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NMR spectroscopy question please help!

Hi can someone please help me with a question I am stuck on. I will add it to this tread! Thank you.
Reply 1
Quite what are you stuck on?

I suggest that we don't just do it for you.
Original post by jadz91
Hi can someone please help me with a question I am stuck on. I will add it to this tread! Thank you.

These questions can be difficult, but there are certain 'tricks' that can help you solve them quickly. You should know how to work out the empirical formula from the given elemental analysis. Then, they've given you the molecular ion peak which indicates that this compound has an Mr. of 102.
These two pieces of data will allow you to determine the molecular formula.

Now onto the IR spec. What absorptions are present? Equally, what absorptions are absent? From this spectrum I can immediately rule out two functional groups that contain an oxygen. The absorption at 1750cm-1 is rather important. I would say that the three peaks at 1750, 1250 and 1100 cm-1 are very characteristic of this type of compound. If you see three long thin 'fingers' at these points in the spectrum the compound is an ester (see what I did there?).

As you can see, the IR has pretty much told us what type of compound we're dealing with. The NMR has only three peaks. A doublet integrating for six protons, and a multiplet integrating for one proves the presence of a particular group. Try and draw out this part of the molecule. The singlet integrating for three protons is also characteristic.

The hint that the peak at 4.9ppm would normally be at 3.9ppm is useful, but you should already have enough imformation to draw the molecule. Peaks shift downfield when the protons are near an electronegative element or anything else that can withdraw electron density by any means such as benzene rings or double bonds that are stabilized by resonance (downfield=deshielded).

The biggest mistake, in my opinion, is to spend ages on the chemical shift numbers trying to find an exact match. These are only ranges of numbers after all. First look at the integration, and then the splitting. The chemical shift can then be used to double check the structure.
(edited 1 year ago)

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