Protons NMR : Official Guide

[SaintJude]

How many types of protons will most likely be dected by high-resolution 1H-NMR at room temperature? Answer: 7

How many types of protons will be detected when the H on the stereogenic carbon is replaced with an methyl group? Answer : 4
I'm surprised that suddenly the two H of the CH2 groups are equivalent?

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[dmf2682]

How many types of protons will most likely be dected by high-resolution 1H-NMR at room temperature? Answer: 7

How many types of protons will be detected when the H on the stereogenic carbon is replaced with an methyl group? Answer : 4
I'm surprised that suddenly the two H of the CH2 groups are equivalent?

Yeah. So if you make that h a methyl you create a plane of symmetry in the plane of the ring. That makes them equivalent. Good question though.

 

[SaintJude]

Yeah, what was important was to see that if you replace the CH3 with a H, then the molecule is no longer chiral.

 

[pm1]

Yeah, what was important was to see that if you replace the CH3 with a H, then the molecule is no longer chiral.

han.. why is it 7 and not 5 for the first question? don't we just count it as one type when it is CH2?

 

[Dasypus]

Seems like it's because the molecule has a top side and a bottom side (determined by that methyl group), so top Hs are different from bottom Hs.

 

[gettheleadout]

han.. why is it 7 and not 5 for the first question? don't we just count it as one type when it is CH2?

Seems like it's because the molecule has a top side and a bottom side (determined by that methyl group), so top Hs are different from bottom Hs.

Yeah, I believe the chirality makes those protons diastereotopic.

 

[johnwandering]

Yeah, I believe the chirality makes those protons diastereotopic.

Wow, I was absolutely sure that it was 5~



Wait, but those two CH2 on the ring are not chiral. You can swap the H from the bottom and the top and the stereoisomers are perfectly superimposable.

 

[dmf2682]

Wow, I was absolutely sure that it was 5~



Wait, but those two CH2 on the ring are not chiral. You can swap the H from the bottom and the top and the stereoisomers are perfectly superimposable.

Yeah, that one kinda threw me too, but then I realized that what makes the protons swappable normally is free rotation about a single bond. When you have a carbon attached to a ring like that it can't really turn around because it's fixed in place (relative to the ring).

The way I figure each one is unique in terms of an HNMR is that if we consider the ring to be in a horizontal plane, with the methyl protruding on the bottom, and the proton protruding on the top, then the H's on the two CH2's "see" different things. One will be closer to the opposite methyl and the other will be closer to the opposite proton. It's slight, but I suspect that's what's causing the difference. Maybe GTLO or medPR can give you a more technical answer- I use my gut for orgo and that's about it