Solution Found!
Dimethylformamide (DMF) is a common solvent:
Chapter 21, Problem 21.83(choose chapter or problem)
Dimethylformamide (DMF) is a common solvent: Dimethylformamide H N CH3 CH3 O (a) The 1 H NMR spectrum of DMF exhibits three signals. Upon treatment with excess LAH followed by water, DMF is converted into a new compound that exhibits only one signal in its 1 H NMR spectrum. Explain. (b) Based on your answer to part a, how many signals do you expect in the 13C NMR spectrum of DMF?
Questions & Answers
QUESTION:
Dimethylformamide (DMF) is a common solvent: Dimethylformamide H N CH3 CH3 O (a) The 1 H NMR spectrum of DMF exhibits three signals. Upon treatment with excess LAH followed by water, DMF is converted into a new compound that exhibits only one signal in its 1 H NMR spectrum. Explain. (b) Based on your answer to part a, how many signals do you expect in the 13C NMR spectrum of DMF?
ANSWER:Step 1 of 2
(a) A resonance structure can be drawn for DMF that shows there is significant double bond character between the nitrogen atom and the carbonyl carbon. This leads to restricted rotation around this bond, and so the two methyl groups are in different electronic environments. This means that they’re not chemically equivalent and so they give rise to two different signals in the 1H NMR. These, plus the proton on the carbonyl carbon, make up the three different signals seen in the 1H NMR spectrum of DMF.
Once the amide has been reduced to an amine using LAH, the resonance structure is no longer possible and so there is relatively free rotation around the nitrogen - carbon bond. The methyl groups become chemically equivalent and so only one signal is seen in the 1H NMR.
