Chemistry - chromatography and spectroscopy

Can anybody help explain! Im stuck with trying to figure out the compound with NMR and IR for questions 3c,d and 5e,f

The NMR shows you CH3CH2
but the empirical formula gives you too few hydrogen atoms,
therefore the molecular formula = C4H8O2, which adds up to the 88 shown
Now remove CH3CH2 and you are left with COO and CH3
All you have to decide is which way round the ester is:
CH3CH2COOCH3
or
CH3CH2OOCCH3

I'll leave that up to you...

Thank you so much - I see it clearly now!

Could you still help explain the other attached question (the one with the 2 IR spectras above)- This one I'm finding challenging.

I get that they're telling me Mr=48, and for part c, there are 3 carbon environments, and after it's refluxed, it becomes a carboxylic acid so it must be a primary alcohol, but the only formulas I can come up with to have 3 carbon environments and and alcohol give me an Mr larger than 48.

and for part d, its telling me there are 2 carbon environments, and that it is a secondary alcohol because when refluxed a ketone is made, but again the only formulas I can come up with don't satisfy all the requirements.

Thank you so much - I see it clearly now!

Could you still help explain the other attached question (the one with the 2 IR spectras above)- This one I'm finding challenging.

I get that they're telling me Mr=48, and for part c, there are 3 carbon environments, and after it's refluxed, it becomes a carboxylic acid so it must be a primary alcohol, but the only formulas I can come up with to have 3 carbon environments and and alcohol give me an Mr larger than 48.

and for part d, its telling me there are 2 carbon environments, and that it is a secondary alcohol because when refluxed a ketone is made, but again the only formulas I can come up with don't satisfy all the requirements.

You are correct in saying that the information given is not possible.
3 carbon environments has at least 3 carbon atoms, Mr = 36
1 OH environment has at least 1 oxygen atom, Mr = 16
---------------------------------------------------------------------------------- total
minimum Mr = 52

I strongly suspect a misprint and that the Mr is not 48

CnH(2n+2)O is the empirical formula of an alcohol.
If n=3 this gives an Mr = 60
If n=4 this gives an Mr = 74
If n=5 this gives an Mr = 88

The second spectrum is a secondary alcohol with only two carbon environments. This could be propan-2-ol, or pentan-3-ol (or other more complex structures, such as alicyclic alcohols, cyclobutanol for example)

If the Mr is 88 then spectrum 3 could be due to methylpropan-1-ol, (CH₃)₂CHCH₂OH and spectrum 4 due to pentan-3-ol, CH₃CH₂CHOHCH₂CH₃

for spectrum 3 would it not be 2-dimethylpropan-1-ol ? as it would have to have 5 carbons?

And I'm assuming for spectrum 4 with pentan-3-ol, we don't count the carbon attached to the OH as it's own environment? because then it would be 3

Thanks for explaining it all!

Hi again Larissa.
Yes, you are correct - I was rather hurried anddid not check my answer - a classic error!

(c) Three environments for carbon and a primary alcohol must be: (CH₃)₃CCH₂OH (Mr = 88)
not the skeletal formula, but you can do that.

(CH₃)₂CHCH₂OH also fits (Mr = 74)


(d) With two peaks in the 13C NMR and a relative mass of 88 it is actually not possible, UNLESS the specification suggests that CH3 and CH2 groups have the same environment (which of course they don't). I think that they are expecting pentan-3-ol.

Propan-2-ol has only two 13C NMR peaks, but of course the Mr = 60

Nothing fits with a Mr = 48

Conclusion
---------------
Either the question is deeply flawed or there is something that I am just not seeing. I'll continue thinking, but with little hope of success.





es,