Question: (a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there? (b) Use the IR spectrum to determine the functional group(s), if any. (c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain? (d) Propose structures for the major fragments at 39, 67, 81, and 95 in the mass spectrum.

From mass spectrum M⁺ Peak =110

110/13=8+(6 remainder)

n=8

molecular mass of H=1

6 remaining that is molecular mass of 6H

molecular formula= C₈H₈+6H

molecular formula= C₈H₁₄

Element of unsaturation for C_nH_X= n-(x/2)+1

= 8-(14/2)+1

=2

Elements of unsaturation value two.

In IR spectrum of given compound are sharp peaks at 3300 and 2120cm^-1 to 3300cm^-1 indicating C-H of alkyne frequency and 2100cm^-1 indicating stretching frequency of triple bonded carbon and hydrogen bond of terminal alkyne. Hence, the functional group is terminal alkyne.

Molecular formula C₈H₁₄

Unsaturation in the given compound is two and the functional group is terminal alkyne for that unsaturation.

The possible structures are:

Possible structures

Uncertain part of structure of saturated chain of compound C₆H₁₃-

If we know hydrogenation C₈H₁₄givesn-octane, there will be no uncertainty in the structure of the compound. The structure will be:

Major fragments of given compound are:

The major fragmentation peaks are: