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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?

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  • $\begingroup$ 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 ♦
    Commented Oct 14, 2014 at 7:18

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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.

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  • $\begingroup$ cis and trans coupling constants in vinylic systems are usually different. OTOH, the chemical shift is distinctly vinylic. $\endgroup$
    – Abel Friedman
    Commented Oct 13, 2014 at 21:46
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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.

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