# HNMR Assignment

I have a molecule

and i need to assign the hnmr. I have a printed spectrum

chemical shift_______splitting pattern_______coupling constant

 2.32                quartet            6.6 Hz & 13.2 Hz

2.66                singlet                   -

3.66                triplet                6.5 Hz

5.11         triplet of doublets         1.5 Hz & 16.8 Hz

5.81   doublet of doublet of triplets   6.9 Hz & 10.2 Hz & 13.8 Hz & 17.2 Hz


I am having trouble assigning them to the hydrogens. I think the H on the OH is a singlet and the H on the right hand side of the double bond is a ddt as it has cis (13.8 Hz) and trans (17.2 Hz). Then on the left of the double bond, the cis H would be the quartet as it has cis coupling (13.2) and the trans H would be the td as it has trans coupling (16.8 Hz).

But if these are correct then does that mean the four hydrogens on carbons 2 and three are the triplet group?

You haven't quite rationailsed the spectrum correctly; If you look at your molecule you will see that you have 6 distinct 1H species, but you have only described 5. Take particular note of the signals that you have described as a td at 5.11ppm. This should be symmetrical about the centre peak if it was a true td, and this signal clearly is not. So, it actually comes from two overlapping signals; one with a large coupling (~17Hz) and the other a medium coupling (~10Hz). Also, a quartet will only have one coupling constant - this will be the separation of any of two adjacent peaks in the quartet, and so is 6.6Hz, as you have described. So perhaps this is not a true quartet.

When assigning your peaks, you should first look at your chemical shifts;

• Vinylic protons typically come in the range 3.5-8.5, but commonly 5-7ppm
• alkyl protons alpha to a heteroatom 3-4.5ppm
• alkyl protons alpha to vinyl group 1.5-2.5ppm
• hydroxyl protons can come just about anywhere, but true alkyl-OH 1H will typically come 1-5 ppm

So from this information, you should be able to broadly assign the 6 protons in your molecule.

You then need to look at coupling constants to further refine your assignments. For vinylic groups, you can rely on the following general coupling constants:

• trans coupling Jbc 14-19Hz
• cis coupling Jac 4-12 Hz
• geminal coupling Jab -4-4 Hz
• vicinal coupling Jcd 4-10 Hz

This should allow you to complete the assignment of this spectrum. See how you go from here.

A useful approach to these type of assignment problems, especially if you know the structure, is to look at the molecule and try to predict the spectrum you'd expect, rather than looking at the spectrum and working out what's what.

Looking at your molecule, I'd expect:

• Ha to come in the vinylic region, and appear as a dd (cis and geminal coupling to Hb and Hc)
• Hb to come in the vinylic region, and appear as a dd (trans and geminal coupling to Ha and Hc)
• Hc to come in the vinylic region, and appear as a ddt (cis, trans and vicinal coupling to Ha, Hb and Hd)
• Hd to come in the alkyl region alpha to a double bond, and appear as a dt (coupling to Hc and He)
• He to come in the alkyl region alpha to a heteroatom, and appear as a t (coupling to Hd)
• Hf to come somewhere and appear most likely as an exchange broadened singlet

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