Once the molecular formula of an unknown compound is known, structural information can be elucidated (for example,
what functional groups are probably present, is the compound cyclic or linear...). The first step involves the
calculation of the index of hydrogen deficiency, or IHD. The IHD tells us the number of p
bonds and/or rings a molecule contains. This information is based on the following general molecular formulas:
| alkane | CnH2n+2 |
| cycloalkane or alkene | CnH2n |
| alkyne | CnH2n-2 |
Notice that each time a ring or p bond is introduced into a molecule, the number of hydrogens is reduced by two. Therefore, if the molecular formula of an unknown compound has two less hydrogens than a straight chain alkane with the same number of carbon atoms would have, then the IHD is 1 and the unknown compound must have one p bond or one ring. Note that interesting structural information has been gleaned from just knowing the molecular formula of the compound in question.
There are two methods for calculating the index of hydrogen deficiency in a molecule as described below.
| |
| |