Alkenes (SQA National 5 Chemistry): Revision Note

The alkenes

What are alkenes?

• Alkenes are another family of hydrocarbons

  • For a full table of the first alkenes and their structures, see the Full & shortened structural formulae

• Alkenes are a homologous series with four key facts:

1. They are unsaturated hydrocarbons

• This means they contain only carbon and hydrogen atoms

• At least one of the carbon-carbon bonds in their structure is a double bond (C=C)

  • The C=C double bond is the functional group

  • It is the site of most of their chemical reactions and makes alkenes more reactive than alkanes

2. They are represented by a general formula

• The general formula for the alkene homologous series is C_nH_2n

• This is the same general formula as the cycloalkanes, which means that alkenes and cycloalkanes with the same number of carbon atoms are isomers.

  • For example, propene and cyclopropane both have the molecular formula C₃H₆

3. They are used to make polymers and alochols

• Alkenes are incredibly important industrial chemicals

• They are the monomers used to make many common plastics (polymers) in addition polymerisation reactions

• They can also be reacted with water (in a process called hydration) to manufacture alcohol

4. They are insoluble in water

• Just like other hydrocarbons, alkenes do not mix with water

Names & formulae of alkenes

• Like other organic compounds, alkenes are named systematically

• There are three skills for alkenes:

  • Naming alkenes from their structure

  • Drawing alkenes from their name

  • Determining the formula of an alkene

1. Naming alkenes

• The name is made of three parts:

  1. A prefix to show the number of carbon atoms

  2. A number to show where the double bond starts

  3. The ending -ene to show it is an alkene

• The smallest possible alkene must have at least two carbon atoms to form the C=C double bond

  • Therefore, the first member of the homologous series of alkenes is ethene

• The rules for naming alkenes:

  1. Find the longest continuous chain of carbon atoms

     • This chain must contain the carbon-carbon double bond, C=C

     • The longest continuous chain gives you the prefix of the main chain

       • For example, if the longest chain is 4 carbons, the name starts with "but-"

     • The carbon-carbon double bond, C=C, means that the name ends with "-ene"

  2. Number the carbon atoms

     • This is only needed if there are 4 or more carbon atoms

     • Start from the end that gives the C=C double bond the lowest possible number

     • The number in the name refers to the first carbon the double bond is attached to

     • For example, if the carbon-carbon double bond, C=C, is between carbons 4 and 5 in the chain, then the name contains "-4-"

  3. Combine the parts to make the full name

     • Write the main chain prefix

     • Add a hyphen

     • Write the number of the first carbon the double bond is attached to

     • Add a hyphen

     • Write the "-ene" ending for an alkene

  4. If there are branches, you name them in the same way as for alkanes

2. Drawing alkenes

• You can work backwards from the name to draw the structure of an alkene

• The rules for drawing alkenes:

  1. Identify the longest carbon chain

     • Identify the number of carbons in the longest chain from the alkene part of the name

     • For example, "-propene" means a 3-carbon chain

  2. Draw the main carbon chain

     • Initially, make all the carbon-carbon bonds single

  3. Identify the carbon-carbon double bond:

     • Identify the first carbon atom of the double bond from the number in the name

     • For example, but-1-ene means the double bond starts on carbon 1, which means it is between carbons 1 and 2

  4. Add the carbon-carbon double bond

  5. Add the hydrogens to complete the structure

     • Make sure every carbon atom has exactly four bonds

3. The molecular formula of alkenes

• The general formula of alkenes can be used to determine the molecular formula