[looza]

Hello, a new member here.


I was given the chemical formula of C6H10O, with its NMR and IR structures. From these spectrum, I deduced that its a cyclohexanone molecule. But the NMR spectrum shows only singlet signals for all three distinct signals. Aren't these signals supposed to be split by the adjacent hydrogens?
The NMR looks exactly like this:


http://www.google.com/imgres?imgurl=http://orgchem.colorado.edu/labquizzes/oxal/cyclohexanone.jpg&imgrefurl=http://orgchem.colorado.edu/labquizzes/oxal/oxalquiz.html&h=452&w=640&sz=14&tbnid=aTuZDca74xPUnM:&tbnh=97&tbnw=137&prev=/search%3Fq%3Dcyclohexanone%2BNMR%26tbm%3Disch%26tbo%3Du&zoom=1&q=cyclohexanone+NMR&hl=en&usg=__UPxsTlFYeynGW0MrucjcIt3vbVk=&sa=X&ei=P3uqTcXNNYPUtQPCs9H5DA&ved=0CD8Q9QEwBw

The IR looks like this:

http://www.google.com/imgres?imgurl=http://orgchem.colorado.edu/labquizzes/oxal/cyclohexanoneIR.gif&imgrefurl=http://orgchem.colorado.edu/labquizzes/oxal/oxalquizans.html&h=545&w=922&sz=44&tbnid=Ro9kryosxBzNEM:&tbnh=87&tbnw=147&prev=/search%3Fq%3Dcyclohexanone%2BIR%26tbm%3Disch%26tbo%3Du&zoom=1&q=cyclohexanone+IR&usg=__xHUgh0EDV4n-xdoREKMYVr4nPNg=&sa=X&ei=eXuqTYP6DJH6swO-psz5DA&ved=0CBoQ9QEwAQ

Again, both the IR and NMR spectrum tell that its a cyclohexanone. But why aren't there any splittings on the hydrogens?

Thank You.

[Honclbrif]

What are you talking about? The linked NMR spectrum definately shows splitting.

[looza]

I am sorry. I posted a wrong picture for the NMR. I couldn't find the right picture online. I guess I will just describe how the NMR looks like in the hard copy.

The chemical structure is C6H10O.

There are three signals for NMR: at 1.667, 1.799, and 2.272. All singlets, clear singlet lines.

I could be wrong in my determination. They could be some other molecule rather than cyclohexanone. If so, what molecule do you think it is?

The IR spectrum is exactly like the one I posted though.

[looza]

I was thinking about the effects of symmetry? Does symmetry cause these signals to come out as singlets?
Thanks.

[Honclbrif]

What's the field strength of the instrument the spec was acquired on? If it was an old 90 MHz instrument, then the reason you don't see splitting is because its a crappy instrument that lacks the power to resolve fine structures.

Nonaromatic cycles tend to give pretty bad looking 1H spectra because their protons are conformationally restricted, which places them in distinct electronic environments. In a cyclohexane, for instance, the axial and equatorial protons have different environments and will yield different chemical shift values. Furthermore, the axial and equatorial protons which share a carbon can couple and will exhibit splitting, as well as distinct splitting from the neighboring axial and equatorial protons. Combine this with ring flips, and you've a recipe for a really bad looking spectrum. An instrument with a weak magnet won't be able to resolve this splitting and they will just look like broad singlets.

[looza]

Thank You.

Unfortunately, no information was provided on the instrument. We were just provided with a hard copy of IR, NMR and unknown molecular formula. That was our "experiment" so to say.

As you predicted, the signals are relatively broad compared to other NMR spectrum I have seen.
Does symmetry have any effect on these? The ortho-ortho and meta-meta bonded hydrogen are expected to show up around the same field, right?

Also, there are 2 signals in close proximity, ie 1.667 and 1.799. I predicted that one signal was from the hydrogens bonded to ortho-carbons and other to meta carbons. My reasoning was that their proximity to the carbonyl groups further deshielded the ortho hydrogens. Am I on the right track?

[Honclbrif]

Ortho, meta, and para are only proper to use when referring to benzene derivatives. Do not use those terms when referring to cyclohexanes; use numbering instead. However, you seem to be on the right track. The closer something is to an electronegative group, the more deshielded it (usually*) becomes.

As for the symmetry question, since this molecule is symmetric, protons from one side of the plane of symmetry will be indistinguishable from their mirror image.

*Funny stuff can happen when ring currents get involved, but don't worry about that now.

[looza]

Thanks for the suggestions. i turned in the work today.