L3 - Index of Hydrogen Deficiency

objectives

Understandings

• The degree of unsaturation or index of hydrogen deficiency (IHD) can be used to determine from a molecular formula the number of rings or multiple bonds in a molecule.

Applications and Skills

• Determination of the IHD from a molecular formula.

Unsaturation

• This refers to the ability of a molecule to add more hydrogen. This is usually due to the presence of double bonds:

CH2=CH2 + H2 --> CH3CH3

• The ethene molecule in the equation above is said to be unsaturated; it is able to add one molecule of hydrogen.
• The opposite term, 'saturated' means that a molecule is unable to add any more hydrogen as it does not have any multiple bonds.

The Index of Hydrogen Deficiency (IHD)

• The index of hydrogen deficency (IHD) is similar to the concept of unsaturation with one important difference - it includes cyclic systems (rings).
• Here is a summary of how the index of hydrogen deficiency works.

1. 1 A double bond and ring each counts as one IHD.
2. 2 A triple bond counts as two IHD.

Hydrocarbons (CxHy):

• In a saturated hydrocarbon the number of hydrogen atoms = (2x + 2), when the number of carbon atoms is x.
• Hence, the number of hydrogens 'missing' is given by (2x + 2) minus the actual number of hydrogens.
• And the number of double bond equivalents is half this value (as a double bond has two fewer hydrogen atoms). Hence the Index of Hydrogen Deficiency is given by:

IHD=[(2x+2)-y)]/2

(where x and y stand for # of C and H respectively.)

• This information is then used to determine the number of possible double bonds or rings in the molecule under analysis.

Effect of Hetero-atoms

• Many organic molecules contain atoms other than carbon and hydrogen. They are known, collectively, as heteroatoms, i.e. atoms that are different to carbon and hydrogen.

Oxygen

• Oxygen atoms do not affect the index of hydrogen deficiency.
• In other words you can forget them when carrying out the calculation.

Nitrogen

• Nitrogen atoms form three bonds, so for each nitrogen atom you must add one (as you would expect to squeeze one more hydrogen atom into the structure).

Halogens

• These are treated as if they are hydrogen atoms and subtracted in the equation.