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

Nucleophiles:

• 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…