Functional groups in organic chemistry (A level chemistry)

Functional groups in organic chemistry are special arrangements of atoms in a part of an organic molecule that determines its characteristic chemical properties.

Organic molecules with the same functional group react in the same
way, no matter the length of the hydrocarbon chain. This means that if you learn the reactions of one alkane, such as ethane, you can perfectly well apply this knowledge to predict the reactions of any other alkane.

The table below shows some families of organic compounds and their functional groups.

FamilyFunctional groupName prefix or suffixExample
alkanesCnH2n+2-aneethane
alkenesC=C-enepropene
halogenoalkanes-X (whereby X is a halogen atom)(prefix is specific to the halogen in the molecule)chloromethane
alcohols-OH-olethanol
aldehydes-CHO

Functional group

-alethanal
ketones-CO-

Functional group

-onepropanone
carboxylic acids-COOH

Functional group

-oic acidethanoic acid
amines-NH2-amineethylamine
esters

Functional group

-oateethyl ethanoate

Homologous series and functional groups

A homologous series is a group of organic compounds with the
same functional group.

Any homologous series has the following characteristics:

  • compounds in the homologous series have the same general formula.
  • each member of the homologous series differs from the next by CH2.
  • the length of the carbon chain has little to no effect on the chemical reactivity of the members of a series.
  • while the functional group affects the chemical properties, the length of the carbon chain affects physical properties, such as melting point, boiling point, and solubility.
  • melting points and boiling points of members of a homologous series tend to increase by a small amount as the number of carbon atoms in the chain increases. This is due to the increase in the intermolecular forces.
  • branching of the carbon chain reduces the melting and boiling points.

General formula of organic compounds

Each member of a homologous series adheres to a certain general formula which shows the ratio of its atoms. The table below shows the general formulae of some functional groups.

FamilyGeneral formula
alkanesCnH2n+2
alkenesCnH2n
halogenoalkanesCnH2n+1 X (whereby X is a halogen atom)
alcoholsCnH2n+1OH
aldehydesCnH2n+1CHO
ketonesCnH2n+1COCmH2m+1
carboxylic acidsCnH2n+1COOH
aminesCnH2n+1NH2
estersCnH2n+1COOCmH2m+1