[qw098]

Hi guys,

For the compound shown below, how many different absorptions in the H-NMR would you expect?

I said six of them, but for some reason my textbook says five?!

Please tell me who is right!

Thanks!

[sjb]

Hi guys,

For the compound shown below, how many different absorptions in the H-NMR would you expect?

I said six of them, but for some reason my textbook says five?!

Please tell me who is right!

Thanks!

(Butan-2-ol)

Can you let us know how you have assigned the 6?

[qw098]

Well the two methyls each have one distinct proton. The hydroxyl proton makes three, the proton adjacent to the hydroxyl makes four, and then the methylene has two chemically distinct protons as they are diastereotopic; they are adjacent to a chiral center.

So that makes six in total!

[discodermolide]

Well the two methyls each have one distinct proton. The hydroxyl proton makes three, the proton adjacent to the hydroxyl makes four, and then the methylene has two chemically distinct protons as they are diastereotopic; they are adjacent to a chiral center.

So that makes six in total!

The text book may well be correct!

[qw098]

So I am wrong by saying six? Why?

[Honclbrif]

SDBS: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/img_disp.cgi?disptype=disp3&imgdir=hsp&fname=HSP00002&sdbsno=507

slightly better spectrum from Aldrich: http://www.sigmaaldrich.com/spectra/fnmr/FNMR003935.PDF

So why do you think it is 5 instead of 6?

[qw098]

Thanks Honclbrif.

The thing is, aren't the methylene hydrogens diastereotopic so I should see two distinct signals for those protons?

[discodermolide]

Thanks Honclbrif.

The thing is, aren't the methylene hydrogens diastereotopic so I should see two distinct signals for those protons?

They are diastereotopic but in this case the bonds can freely rotate so the protons are in an identical environment.

[qw098]

They are diastereotopic but in this case the bonds can freely rotate so the protons are in an identical environment.

Go to this http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/couplingtopic.html and scroll to the bottom. It discusses diastereotopic protons, and in their example, the bonds can freely rotate as well.

Also, I said I expected 6, and not 5 distinct signals Honclbrif.

[Honclbrif]

I apologize, a typo missed in the proofreading stage.

Anyway, seems like you've got it worked out.

[discodermolide]

Sorry my mistake.

[sjb]

Depending on the exact experiment you carry out, you may not see the OH proton that easily.