Thanks thushan! Do you know if there are any rules regarding this that we should be familiar of? Perhaps an explanation or even a link to one would be appreciated!
Hey - sure. So this is about working out the number of double bond equivalents (DBEs), which is basically the number of double bonds and rings.
DBE = 1/2 x (2(c+1) - h/cl/f/br/i + n)
The H/Cl/F/Br/I are just atoms that form one bond. Note that oxygen is not included in the equation.
Alternatively, another way to do it is to find a hydrocarbon with the same number of double bonds. Eg.
C3H6O will have the same number of double bonds as C3H6 (noting that O doesn't appear in the DBE formula), which is 1. Note here that we can effectively "ignore" oxygen atoms in looking for an equivalent hydrocarbon with the same number of double bonds.
C3H9N will have the same number of double bonds as C3H8, which is 0. To account for the N, add 1 to the number of H atoms.
C3H7Cl has the same number of double bonds as C3H8, which is 0. To account for the Cl/F/Br/I, subtract 1 from the number of H atoms.