smoke 3108065 640 Functional groups in organic chemistry (A level chemistry)

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.

Family Functional group Name prefix or suffix Example
alkanes CnH2n+2 -ane ethane
alkenes C=C -ene propene
halogenoalkanes -X (whereby X is a halogen atom) (prefix is specific to the halogen in the molecule) chloromethane
alcohols -OH -ol ethanol
aldehydes -CHO

Functional group

-al ethanal
ketones -CO-

Functional group

-one propanone
carboxylic acids -COOH

Functional group

-oic acid ethanoic acid
amines -NH2 -amine ethylamine

Functional group

-oate ethyl 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.

Family General formula
alkanes CnH2n+2
alkenes CnH2n
halogenoalkanes CnH2n+1 X (whereby X is a halogen atom)
alcohols CnH2n+1OH
aldehydes CnH2n+1CHO
ketones CnH2n+1COCmH2m+1
carboxylic acids CnH2n+1COOH
amines CnH2n+1NH2
esters CnH2n+1COOCmH2m+1