

21

Excalibur

Thanks to you both :smile:

Yes, I am talking about low res NMR. So basically you'd consider the groups either side of the carbon (which the H is bonded to), and if they're the same, they're more or less in similar chem environments. That's probably a simplification anyway, but it's A-level, so.... :p:

Essentially yes, so in CH3CH2OCH2Cl the two CH2 groups are different, so expect 3 different signals (for bold hydrogens);

CH3CH2OCH2Cl = triplet
CH3CH2OCH2Cl = quartet
CH3CH2OCH2Cl = singlet

Reply 8

georgeElsworth

If you had a carboxylic acid which also had an alcohol group on another carbon, would the 2 OH groups be 2 differnt envioronments?

Reply 9

EierVonSatan

21

georgeElsworth

If you had a carboxylic acid which also had an alcohol group on another carbon, would the 2 OH groups be 2 differnt envioronments?

yes very different in fact

Reply 10

georgeElsworth

Right, can someone tell me how many peaks glycine will have and how many peaks serine will have (amino acids)..

Reply 11

EierVonSatan

21

for proton nmr glycine has 3 and serine has 5 :smile:

Reply 12

georgeElsworth

yay, was in my unit 6 synoptic, cheers for that

Reply 13

Golden_Gloves

georgeElsworth

yay, was in my unit 6 synoptic, cheers for that

Did you pick 'Y' , the 2nd graph?

yes i did!

http://eiewebvip.edexcel.org.uk/Reports/Confidential%20Documents/0806/6256_01_que_20080619.pdf

i think one of you chemistry genious should make us a dummed down mark scheme for this :biggrin:

Reply 16

EierVonSatan

21

georgeElsworth

http://eiewebvip.edexcel.org.uk/Reports/Confidential%20Documents/0806/6256_01_que_20080619.pdf

i think one of you chemistry genious should make us a dummed down mark scheme for this :biggrin:

lol sure you going to pay me? (i was a specialist marker for edexcel last year :wink:

)

Reply 17

aster100

Excalibur

Thanks to you both :smile:

Yes, I am talking about low res NMR. So basically you'd consider the groups either side of the carbon (which the H is bonded to), and if they're the same, they're more or less in similar chem environments. That's probably a simplification anyway, but it's A-level, so.... :p: