5
$\begingroup$

I am trying to find the structure of an unknown molecule. What is the splitting pattern of these sets of lines?

enter image description here

I think my unknown molecule is butanol and the hydrogens this splitting pattern is referring to are the middle ones, i.e. $\ce{H3C-CH2-C\color{\red}{\mathbf{H}}_2-CH2-OH}$. They would couple with the hydrogens on the right as a triplet and the ones on the left as a triplet, but would they also maybe do a W-coupling and couple with the methyl on the left.

So what would the coupling pattern be?

$\endgroup$
  • $\begingroup$ I encourage you to include the numeric positions of the peaks, the next time you have a similar question. It will make things easier to determine. $\endgroup$ – Martin - マーチン Oct 14 '14 at 7:18
7
$\begingroup$

This is the a peak from one of the vinyl protons on an allyl group. This splitting pattern is an ABX pattern from the bold hydrogen below. It is technically a triplet of doublets of doublets.

$$\ce{R-CH2-C}\textbf{H}\ce{=CH2}$$

Check out the following simulated NMR spectrum of 3-chloro-1-propene (allyl chloride) from nmrdb.org enter image description here

Now image if the coupling constants are a little different and the structure collapses into what you have.

|improve this answer|||||
$\endgroup$
  • $\begingroup$ cis and trans coupling constants in vinylic systems are usually different. OTOH, the chemical shift is distinctly vinylic. $\endgroup$ – Abel Friedman Oct 13 '14 at 21:46
4
$\begingroup$

This is either a quartet of triplets or quartet of doublets of doublets, with both doublets of equal magnitude. It is most definitely not butanol. The pattern you see is from a double bond, and shows 3J coupling to a methyl group. I suspect you have another set of signals near this at about 5.5, and probably a methyl signal near 2ppm.

Generally, W-coupling (or M-coupling) is only observed in rigid systems, and is typically a 4J phenomenon, although can occur over longer bond orders if the bond overlap allows.

|improve this answer|||||
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.