NMR spectroscopy - structure elucidation

Question

What are the possible structures for molecular formula C3H8Cl That shows one doublet And one septet In proton NMR spectrum?

Answer 1

First of all, you got the chemical formula wrong. Its C3H7Cl.

Here's how you approach an NMR problem when given the molecular formula:

First, determine the index of hydrogen deficiency (IHD), which tells you how many rings or double bonds you have. All you're doing here is figuring out how many hydrogens are missing from a fully saturated compound, which tells you the number of rings/double bonds in the compound. For every 2 missing hydrogens, the compound has one ring/double bond.

Remember that an alkane has (2 x (number of carbons)) + 2 hydrogens. This isn't the end of the story though. Heteroatoms (elements other than carbon and hydrogen affect the number of hydrogens we expect to be present. Just remember these rules:

A halogen (anything in fluorine's column) means you expect one fewer hydrogen

A chalcogen (anything in oxygen's column) does not change the number of hydrogens you expect

A pnictogen (anything in nitrogen's column) means you expect one more hydrogen

So now we'll look at the compound we're talking about. Taking the rule above, for three carbons we would expect (3*2)+2=8 hydrogens. However, We have a chlorine atom, which is a halogen, so we would expect one fewer hydrogen (so 7 total). We have 7 in our compound, so there are no rings or double bonds in our compound.

Now we get into analysis of the NMR spectrum. For each peak we care about three things; the integral, the multiplicity, and the chemical shift.

The integral tells us how many hydrogens are contributing to the peak. Your question doesn't specify but in this case the septet integrates for one and the doublet integrates for six, meaning the septet represents one hydrogen and the doublet represents six.

The multiplicity tells us how many hydrogens are within three bonds of the hydrogen represented by a peak. A singlet means no hydrogens within three bonds, a doublet means 1 hydrogen within three bonds, etc..So the doublet that integrates for 6 means those six hydrogens are within three bonds of one hydrogen. The septet integrating for one would mean that one hydrogen is within three bonds of six hydrogens.

Finally, the chemical shift tells us what kind of environment the hydrogen is in (there is a table, look it up and learn it). For example, if a hydrogen is attached to the same carbon as a halogen, we expect it to be further downfield (the chemical shift will be a bigger number. In this case the septet would be further downfield than the doublet.

So with all this we can figure out the structure is 2-chloropropane. The one hydrogen on the middle carbon is within three bonds of the methyl hydrogens for a total of 6. You may ask why the two methyl group's hydrogens are represented by the same peak. The answer is they are chemically identical.

NMR does eventually get more complex than this but this should be all you need for now.

If you have any questions or want some practice problems feel free to get in touch.