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Useful Concepts |
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Index of Hydrogen Deficiency (IHD)
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 + r).
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 * [2c+2-h-x+n]
Where does this equation come from ?
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Well, the maximum number of hydrogen atoms for "c" carbon
atoms is 2c+2 (think of the formulae of saturated hydrocarbons such as
ethane, propane etc.).
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From this number, subtract the "h" hydrogens that you have.
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Since, like hydrogen, a halogen only forms one bond, then they can be treated
as if they are hydrogens, so subtract them as well.
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Oxygen forms two bonds, therefore it has no impact (compare H count
for methane, CH4, and methanol, CH3OH).
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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".
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The factor of 0.5 accounts for us counting H atoms, but adding
hydrogen, H2 , molecules. OK ?
Determining the IHD for molecules can be useful for the following reasons:
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Seeing what types of structural units maybe possible
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Quickly checking structures to see if they fit the molecular formula rather
than simply counting H (when a mistake is possible)
What is the IHD of the following molecular formulae ?
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C6H10
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C6H6
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C4H8O
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C4H9N
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C2Cl2
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