Q74P Question. (A true story.) The ch... [FREE SOLUTION]

Chapter 20: Q74P (page 1038)

Question. (A true story.) The chemistry department custodian was cleaning the organic lab when an unmarked bottle fell off a shelf and smashed on the floor, leaving a puddle of volatile liquid. The custodian began to wipe up the puddle, but he was overcome with burning in his eyes and a feeling of having an electric drill thrust up his nose. He left the room and called the fire department, who used breathing equipment to go in and clean up the chemical. Three students were asked to identify the chemical quickly so that the custodian could be treated and chemical could be handled properly. The students took IR and NMR spectra, which follow. The UV spectrum showedat 220 _nm(E= 16,000). The mass spectrometer was down, so no molecular weight was available. Determine the structure of this nasty compound, and show how your structure fits the spectra.

Short Answer

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Answer.The strong UV absorption at 220 _nmindicates conjugated aldehyde or ketone. Further, IR spectrum confirms that it is an aldehyde. The NMR spectrum shows that aldehyde proton is split into doublet, so it has one neighbouring hydrogen and further information from the given spectrum leads to identification of structure that is, structure name is crotonaldehyde.

Step by step solution

Determination of the structure through UV and IR spectra:

The strong UV absorption at 220 _nmindicates a conjugated aldehyde or ketone. The IR shows a strong carbonyl at 1690 cm^-1, alkene at 1625 cm^-1, and two peaks at 2720 cm^-1and 2810 cm^-1which clearly indicates towards an aldehyde.

Determination of the structure through NMR spectra

The NMR shows the aldehyde proton at chemical shift value 9.5 which is split into doublet so it must be having one neighbouring hydrogen. There are two vinyl protons, so there must be an alkyl group coming from the beta-carbon. The only other NMR signal is a 3H doublet. Thus, the 鈥楻鈥� group attached to the alkene carbon must be methyl. This gives the structure 鈥渃rotonaldehyde鈥�.