Introducing doubly-bonded functional groups
Monovalent groups like OH and Cl replace one hydrogen at a carbon atom of a hydrocarbon chain.
Double bonds replace two hydrogen atoms.
For a carbon-carbon double bond the two hydrogen atoms are on adjacent carbons.
A carbon-oxygen double bond replaces two hydrogen atoms from the same carbon atom.
A carboxylic acid group (
) has three bonds from oxygen to carbon and must be at the end of the chain.
Depending on the number of carbon atoms at the carbon-bearing OH (or Cl), alcohols and alkyl halides are classified as either primary, secondary or tertiary.
 |  |  |
| tertiary (3°) | secondary (2°) | primary (1°) |
Double bonds replace two hydrogen atoms.
For a carbon-carbon double bond the two hydrogen atoms are on adjacent carbons.
Double bonds can be between any pair of carbon atoms in a chain, provided each carbon atom bears one hydrogen. The possible alkene isomers for the C5 skeleton above are shown.
 |  |  |
A carbon-oxygen double bond replaces two hydrogen atoms from the same carbon atom.
Thus, carbonyl groups can be at either the end of a chain or in the middle of a chain, but cannot be at branch points (because these carbon atoms have a maximum of one hydrogen atom).
 ketone C=O in the chain |  aldehydeC=O at the end of the chain. |
) has three bonds from oxygen to carbon and must be at the end of the chain. |  |
| carboxylic acid | carboxylic acid |