Degrees of Unsaturation

1
Degrees of Unsaturation
The molecular formula can give us hidden
structural information about a compound.
C6H14
Molecules that contain the maximum number or
hydrogen atoms per carbon atom are said to be
saturated.
2
Degrees of Unsaturation
Having fewer than the maximum number of hydrogens
is called unsaturation.
C6H14
C6H12
3
Degrees of Unsaturation
Compounds can be characterized by the degrees of
unsaturation (doU) they bear.
For a generic molecular formula CnHm
S the number of hydrogens
predicted for a saturated compound 2n 2
doU (S m)/2
C6H14
C6H12
S 14 m 14 doU 0
S 14 m 12 doU 1
4
Degrees of Unsaturation
Phenyl rings have four degrees of unsaturation
one for each p bond and one for the ring
3 p bonds
1 ring
C6H6
S 14 m 6 doU 4
5
Degrees of Unsaturation
Modifications must be made for nitrogen-containing
molecules.
For a generic molecular formula CnHmNN
SN the number of hydrogens predicted for
a saturated N-containing compound 2n 2
N doU (SN m)/2
C4H11N
C4H10N2
SN 11 m 11 doU 0
SN 12 m 10 doU 1
6
Degrees of Unsaturation
Modifications must also be made for
halogen-containing molecules.
For a generic molecular formula CnHmXX
SX the number of hydrogens predicted for
a saturated N-containing compound 2n 2
- X doU (SX m)/2
C4H9Br
C6H9Cl
SX 9 m 9 doU 0
SN 13 m 9 doU 2
7
Degrees of Unsaturation
When a compound contains both nitrogen and
halogens, both species have to be accounted for.
No modification is necessary for oxygen.
For a generic molecular formula CnHmXXNN
SE the total number of hydrogens
predicted for the compound 2n 2 X
N doU (SE m)/2
SE 2n 2 X N 12 2 1 1 14
C6H6ClNO
SE 14 m 6 doU 4