EK Ochem #945

[drillers]

In the NMR spectrum of Morpholine (not Morphine), how many triplet peaks are present?
a. 1
b. 2
c. 3
d. 6

I have no picture, so I will refer to the wiki picture: http://en.wikipedia.org/wiki/Morpholine

Since there is a line of symmetry, only C2, C3 (as pictured in Wiki) and Nitrogen will have hydrogens that cause a peak: thus 3 peaks.

I got the right answer (B), but EK said the two triplets came from C2 and C3, whereas I had my two triplets coming from C2 and the Nitrogen.

C2: Triplet, because it "sees" C3 which has 2 adjacent non-chem. equivalent Hydrogen (2+1)

C3: Quartet, because it sees C2 which has 2 adjacent non-chem eq Hydrogens and Nitrogen has 1 (3+1)

N: Triplet, because it sees C3 which has 2 adjacent non-chem eq Hydrogens (2+1)


Is it because of the lone pair on Nitrogen as to why I am incorrect? Maybe the lone pair technically makes C3 and Nitrogen not adjacent to each other:

N: singlet
C3: triplet
C2: triplet

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[IcemanMD]

Shouldn't N be a quintet because it sees 2H from C3 and 2H from C5?

Regarding the original question, I agree with two triplets but I think they come from C2 and C6 (referring to the wiki) rather than C2 and C3. C3 should be a quartet should it not?

 

[FeinMS]

NH and OH doesn't usually couple -> usually singlet

 

[ashtonjam]

NH and OH doesn't usually couple -> usually singlet

I thought this was the case but Berkeley Review seems to make NH protons couple with nearby hydrogens. Other sources seem to contradict it.

 

[drillers]

I don't think so because of the line of symmetry... you would disregard C6, C5 (depending on which side you want to disregard)

 

[sps27]

How do you recognize equivalent hydrogen's? I can figure out simple straight chain structures but when it comes to rings, it becomes complicated. Is there a rule book here? Thanks for the help.

 

[young_gohard]

For anyone else having trouble with this question: as I understand it, the hydrogens in N-H and O-H bonds don't usually count in splitting (so they're usually singlets and you also don't factor them in when counting the neighboring hydrogens of a carbon).

They break down the NMR of morpholine here: http://orgchem.colorado.edu/Spectroscopy/specttutor/ex18.html

The concept is also explained a bit here: ("However, coupling is almost always lost on hydrogens bound to heteroatoms (OH and NH). The lack of communication between an OH or NH and its neighbours is related to rapid proton transfer, in which that proton can trade places with another OH or NH in solution.")

And here:

("The lack of splitting with -OH groups

Unless the alcohol is absolutely free of any water, the hydrogen on the -OH group and any hydrogens on the next door carbon don't interact to produce any splitting. The -OH peak is a singlet and you don't have to worry about its effect on the next door hydrogens.")