Q.46P Question. An unknown compound gi... [FREE SOLUTION]

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Chapter 18: Q.46P (page 970)

Question. An unknown compound gives a molecular ion of m/z 70 in the mass spectrum. It reacts with semicarbazide hydrochloride to give a crystalline derivative, but it gives a negative Tollens test. The NMR and IR spectra follow. Propose a structure for this compound, and give peak assignments to account for the absorptions in spectra. Explain why the signal atin the IR spectrum appears at an unusual frequency.

Short Answer

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Answer

Cyclobutanone

Step by step solution

Step-1: Introduction

A molecular ion of m/z value 70 indicates that the size of a molecule is small. A solid semicarbazone derivative and a negative tollens test indicates that it is a ketone.

The carbonyl has mass 28, which means 70-28= 42, enough mass for three more carbon atoms. Molecular formula is probably C₄H₆O with unsaturation, so we can infer that double bond or a ring must be present in addition to the carbonyl.

IR shows a strong absorption at 1790cm^-1 which indicates ketone in a small ring. No peak 1650-1600cm^-1 present in the region shows the absence of an alkene.

Only possible structures in context with above information are cyclobutanone and methylcyclopropanone.

CyclobutanoneMethylcyclopropanone

Step-2.Explanation

The proton NMR can distinguish these, as no methyl doublet found in NMR spectrum rules out the possibility of methylcyclopropanone.NMR does a triplet at 3.1 chemical shift of and this signal comes from two methylenes, carbon number 2 and 4, adjacent to the carbonyl in cyclobutanone. The signal for methylene at the third carbon appears at 2.0 chemical shift value which is quintet because of splitting by four neighbouring protons. Hence, the unknown is cyclobutanone. Also, the symmetry indicated by carbon NMR rules out the possibility of methylcyclopropanone.