Series of Organic Compounds.

Organic compounds are best classified into groups according to their reactions, which means essentially according to the bond and atom types that they contain. Thus all compounds which contain only singly bonded carbon and hydrogen have similar chemical properties.

Examination of the formulae for such a series shows that as the number of carbon atoms increases, so does the number of hydrogens in the proportion two hydrogen to each added carbon (2/1::H/C).

Any series of organic compounds where successive members differ from each other by a CH₂ unit is termed an homologous series and members are called homologs.

Following is the homologous series of compounds containing only singly bonded carbon and hydrogen. (Only those members which have all carbons linked in a continuous chain are shown.)

Note that going from one member to the next involves adding CH₂ to the molecular formula.

# of carbons	Molecular formula	Name	Condensed structure	Molecular model
1	CH4	methane	CH4
2	C2H6	ethane	CH3CH3
3	C3H8	propane	CH3CH2CH3
4	C4H10	butane	CH3CH2CH2CH3
5	C5H12	pentane	CH3(CH2)3CH3
6	C6H14	hexane	CH3(CH2)4CH3
7	C7H16	heptane	CH3(CH2)5CH3
8	C8H18	octane	CH3(CH2)6CH3
9	C9H20	nonane	CH3(CH2)7CH3
10	C10H22	decane	CH3(CH2)8CH3
n	CnH2n + 2	alkane	CH3(CH2)n-2CH3

Family Formulae and Names.

Since molecular formulae are related in each homologous series, there is a general formula for each series. You can see it at the bottom of the two tables on this page.

A quick look at the names shows you that there is a system to them. In fact each name above consists of a root which indicates the number of carbons followed by the suffix -ane, which is indicating the family. Note the special root alk- to indicate the whole family.

Another Example.

Different homologous series contain other types of atoms and/or bonds. As above, though, the difference from one compound to the next must be a CH₂ unit.

Following is an homologous series of compounds that contain a double bonded oxygen on the end carbon. Again, only those members of the family which have all the carbons in a line are included.

The Aldehyde or Alkanal Family.
# of carbons	Molecular formula	Name	Condensed structure	Molecular model
1	CH2O	methanal	CH2O
2	C2H4O	ethanal	CH3CHO
3	C3H6O	propanal	CH3CH2CHO
4	C4H8O	butanal	CH3CH2CH2CHO
5	C5H10O	pentanal	CH3(CH2)3CHO
6	C6H12O	hexanal	CH3(CH2)4CHO
7	C7H14O	heptanal	CH3(CH2)5CHO
8	C8H16O	octanal	CH3(CH2)6CHO
9	C9H18O	nonanal	CH3(CH2)7CHO
10	C10H20O	decanal	CH3(CH2)8CHO
n	CnH2nO	alkanal	CH3(CH2)n-2CHO