8. Why are there many families of organic compounds? - variety !

(gcse revision notes on homologous series and functional groups for GCSE 9-1 chemistry exams)

Homologous Series and Functional Groups

(gcse revision notes on homologous series and functional groups for GCSE 9-1 chemistry exams)

You need to be able to explain that the vast array of natural and synthetic organic compounds occur due to the ability of carbon to form families of similar compounds, 'linear' straight chains, branched chains and rings of carbon atom all strongly bond in a variety ways with other carbon atoms and (usually) non-metallic elements including hydrogen, oxygen and nitrogen in particular e.g.

a linear chain alkane hydrocarbon, just carbon and hydrogen atoms in this organic molecule

a branched chain hydrocarbon,  just carbon and hydrogen atoms (also an alkane, methylpropane)

a hydrocarbon ring compound of carbon and hydrogen atoms (its called cyclohexane, a cyclic alkane)

an organic compound of carbon, hydrogen and oxygen atoms (ethanoic acid, a carboxylic acid)

an organic compound of carbon, hydrogen, nitrogen and oxygen atoms (an amino acid, aminoethanoic acid)

Most food is chemically organic in nature, apart from some minerals, and many drugs and plastic materials are composed of organic molecules, consequently, organic compounds and organic chemistry is rather important to us!

The term organic compound comes from the fact that most of the original organic compounds studied by scientists-chemists came from plants or animals, i.e. of natural origin. These days most organic compounds are synthesised from raw materials, in particular the physical separation and chemical manipulation of the products of crude petroleum oil.

The vast array of organic compounds, both natural and synthetic carbon compounds, occur due to the ability of carbon atoms to form families of similar compounds.

The compounds in each family have a similar chemical structure and a similar chemical formula. Each family of organic compounds forms what is called a homologous series. and all the members behave in the same chemical way i.e. they have a common set of reactions.

Different families arise because carbon atoms readily join together in chains (catenation) and strongly bond with other atoms such as hydrogen, oxygen and nitrogen.

The result is a huge variety of 'organic compounds' which can be classified into groups of similar compounds i.e. different homologous series with particular similarities (more on this below).

'Probably' before reading this page, you should have already been introduced to at least two series of organic molecules, such as the hydrocarbons called alkanes and alkenes, which illustrate the fact that organic chemistry is the chemistry of carbon based compounds, but first we need to explain what we mean by the terms homologous series and functional group.

• A homologous series is a family of compounds which have the same general formula (*) and have a similar molecular structure and similar chemical properties because they have the same functional group of atoms (e.g. C=C alkene, C-OH alcohol or -COOH carboxylic acid).
  • Members of a  homologous series have similar physical properties such as appearance, melting/boiling points, solubility etc. BUT show trends in them e.g. steady increase in melting/boiling point with increase in carbon number or molecular mass.

  • The functional group is a group atoms common to all members of a particular homologous series that confer a particular set of characteristic chemical reactions on each member molecule of the series.

• From one member of a homologous series to the next, you add on an extra -CH₂- unit

• (*) Some examples of general formula and the functional group for four homologous series of organic molecules
  • These examples are based on a chain of just two carbons atoms, but the amply illustrate the four homologous series described.
  • Homologous series of alkanes C_nH_2n+2, where n = 1, 2, 3 etc. number of carbon atoms in the molecule
    • The diagram shows ethane, the 2nd in the series
    • Alkanes don't really have a functional group like most other homologous series.
  • Homologous series of alkenes C_nH_2n, where n = 2, 3, 4 etc. number of carbon atoms in the molecule
    • Here the functional group is the carbon - carbon double bond, >C=C<
      • The diagram shows ethene, the 1st in the series
  • Homologous series of alcohols C_nH_2n+1OH, where n = 1, 2, 3 etc. number of carbon atoms in the molecule
    • Here the functional group is the hydroxy group attached to a carbon atom, C-O-H
      • The diagram shows ethanol, the 2nd in the series
  • Homologous series of carboxylic acids C_nH_2n+1COOH, where n = 0, 1, 2 etc. number of carbon atoms in the molecule minus 1
    • Here the functional group is, COOH, a combination of one carbon, two oxygen and one hydrogen atom.
      • The diagram shows ethanoic acid, the 2nd in the series

Some more examples of the molecular structure of some homologous series you will come across ...

(e.g. in your GCSE/IGCSE chemistry science courses).

Hydrocarbons like alkanes like butane, 4th in the alkane homologous series,

and alkenes like propene, the 2nd in the alkene homologous series

which form two so called 'homologous series' or organic chemical compounds.

Advanced A Level revision notes on the structure & naming of ALKANES

Advanced A Level revision notes on the structure and naming of ALKENES

but there are many more series of organic molecules, so why such variety of organic molecules?

(gcse revision notes on homologous series and functional groups for GCSE 9-1 chemistry exams)

Organic compounds belong to different families called homologous series, though all organic compounds are based on carbon C, and usually hydrogen H, the carbon atoms do combine with other elements e.g.

oxygen O e.g. alcohols like methanol , the 1st in the alcohol homologous series, hydroxy functional group -OH

Advanced A Level revision notes on the structure and naming of ALCOHOLS

carboxylic acids like propanoic acid, 3rd in the carboxylic acid homologous series, functional group is -COOH

Advanced Level revision notes on the structure and naming of CARBOXYLIC ACIDS & ESTERS

and esters like ethyl ethanoate, in the ester homologous series

nitrogen N e.g. in H₂NCH₂COOH the simplest of the amino acids

halogens like fluorine, chlorine, bromine and iodine can combine with carbon e.g.

chloromethane, no hydrogen!,

bromoethane, iodopropane

producing lots of homologous series like chloro-alkanes etc.

Advanced Level revision notes on the structure and naming of HALOGENOALKANES

Simplified displayed formula for the first four or five members of the four homologous series of organic compounds you are likely to come across (UK GCSE level chemistry students)

alkanes have no functional group, alkenes have the >C=C< double bond functional group,

alcohols have the C-OH functional group and carboxylic acids have the -COOH functional group

Notice as you move from one member of a homologous series to the next, you add on an extra -CH₂- unit

Carboxylic acids

The two series of displayed formulae for alkenes and carboxylic acids are a more accurate representation of the structure of the carboxylic acid molecules

Some notes on molecular formula and molecular structure

(gcse revision notes on homologous series and functional groups for GCSE 9-1 chemistry exams)

• The molecular formula represents a summary of all the atoms in the molecule e.g. butane is C₄H₁₀.
• The structural formula or displayed formula shows the full structure of the molecule with all the individual bonds and atoms shown (though there are different 'sub-styles' of varying detail)
  • e.g.   all represent ethane
  • The full displayed formula must show all the bonds in the molecule so that connection between every atom can be clearly seen, as in the 3rd and 4th diagrams above, and the other examples below.
• When a specific group of atoms in a molecule give it a particular set of characteristic chemical reactions, that group of atoms is called the functional group of the molecule. Examples of functional groups include:
  • the double carbon-carbon bond >C=C< in alkenes,
    • eg and
  • the oxygen-hydrogen atom group of the C-OH structure in alcohols,
    • eg   and 
  • and the group of four atoms constituting the -COOH and -COOC- groups in carboxylic acids and esters respectively.
    • eg carboxylic acid  and  ester
• NOTE: Inorganic compounds are all the other compounds not based on carbon, except for carbon monoxide CO, carbon dioxide CO₂ and carbonates, which are also considered to be inorganic.
  • Examples of inorganic compounds: water H₂O, ammonia NH₃, sodium chloride NaCl, calcium carbonate CaCO₃

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... Ex OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Ex Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes