Beginning Organic Chemistry (BOC)
4. Organic Families.

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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

Date created: 2005 06 10.