A student miscalculated the quantities of starting material required for the rea
ID: 929395 • Letter: A
Question
A student miscalculated the quantities of starting material required for the reaction and mistakenly used 2X the amount of 3-methoxyacetophenone necessary (that is 2 moles of the acetophenone to 1 mole of 4- chlorobenzaldehyde). When his NMR was returned there was clearly a 1:1 mixture of the desired Chalcone and another compound. The student separated the mixture using high performance liquid chromatography and obtained an NMR of the second compound contaminating his product. (See last NMR spectrum attached). He proposed that the product might be the result of a Michael reaction and came up with the structure given on the NMR.
Please label the protons on the NMR
Oranic-RBF University of Minnesota Department of Chemistry VAC-200 Pulse Sequence: s2pul User: Date: Solvent: CDC13 File: Starting Time: 15:18:52 Completion Time: 15:26:34 Total acq. time 1 minute UNITYp lus- 500 "field" Ambient temperature PULSE SEQUENCE zzzusr Jun. 23, 2008 080623v2 0802 Relax. delay 1.500 sec Pulse 45.0 degrees Acq. time 2.000 sec Width 4002.4 Hz 16 repetitions H1, 200.1209120 MHz OBSERVE DATA PROCESSING Line broadening 0.1 Hz FT size 32768 OCH3 Ha He ignore peaks with X 10 4 16.03 15.86 28.93 0.20 17.80 0.66 15.51 0.EExplanation / Answer
The peaks od COCH2 protons are absent in the NMR at the range of 2.1-3. Hence the structure given by you is ruled out from the race. There are 7 set of protons in the structure and we could see only 5 sets in the NMR. Expected Aromatic protons in the structure are more as we see fewer protons in NMR. Hence given structure is not fit in to the NMR. Predictions are bit tough for me. Since the structure is symmetric with less number of protons.
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