AQA AS Chemistry Unit 2: Alkenes

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Chemistry Unit 2: Alkenes

Alkenes are unsaturated hydrocarbons

  • Made only of Carbon and Hydrogen

  • They have one or more Carbon Double Bonds

  • The double bond makes the molecule more reactive then alkanes

  • The general formula: CnH2n

Shape of Alkenes

  • Ethene is a planar molecule so its angles between each bond is 120 degree

  • There is no rotation around the double bond

  • There is no rotation due to the pi bond formed from the cloud of electron density above and below the single bond


  • Alkenes with more than 3 carbons can form different isomers

  • Two types of isomers can be formed involving the double bond:

    • Position Isomers

    • Geometrical Isomers

  • Position isomers are where the double bond is positioned differently

  • Geometrical isomers are a form of stereoisomerism, they stereoisomers have the same structural formula but different bonds are arranged differently, it occurs around the double bond

  • When two types of the same group e.g. -CH3 are on the same side of a double bond this is called Z and when the groups are on opposite sides this called E

    • When atoms of higher atomic number are on the same side this is Z

  • When atoms of higher atomic number are opposite sides this is E

Physical Properties

  • Van der Waals are the only intermolecular forces acting between alkene molecules

  • Physical properties are similar to alkanes

  • The melting and boiling point increase with the number of carbon atom

  • Alkenes are not soluble in water as they are non-polar

How Alkenes React

  • The bond enthaply for a C=C bond is almost double of a C-C bond

  • Alkenes are more reactive than alkanes

  • C=C forms an electron rich area in a molecule which can be easily attacked by a positively charged reagent called an electrophile

  • Electrophiles are electron pair acceptors such as H+

  • The reaction is known as electrophilic addition

Reactions of Alkenes

Alkenes can burn in air to form CO2 and H2O

Electrophilic Addition Reaction

  • The four electrons in the C=C bond make an alkene a centre of high electron density

  • Electrophiles are attracted to it and form a bond by using 2 of its 4 electrons in the C=C bond




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