Index of Hydrogen Deficiency

The degree of unsaturation or index of hydrogen deficiency provides information about the number of double or triple bonds in a molecule
The IHD is the number of hydrogen molecules, H₂, needed to convert the molecule to the corresponding saturated, non-cyclic molecule
There are two ways to solve IHD problems. One way is to draw the structure and identify rings and double and triple bonds, counting each one as an IHD value of 1.
The second way is to use a formula,
- For a compound containing C_xH_y, IHD= (2x+2-y)/2
- This is a little complicated, since for the formula to work you need to:
  - ignore O and S
  - count halogens as hydrogen
  - add one C and one H for every nitrogen in the formula

Index of Hydrogen Deficiency Examples

Deduce the Index of Hydrogen Deficiency in ethyne, C₂H₂, methyl benzene, C₆H₅CH₃, and propanone, CH₃COCH₃

Answer:

Answer 1: Ethyne, C₂H₂IHD= 2

Answer 2: Methylbenzene, C₆H₅CH_3,IHD= 4

Answer 3: Propanone,CH₃COCH₃IHD = 1

Drawing out the structure is a much faster way to solve IHD problems, but using the formula is helpful when you are struggling to draw the structure

Index of Hydrogen Deficiency (DP IB Chemistry: HL)