We would number from the left end and go toward the right end of the contineous chain. If more than one of the same kind of branched group is attached to the chain, identify the number carbon each group is attached to as a series of numbers separated by commas between each number then a hyphen and finally use a greek prefix attached to the branch name. We see that for a ring structure or a double bond there is a difference of two hydrogens compared to the alkane structure with the same number of carbons. We say that the Index of Hydrogen Deficiency is equal to the number of pairs of Hydrogens that must be taken away from the alkane to get the same molecular frmula of the compound under investigation. Every Pi bond in the molecule increases the index by one.
Any ring structure increases the index by one.
We can use the index and the molecular formula to identify possibilities as to the exact nature of the molecule. Identify the Alkane molecular formula for six carbons. That would be C 6 H 14 . Using the given Index determine the molecular formula. There are two kinds of addition type reactions where a Pi bond is broken and atoms are added to the molecule.