Ochem q from DAT achiever

[Dentist2be]

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Here is the question:

Q. Describe the 1H NMR spectrum for p-xylene (it's 1,4-dimethylbenzene).

A. Two singlets and four doublets
B. One singlet and four doublets
C. One singlet and one doublet
D. Two doublets
E. Two singlets

I thought the answer was A but it was actually E. Why are ring protons not counted? In the explanation, it says "The ring protons, on the other hand, have their signals superimposed on the 200 MHz 1H NMR spectrum, and as such do not give rise to any supposed splitting". Could any of you please elaborate this?

Thanks for your help! 🙂

 

[GoDucks69]

I think all of the protons on the ring are equivalent...so one signal

 

[Envision]

well, there is 1 signal produced by the 6 protons off of the 2 methyl groups, they are all equvilent producing your first singlet. and then there is the 4 on the benzene ring, these are also all equvilent making ur second singlet. remember that if they are equivilent, they produce only one signal. hope that helps

 

[issa]

this question is very easy. i never counted the hydrogen for the aromatic ring as a signal. if you practice more you will get every question about nmr correct.

 

[xxxTheDonxxx]

this question is very easy. i never counted the hydrogen for the aromatic ring as a signal. if you practice more you will get every question about nmr correct.

Your technique might not work everytime. Benzenes with methyl groups around it (at position 1,3), will produce more than one signal from its hydrogens around the aromatic ring because it's not symmetrical. I think this example is from dat achiever. I got one for my dat and it was real simple, look at 4 of them and see which has only 1 signal.
-you must understand why it produces a certain signal and what kind of signal it is (singlet, triplet, doublet) to be on the safe side.

 

[Envision]

Your technique might not work everytime. Benzenes with methyl groups around it (at position 1,3), will produce more than one signal from its hydrogens around the aromatic ring because it's not symmetrical. I think this example is from dat achiever. I got one for my dat and it was real simple, look at 4 of them and see which has only 1 signal.
-you must understand why it produces a certain signal and what kind of signal it is (singlet, triplet, doublet) to be on the safe side.

very good point. the problem the OP gave was conviniently symmetrical therefore, all 4 protons are equvilent. if it were at the 1,3 position, then certain hydrogens will be effected differently (for example the 2 H on the 2 position) and this would result in a different HNMR spectrum.

 

[Gasedo]

very good point. the problem the OP gave was conviniently symmetrical therefore, all 4 protons are equvilent. if it were at the 1,3 position, then certain hydrogens will be effected differently (for example the 2 H on the 2 position) and this would result in a different HNMR spectrum.

So, for Benzene with methyl groups around it (at position 1,3), will produce how many signals? Four?

 

[Envision]

So, for Benzene with methyl groups around it (at position 1,3), will produce how many signals? Four?

Ya, i believe there is 4 signals. oh ya, i made a mistake before when i said 2 hydrogens on the 2 position, there is only one hydrogen, sorry.

 

[Dentist2be]

Thanks much for your help guys! I think I've got it now.