Question

1. A compound composed of carbon, hydrogen & oxygen has a molecular ion at m/z=112 amu in its mass spectrum. The base peak is at m/z=28 amu. The infrared spectrum shows strong absorption in the 2850 to 2980 cm-1 region, and very strong absorption at 1717 cm-1. The 1H nmr shows a single sharp signal at ? = 2.7 ppm, and the 13C nmr has two signals ( ? = 37 & 208 ppm ).

Answer 1

Info from mass spectroscopy: In mass spectroscopy the molecular ion ia the largest ion which is formed due to loss of an electron by the compound. Hence the m/z value of the molecular ion will be equal to the molar mass of the compound. Hence the molecular mass of the compound is 112 amu.

Since it contains C,H and O, the molecular formulae of the compound could be either C7H12O or C6H8O2 or C5H4O3.

Information from infrared spectrum:

A strong absorption spectrum at 1717 cm-1 conforms the presence of a carbonyl ( - C=O) group. However the absence of any considerable peak beyond 3000 cm-1 conforms the absence of alcoholic or phenolic -OH and carboxylic -OH group. Hence the compound is not an alcohol, phenol, or carboxylic acid.

Hence the compound is either an aldehyde or ketone.

A strong absorption peak in the 2850 to 2980 cm-1 region conforms the presence of alkyl group.

Info from 1H NMR spectroscopy: A sharp signal at 2.7 ppm conforms that the carbonyl carbon is a ketone. Since the signal is a single sharp signal conforms that all the hydrogens are alike. This is possible only when 6 C-atoms form a cyclic ring. Hence a cyclohexane ring is present.

Info from C-13 NMR spectroscopy: A signal at 208 ppm conforms the presence of ketone carbon. A signal at 37 ppm conforms the presence of alkyl carbon. Also two different signal conforms that there are only 2 different types of C - atom. This conforms that the two ketone groups are attached to the cyclohexane ring at symmetrical position i.e. at 1 and 4 position. Hence the compound should be 1,4 - cyclohexanedione.