TBR Organic Chemistry H NMR Example 2.26

[CookieZine]

What is the common name of the compound represented by the following 1H NMR spectrum, whose molecular formula is C8H10O2?

A. Para-ethoxy phenol (H3CH2COC6H4OH)
B. Ortho-ethoxy phenol (H3CH2COC6H4OH)
C. Para-methoxy anisole (H3COC6H4OCH3)
D. Ortho-methoxy anisole (H3COC6H4OCH3)

The answer solution states that the symmetry in the 1H NMR spectrum is associated with a structure that is also symmetric. How exactly is the symmetry in the NMR seen? The 2 hydrogen singlet appears to have a chemical shift that is twice that of the 3 hydrogen singlet. Is this what they are talking about? How do you know if a NMR spectrum is symmetrical and is this always associated with a structure that is symmetric?

Can anyone tell me if the unique carbons are correctly labeled for para-ethoxy phenol? I count 5 unique hydrogens so we should expect 5 signals in a 1H NMR spectrum. Is my thinking correct?

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[sillyjoe]

The answer to the question is C not A. Draw out A and you will see the symmetry as compared to para-ethoxy phenol.

Here is a picture of para-methoxy anisole:

And here is the plane of symmetry with the unique hydrogens labeled. Note the green dots are all equivalent so they come up as singlets between 6.5 and 8.5 ppm (benzene). The purple dots are equivalent and come up as 3H singlets.

 

[orangetea]

The answer to the question is C not A. Draw out A and you will see the symmetry as compared to para-ethoxy phenol.

Here is a picture of para-methoxy anisole:

And here is the plane of symmetry with the unique hydrogens labeled. Note the green dots are all equivalent so they come up as singlets between 6.5 and 8.5 ppm (benzene). The purple dots are equivalent and come up as 3H singlets.

Why don't the green Hs split and give you a 1(H) doublet?

 

[orangetea]

TBR lies. I wish they drew that accurately

 

[sillyjoe]

Why don't the green Hs split and give you a 1(H) doublet?

Honestly, I don't know the answer, but this makes the most sense for THIS question. I am curious as well to know why!

 

[Czarcasm]

Why don't the green Hs split and give you a 1(H) doublet?

My understanding was because they are identical hydrogen environments. Only non-identical neighboring hydrogen environments would cause splitting.

 

[orangetea]

My understanding was because they are identical hydrogen environments. Only non-identical neighboring hydrogen environments would cause splitting.

hmm no there still would be splitting. Take a look.

http://www.chem.uky.edu/courses/che230/MSM/Ortho Meta Para Explainer.pdf

 

[sillyjoe]

hmm no there still would be splitting. Take a look.

http://www.chem.uky.edu/courses/che230/MSM/Ortho Meta Para Explainer.pdf

I can't find an analogous example in this link where you have two equivalent protons on ADJACENT carbons.

 

[orangetea]

I can't find an analogous example in this link where you have two equivalent protons on ADJACENT carbons.

I see what you're saying I guess I just didn't see the two R groups on the molecule up there as "equal" R groups..so I correlated it with the "para" example in the PDF. but yea I see why there wouldn't be splitting now.

 

[sillyjoe]

I see what you're saying I guess I just didn't see the two R groups on the molecule up there as "equal" R groups..so I correlated it with the "para" example in the PDF. but yea I see why there wouldn't be splitting now.

I really think such a conceptual understanding of NMR is out of the scope of the MCAT. Thankfully, for a question like this you can eliminate to find the correct answer.

MY PROBLEM IS REMEMBERING WHAT THE HELL ANISOLE MEANS!

 

[orangetea]

I really think such a conceptual understanding of NMR is out of the scope of the MCAT. Thankfully, for a question like this you can eliminate to find the correct answer.

MY PROBLEM IS REMEMBERING WHAT THE HELL ANISOLE MEANS!

I didn't know until this question

 

[Czarcasm]

hmm no there still would be splitting. Take a look.

http://www.chem.uky.edu/courses/che230/MSM/Ortho Meta Para Explainer.pdf

This was my thought: The X and Y represent two different substituents. If instead it were -OCH3 (of anisole) and an -OCH2CH3 in the para position, then yeah, those hydrogens would be non-equivalent. But in this scenario, the substituent is the same (-OCH3), so you can think of the molecule folding upon itself twice to produce 2 unique hydrogen environments - the hydrogens directly attached to the carbon of the -OCH3 group, and the benzylic hydrogen.