Question

7.   If a hydrocarbon has nine carbon atoms, three double bonds and a ring, how many
hydrogen atoms must have?

Answer 1

For a compound like C_aH_bN_cO_d the Double Bond Equivalence is:

$DBE=a+1-\frac{b-c}{2}$

DBE is the total number of double bonds and ring. Now, for hydrocarbons only H and C are there, so, c = 0.

Thus, the formula reduces to,

$DBE=a+1-\frac{b}{2}$

Now here in our case, DBE = number of double bonds + number of rings = 3+1 = 4, a = 9

$Thus,4=9+1-\frac{b}{2}$

$Or,6=\frac{b}{2}$

$Or,b=12$

So, there must be 12 Hydrogens.