AQA AS Chemistry Unit 2: Haloalkanes


Chemistry Unit 2: Haloalkanes

Haloalkane Introduction

  • General Formula: CnH2n+1X

Bond Polarity

  • Haloalkanes have C-X bonds, C-X bonds are polar due to halogens being more electronegative than carbon

  • Down the group the bonds get less polar

Physical Properties: Solubility

  • The polar bonds aren't polar enough to make haloalkanes soluble in water

  • Intermolecular forces are dipole-dipole attraction and van der Waals forces

Boiling Point

  • Increases with increased chain length and when you go down the halogen group

  • This is due to increased electron number which increases van der Waals forces

  • Melting point is lowered if the haloalkanes are branched

  • Haloalkanes have higher BP to alkanes due to higher molecular masses and being polar

How Haloalkanes react: the reactivity of the C-X bond

  • Factors that determnine how readily the C-X bond reacts:

  • the C-X bond polarity and the C-X bond enthalpy

Bond Polarity

  • Carbon on the C-X bond is partial positive in charge due to the halogen being more electronegative so it is electron deficient

  • It can be attacked by reagents called nucleophiles

  • Nucleophile: an electron pair donor

  • The more positive the charge of C the more easily attacked by a nucleophile

Bond Enthalpies

  • Bond is weaker down the group due to the shared electrons in the C-X bond being further away from the Halogen nucleus

  • Experiments show reactivity increases as we go down the group so bond enthalpy is more important than bond polarity

Nucleophilic Substitution


  • Reagents that attack and form bonds with positively charged carbon atoms

  • It is a negatively charged ion or atom with a delta minus charge

  • It has a lone pair which it can use to form a covalent bond

  • It is a species that can donate its lone pair to an electron deficient carbon atom

  • Common: Hydroxide Ion, Ammonia, Cyanide Ion

  • When they replace a halogen in a haloalkane this in nucleophilic substitution

The Nucleophilic Substitution

  • A curly arrow is drawn from the lone pair on the nucleophile to the partially positive carbon

  • Another curly arrow is draw from the carbon-halogen bond to the halogen forming a halide ion (This is the leaving group)

  • Going down the halogen group the rate of reaction increases as the C-Halogen bond strength decreases

Haloalkanes with Hydroxide Ions

  • Conditions: Room temperature, Ethanol solvent in which the haloalkane and ions mix

  • This reaction is called a hydrolysis reaction

  • It forms an alcohol

Haloalkanes with Cyanide Ions

  • Product: Nitrile

  • On a nitrile there is a triple bond between the carbon and the


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