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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: Winga on July 27, 2004, 12:28:15 AM

Title: NMR spectrum of aldehyde!
Post by: Winga on July 27, 2004, 12:28:15 AM
In a class test. I need to find out the structural formula of a compound with providing the empirical formula (C5 H10 O), IR spect, NMR spect and mass spect.

The answer is 3-methylbutanal.

(CH3)2-CH-CH2-CHO

In NMR sepct, there should be 4 signals. The aldehydic proton has a chemical shift around 9. However, there is no spin-spin coupling between aldehydic proton and methylene protons (nearby protons).

Theorectically, the signal of aldehydic proton should be present as triplet and the methylene protons is also triplet (another methine proton just next to them).

In this spect, the signal of aldehydic proton is singlet while the methylene protons is doublet. An explanation from someone is that, there is oxygen affect that proton. But I don't think this case is as same as the high concentrated / pure alcohol which gives no coupling and the effect of magnetic anistropy by C=O double bond only reinforce the applied magnetic field that the proton feels.

The answer 3-methylbutanal is correct and the NMR spect is made by real experiment. So, how to explain it?
Title: Re: NMR spectrum of aldehyde!
Post by: Organic_lover* on January 21, 2012, 07:57:36 PM
You can find the aswer to your question in this book:

Introduction of Spectroscopy
Donald Pavia and Gary Lampman
Title: Re: NMR spectrum of aldehyde!
Post by: dipesh747 on January 23, 2012, 09:37:26 AM
Look up the Karplus relation
Title: Re: NMR spectrum of aldehyde!
Post by: AWK on January 23, 2012, 10:01:26 AM
It depends on quality of spectrometer
http://www.ymdb.ca/spectras/2496
Title: Re: NMR spectrum of aldehyde!
Post by: orgopete on January 23, 2012, 03:07:33 PM
Quote from: AWK on January 23, 2012, 10:01:26 AM
It depends on quality of spectrometer
http://www.ymdb.ca/spectras/2496

Isn't the J-value of the aldehyde 3Hz? Shouldn't you be able to see that with almost any spectrometer of the last 50 years?
Title: Re: NMR spectrum of aldehyde!
Post by: AWK on January 25, 2012, 03:24:26 AM
Quote from: orgopete on January 23, 2012, 03:07:33 PM
Quote from: AWK on January 23, 2012, 10:01:26 AM
It depends on quality of spectrometer
http://www.ymdb.ca/spectras/2496

Isn't the J-value of the aldehyde 3Hz? Shouldn't you be able to see that with almost any spectrometer of the last 50 years?
but Winga wrote:
Quote
The aldehydic proton has a chemical shift around 9. However, there is no spin-spin coupling between aldehydic proton and methylene protons (nearby protons).
Title: Re: NMR spectrum of aldehyde!
Post by: discodermolide on January 25, 2012, 02:03:01 PM
Is the aldehyde signal not an ABq where the J value is very small and you only observe the middle two lines?
I have spectra of aldehydes on this computer where the aldehyde signal appears to be a doublet. (Sorry I forgot to include the scale but the signal is at 9.5 around)