H1 Nmr

[osims]

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Wow this is really pissing me off, every practice test ive taken so far has asked an H1 NMR question and i really cant answer them. after reading through kaplans explanation, wikipedia's explanation, and some other online explanations i still dont understand how you obtain those freakin singlets, doublets, triplets, and so on. I know it has something to do with different hydrogens but i just dont get how to identify them. I would appreciate it if someone would help me out here with an easy explanation.

 

[dontwakeme]

You just count the different "types" of hydrogens in the compound i.e. hydrogens that have different connectivity. For example, methane would have one signal (peak) on the Proton NMR because there is only one type of hydrogen (all hydrogens are the same). Conversely, a compound such as Acetic Acid (CH3COOH) will have 2 Proton NMR signals (the methyl hydrogens and the hydrogen in the carboxyl group). This is pretty much the basics of it.

 

[osims]

alright that makes sense, but question 74 on topscore ochem test 2, they ask for how many peaks does m-xylene have on its H1 NMR. (m-xylene is basically benzene with 2 methyl substituents). The answer is 4 peaks: 2 singlets, one doublet, and 1 triplet. Now how did they come up with 4, because i can only identify 2 different H's. One from the methyl group and one from the hydrogen's on the benzene ring.

 

[Imperium]

The hydrogens on the benzene aren't the same. The H on C1 (let's say it's between the two methyl groups) is different from the H's on C3 and C4.

 

[osims]

so which one do you identify as a doublet triplet or even singlet?

 

[Imperium]

The two singlets are the H's on the methyl groups and C1 (because their adjacent C's don't have any H's attached, [n+1] rule). The doublet is the H on C3/C5 (again, because the adjacent C4 only has 1 H) and the triplet is C4 (C3 and C5 have 1H each, so total 2H, use the n+1 rule and you get the triplet). There's probably an easier way to explain this lol.

 

[osims]

haha this is so confusing, i hate NMR...but IR is cool haha