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Here's the problem: enter image description here

I've already know the 3600 cm$^{-1}$ line indicates the $\ce{-OH}$ group and the 1640 cm$^{-1}$ line indicates the C double-bond C alkene.

From the $^{13}$C NMR data: the two carbons with chemical shifts of 100+ are sp$^2$ carbons and the remaining two are sp$^3$ carbons.

I have tried to work out compound D, and suggest this structure: enter image description here

I am stuck on figuring out compound C. I suggest the structure below, but am unsure whether if it is correct. enter image description here

Any help is appreciated thanks!

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  • $\begingroup$ What makes you unsure about it? $\endgroup$ – pH13 - Yet another Philipp Aug 10 '15 at 12:12
  • $\begingroup$ @PH13 it's just the 1Hproton shifts that are confusing...especially for the Hb that I have labelled in compound C. I have interpreted 'qn' to mean 'quintet' but if it couples to 1x Ha and 3xHc, technically shouldn't it be a 'doublet of quartets'? $\endgroup$ – justbehappy Aug 10 '15 at 12:31
  • $\begingroup$ That's not a "normal" quintet and thus it should not be named like one ... it's rather a multiplet or as you say at high resolution it would probably be a quarted of a doublet (vice versa). The coupling constants are very close to each other and this makes it a quintet at low resolutions. $\endgroup$ – pH13 - Yet another Philipp Aug 10 '15 at 12:46
  • $\begingroup$ @PH13 ok! I see. Thanks for clarifying this. Do you also agree with my proposed structure for Compound D? $\endgroup$ – justbehappy Aug 10 '15 at 13:08
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Your two proposed structures are correct, although your rationalisation of the 13C spectrum for Cpd D is not quite correct around the double bond. In a nutshell, carbon chemical shifts can be calculated as a function of their α, β and γ contributions. α for what is directly bonded to that carbon, β for substituents one carbon away, and γ for substituents two carbons away. Almost without exception, alkene carbons exhibit a downfield β shift and upfield γ shift for substituted alkenes. So, for both compounds C and D, the carbon closest to the substituents will be shifted furthest downfield. The β and γ contributions for the following substituents are shown (someone more tech savvy than me might tidy this up) :

[Substituent; β ;γ] [-H; 0 ; 0] [-CH2O-; +14.2 ; -8.2] [-CH3 ; +9.0 ; -7.0]

The electronics contributing to β and γ shifts is not well understood, but is more complex than a consideration of just electronegativities/electron withdrawing potential.

Below are the spectra for the two compounds:

Cpd C

Cpd D

Your confusion with the description of the quintet label arises from how people report splitting patterns in the literature. I have discussed this on another question, but essentially the two methods of reporting splittings are to (a) report the observed splitting pattern (here a quintet) and (b) report the calculated/expected splitting pattern (here a doublet of quartets). As you can see, (or least I can), reporting the observed pattern can lead to some confusion, and gives no real information about how this splitting pattern arises.

Further, reporting couplings to the nearest Hz (here 6Hz) might be fine for some occasions where the difference in coupling for the two different partners is less or approximate to the observed linewidth, however, with a good sample and a good operator on a good magnet, what is reported as a quintet may very well appear different, especially once some apodization is applied. For example, below is a expansion of a simulated spectrum for compound C with Jbc=5.8 and Jbe=6.2 (smaller couplings from d and f ignored), and observed linewidth of 0.5Hz. The top spectrum is the normal spectrum, and the bottom spectrum is what it looks like with a gaussian linewidth function applied (gb 0.1, lb -1). They look quite different, and the bottom spectrum is very hard to rationalise as a quintet, but easily recognizable as a doublet of quartets.

enter image description here

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