Instructions
Calculate the index of hydrogen deficiency or IHD from the chemical formula by using: IHD=C-H/2+1, where C is the number of carbon atoms and H is the number of hydrogen atoms. For example: Consider the chemical formula C6H6. To calculate the IHD, substitute the following values above: C=6, H=6. This yields IHD=6-(3/2)+1 = 4.
A ring system, double bond and triple bond each take one IHD. With a value of four, there are several possibilities: one ring and three double bonds or two rings and two double bonds, etc.
Assemble the carbon skeleton chain. Assume all the carbon atoms are linked together and connect them with single bonds represented in a Kekule structure by a single line segment between two atoms. The example had six carbons, so two possibilities are: C-C-C-C-C-C or a ring of carbon atoms where the first carbon is connected to the last carbon in the linear chain above.
Attached, are the remaining hydrogen atoms to the carbon atoms. Each carbon atom needs to have either four single bonds, two double bonds or one single bond and one double bond attached to it. In the example, there are not enough hydrogens to fill the requirements of carbon as stated above. The ring is the only possible choice and by adding the hydrogen atoms to the carbons on the ring, each atom has the required number of linkages. This is the Kekule structure of benzene.