A hydrocarbon of unknown structure has the formula C8H8. On catalytic hydrogenation over the Lindlar catalyst, 2 equivalents of hydrogen are absorbed. On hydrogenation over a palladium catalyst, 5 equivalents of H2 are absorbed.

a) The degrees of unsaturation in the unknown is (are) ?

b) The number of triple bonds is?

c) The number of double bonds is?

d) The number of rings is?

a) Degree of unsaturation: 5.

C8H8 corresponds to the saturated alkane C8H18, octane, which has eight hydrogen atoms per molecule. Theoretically, it will take five hydrogen molecules H2 to convert C8H8 to C8H18. C8H8 therefore has and DoU of 5.

b) Number of triple bonds: 2.

Only triple bonds are hydrogenated over a Lindlar catalyst. A triple bond can be converted only to a double bond, but not a single bond, under such settings. Each triple bond would absorb only one molecule of hydrogen.

Two hydrogen molecules are absorbed for each C8H8 molecule. Each molecule therefore contains two triple bonds.

c) Number of double bonds: 1.

Palladium catalysts convert both triple and double bonds to single bonds while preserving any rings. Each triple bond would consume two hydrogen molecules, whereas each double bond would consume one.

The two triple bonds as determined in b) account for the consumption of four out the five hydrogen molecules. One double bond shall be responsible for the other.

d) There is no ring in C8H18.

The molecular formula of C8H18 indicates an DoU of five. Each double bond contributes to the DoU by one, each triple bond by two. The double bond and the two triple bonds would have accounted for all five degrees of unsaturation. There are thus no spare DoU available for the presence of rings.