[Messi]

Hi guys,

I did a reaction between benzylamine and heptyne to form N-benzylheptanethioamide.

I then did a column, and got three different products and took an NMR of all three. I am pretty sure that one of my NMR is my actual desired product (N-benzylheptanethioamide). I have attached NMR below.

Could you guys let me know if my NMR shown below is my actual product... the integration and chemical shifts are driving me nuts... and I'm not 100% sure it is my actual product... Any help would be GREATLY appreciated!

[Babcock_Hall]

It does not seem to me that your coupling patterns agree with your assignments.  Also, I was surprised when I looked up the chemical shifts of a couple of compounds containing a C=S (carbon-sulfur double bond).  Do you have some good models to estimate chemical shifts?

[Messi]

It does not seem to me that your coupling patterns agree with your assignments.  Also, I was surprised when I looked up the chemical shifts of a couple of compounds containing a C=S (carbon-sulfur double bond).  Do you have some good models to estimate chemical shifts?

I got that spectrum from my NMR machine. I used chemdraw as a model to give me an estimate of what my spectra should look like.

Are you saying my spectrum does not look like my thioamide?

[Babcock_Hall]

Setting aside chemical shifts for a moment, I would be inclined to assign my doublet to the benzylic hydrogens (A) being split by the amide hydrogen.  And the apparent triplet just might be the hydrogens that are alpha to the thioamide (H).

Now we have to worry about chemical shift.  I like to use Hans Reich's site at the University of Wisconsin and also the spectral database of organic compounds.  If one compares propionamide with thiopropionamide, the -CH2- group moves downfield from 2.24 ppm in the former to 2.695 in the latter.  In other words, your peak at 3.8 ppm has approximately the right shift for the thioamide group.  The benzylic hydrogens of benzyl acetamide have a chemical shift of 4.35 ppm, whereas you have a chemical shift of 4.8 ppm and an integral that is larger than the signal at 3.8.  One would need a good model, but I find the difference of about .40-.45 ppm surprising (unless the thio substitution also affects the hydrogen atoms adjacent to the nitrogen in the same way it affects the ones adjacent to carbon.

I would say that some of the data agree with your proposed structure, but not all of the data agree.

thiopropionamide:  SDBS No.: 5289
benzylacetamide:  SDBS No.: 6378

[discodermolide]

Some of your compound may be there. But THF is not the signal at 0ppm. What do the NMR's of the other two products look like?

[Messi]

Some of your compound may be there. But THF is not the signal at 0ppm. What do the NMR's of the other two products look like?

I meant to put TMS, not THF

[discodermolide]

What do the NMR's of the other two products look like?

I also think that thioamides can exist as an equilibrium mixture of tautomers, which will complicate things in the NMR.
Thioketones like to exist as the thioenol form.

[Messi]

Setting aside chemical shifts for a moment, I would be inclined to assign my doublet to the benzylic hydrogens (A) being split by the amide hydrogen.  And the apparent triplet just might be the hydrogens that are alpha to the thioamide (H).

Now we have to worry about chemical shift.  I like to use Hans Reich's site at the University of Wisconsin and also the spectral database of organic compounds.  If one compares propionamide with thiopropionamide, the -CH2- group moves downfield from 2.24 ppm in the former to 2.695 in the latter.  In other words, your peak at 3.8 ppm has approximately the right shift for the thioamide group.  The benzylic hydrogens of benzyl acetamide have a chemical shift of 4.35 ppm, whereas you have a chemical shift of 4.8 ppm and an integral that is larger than the signal at 3.8.  One would need a good model, but I find the difference of about .40-.45 ppm surprising (unless the thio substitution also affects the hydrogen atoms adjacent to the nitrogen in the same way it affects the ones adjacent to carbon.

I would say that some of the data agree with your proposed structure, but not all of the data agree.

thiopropionamide:  SDBS No.: 5289
benzylacetamide:  SDBS No.: 6378

Yes, Babcock, the biggest thing that is confusing me are my integral values.. they make absolutely no sense at all!

[Messi]

What do the NMR's of the other two products look like?

I just scanned them and attached them.

Thanks for willing to look at them! Much apperciated!

[discodermolide]

I think you may have some of the following compounds in the other two NMR's (see picture). As well as the amide rotamers.

[Messi]

May I ask how you know this?

I do tend to agree with you however!

Are you with me though that my original spectra for my compound is a little funny because my integrations don't match up?

[discodermolide]

Yes the original spectrum looks like a mixture as well.
As a general point, when I was running NMR's I always tried to extract as much info. as possible by expanding areas of the spectrum so I could better see the signals. You could do this by expanding the regions between the aromatic protons and the alkyl groups. You will then get a better view of the very small signals in this region.

How do I know this? Well I did work with thioketones years ago, and saw the effects in that they like to exist as the thio enol form. I know you have an amide on the other side, but I think the thio ketone is maybe more relevant in the structure. Just my opinion, I may be wrong.
Perhaps someone else will comment on this.

[Messi]

Excellent!

Thank you kindly discodermolide! Mole cookie here you go!

[OC pro]

The first spec could be the thioamide. Of course, these compounds tend to enolize. You can try also a different solvent for NMR, like DMSO or benzene to get more information. You should confirm the structure by mass-spec.

[Messi]

Great idea OC pro!