Common functional groups
Organic compounds are organized into families. The members of the simplest hydrocarbon family are referred to as alkanes and have only singly bonded carbon ( these can be represented as R–H where R represents a hydrocarbon group).
The general formula of members of other families is shown below. These have functional groups containing atoms other than C and H.
The carbonyl group (C=O) is a common structural element.
(RCOR)
(RCHO)
R = H or hydrocarbon for alkenes, alkynes, carboxylic acids, amides
Otherwise R = hydrocarbon group 
(RCO2H)
(RCO2R)
(RCONH2)
R–NH2
R–OH
R2C=CR2
RC≡CR
alkene
RC≡CR
alkyne
In other hydrocarbon families at least one pair of carbons is joined by a multiple bond. This is the reactive site in the molecule and is known as the functional group.
The general formula of members of other families is shown below. These have functional groups containing atoms other than C and H.
The carbonyl group (C=O) is a common structural element.
(RCOR)ketone
(RCHO)aldehyde
R = H or hydrocarbon for alkenes, alkynes, carboxylic acids, amides
Otherwise R = hydrocarbon group
(RCO2H)carboxylic acid
(RCO2R)ester
(RCONH2)amide
R–X
X=Cl, Br, I
X=Cl, Br, I
haloalkane
R–NH2
amine
R–OH
alcohol