||Organic Chemistry 4e Carey||
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Chapter 13: Spectroscopy
Infra Red |
Nuclear Magnetic Resonance |
Mass Spectrometry |
Getting Structures from Spectra |
Index of Hydrogen Deficiency |
Index of Hydrogen Deficiency
The Index of Hydrogen Deficiency
(IHD), is a count of how many molecules of H2 need to be added to
a structure in order to obtain the corresponding saturated, acyclic species.
Hence it takes a count of how many rings and multiple bonds are present in the
structure, so IHD can also be thought of as (multiple bonds + rings) or (p +
When you look at a structure,
just count them up (but take care not to count any rings twice !)
If you have a molecular formula, CcHhNnOoXx,
then the following equation can be derived:
IHD = 0.5 *
Where does this equation
Determining the IHD for molecules
can be useful for the following reasons:
- Well, the maximum
number of hydrogen atoms for "c" carbon atoms is 2c+2 (think of
the formula of saturated hydrocarbons such as ethane, propane etc.).
- From this number, subtract
the "h" hydrogens that you have.
- Since, like hydrogen,
a halogen only forms one bond, then they can be treated as if they are hydrogens,
so subtract them as well.
- Oxygen forms two bonds,
therefore it has no impact (compare H count for methane, CH4,
and methanol, CH3OH).
- Nitrogen forms three
bonds. This means for "n" nitrogens, "n" extra hydrogen atoms are needed (compare
the H count for methane, CH4, and methyl amine, CH3NH2),
therefore, add "n".
- The factor of 0.5 accounts
for us counting H atoms, but adding hydrogen, H2 ,
molecules. OK ?
What is the IHD of the following
- Seeing what types of
structural units maybe possible
- Quickly checking structures
to see if they fit the molecular formula rather than simply counting H (when
a mistake is possible)