CH610B Sophomore Organic II

Dr. Brian Pagenkopf

Stereochemistry and NMR

The NMR “Sees” Stereochemistry

The ¹H NMR and ¹³C NMR spectra of 3-methyl-2-butanol look like. Write down your predictions and compare it to the tabulated values.

The ¹H NMR shows 6 different resonance signals.

The ¹³C NMR shows 5 different carbons. There specific values are:

Why are the two methyl different? The following shows a Newman Projection of looking down the red bond with the alcohol’s carbon back.

All we need do is pick one reference point.

In conformation A Me is anti to OH, and to its right is Me. Me is gauche to OH and Me.

Let’s compare what Me sees when it’s in the same place.

In conformation C Me is anti to OH, and to its right is H. Me is gauche to H and OH.

This means Me and Me can never occupy the same space: they are stereochemically different, therefore they resonate at different frequencies in the NMR.

How do I tell when looking at an NMR? If you make a change to Me or Me you’ll get diastereomers – two different molecules. If you just got an enantiomer then the starting methyls are the same.

Still not sure, think of two Siamese twins…