9: Sites of Unsaturation

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Calculating Sites of Unsaturation

When trying to determine a structure, it is useful to know how many sites of unsaturation (SU) there are in the molecule. This is also referred to as degrees of unsaturation (DU).

Saturated molecules have all single bonds and no rings. Unsaturated molecules contain double bond(s), triple bond(s) and/or ring(s).

A saturated molecule has the maximum number of hydrogen atoms possible to be an acyclic alkane:

\[\text{H = (2 $\times$ C) + 2}\]

Every pi bond or ring reduces the number of hydrogens by 2.

The sites of unsaturation (SU) can be calculated with the following formula:

\[\text{SU} = \frac{\text{#H on a saturated system $-$ #H on your compound}}{2}\]

N = CH

Ignore O

X (halides) = H

Try a couple:

C₅H₁₂O SU:

C₅H₁₀O SU:

Benzoic Acid (C₇H₆O₂) SU:

C₆H₁₃N SU:

C₄H₉Cl SU:

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