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4.1 - 4.7 Organic chemistry

 4.1 - 4.3. Activity 1. Speaking Organically

DNA is a very complicated organic molecule

Organic chemistry is concerned with the compounds of the element carbon. Carbon atoms can form four covalent bonds with a range of other elements

Carbon chains form longer molecules and branches create a greater complexity. 

Atoms of hydrogen, nitrogen and oxygen will bond readily with carbon atoms and can ultimately form the molecules of life including amino acids and proteins.


 4.1 - 4.3. Activity 2. Alkanes, Alkenes and other terms...

 Students should:

  • 4.1 know that a hydrocarbon is a compound of hydrogen and carbon only
  • 4.2 understand how to represent organic molecules using empirical, molecular, general, structural and displayed formulae
  • 4.3 know what is meant by the terms homologous series, functional group and isomerism

When carbon atoms bond they usually form 4 covalent bonds. Hydrogen atoms form one bond. Thus the simplest hydrocarbon molecule which can form is methane. This has the formula CH4 .

Watch both of the videos carefully pausing where instructed. Use the videos to answer the questions below:

  1. What is a hydrocarbon?
  2. ​Why are alkanes referred to as saturated hydrocarbons?
  3. What is the general formula of the alkanes?
  4. What is the meaning of the term "homologous series".
  5. Why are alkenes known as unsaturated hydrocarbons?
  6. What is the general formula for the alkenes?
  7. What is a functional group?
  8. What can you say about the properties of the compounds in a homologous series?
  9. What is organic chemistry?

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 4.3 Activity 3. Isomers: Same atoms, different arrangements

a mixture of some of the hydrocarbons in crude oil

​Isomers are two or more molecules which have the same atoms ( and therefore the same molecular formula) but a different arrangement of atoms. We need to use structural or displayed formulae to show the differences between isomers:

Molecule d is methane - this has the molecular formula CH4

Its displayed formula can be written as shown here.

Displayed formulae show all the bonds in a molecule and can therefore be used to distinguish two or more isomers from each other.

When hydrocarbon molecules contain more than three carbon atoms branching can occur and so isomers can exist

​The large image shows models of a number of hydrocarbon molecules. 

  • Work out and write down the molecular formula for each of the molecules.
  • Which two molecules are isomers of one another?
  • Draw displayed formulae of each of these two isomers to show their differences

​Please log in to see the answers and explanations.


 4.3 Activity 4. Functional Groups

ethanol - ball and stick model
ethanol - displayed formula

Ethanol has the molecular formula C2H6O.  The molecule contains the - OH functional group. This is shown by the semi structural formula ;C2H5OH and the fully structural formula CH3CH2OH


​ethanoic acid has the -COOH functional group

Functional groups are specific groupings of atoms within molecules that have their own characteristic properties. The functional_group in an alcohol is the "OH" group  -OH . The functional group in a carboxylic acid is -CO2H  ( sometimes written COOH).

Molecules can contain more than one functional_group. An example of such molecules are amino acids. These contain both amine (- NH2) functional_groups and carboxylic acid functional groups. Because of the two functional groups  these molecules can join up with on another to form complicated and important molecules called proteins. 

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 4.4 Activity 5. It's in the name

butane : molecular formula C4H10

3D ball and spoke model 

 fully displayed formula ( straight chain)

methylpropane : molecular formula C4H10

3D ball and spoke model  

fully displayed formula ( branched chain) 

 Students should:

  • 4.4 understand how to name compounds relevant to this specification using the rules of International Union of Pure and Applied Chemistry (IUPAC) nomenclature

​Organic molecules can become complicated. Because of this we have an internationally agreed system for naming them. 

Organic molecules are named using a prefix according to the number of carbon atoms in the longest carbon chain in the molecule. See table below. For the alkanes the suffix is "ane". Thus the alkane with one carbon atom is called methane

​number of carbon atoms ​prefix used ​formula of alkane name​​number of isomers
​1 ​meth ​CH4 methane​​1
​2​ethC2H​6ethane​1
​3​propC3H​8propane​​1
​4​butaneC4H10​butane​2
​5​pentC5H12​pentane3
​6​hexC6H14hexane​5
​n-​CnH2n+2​general formula​

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 Activity 6

 Students should:

  • 4.5 understand how to write the possible structural and displayed formulae of an organic molecule given its molecular formula

​Molecules are three dimensional particles whose shapes can have important effects . Formulae are used to show the number and type of atoms present in a molecule.  The best type of formula to use depends upon what you are trying to show. 

The image shown below is a 3D representation of a glucose molecule.

Watch the video carefully then tackle the question.

Look at this 3 dimensional representation of a molecule. Use your cursor to rotate and your scroll wheel to zoom in. 

Represent this molecule as a :

  1. displayed formula
  2. structural formula 
  3. molecular formula
  4. empirical formula
please log in to see answers and explanations
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