AQA AS Chemistry Unit 2: Haloalkanes
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- Created on: 01-05-13 23:49
Chemistry Unit 2: Haloalkanes
Haloalkane Introduction
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General Formula: CnH2n+1X
Bond Polarity
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Haloalkanes have C-X bonds, C-X bonds are polar due to halogens being more electronegative than carbon
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Down the group the bonds get less polar
Physical Properties: Solubility
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The polar bonds aren't polar enough to make haloalkanes soluble in water
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Intermolecular forces are dipole-dipole attraction and van der Waals forces
Boiling Point
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Increases with increased chain length and when you go down the halogen group
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This is due to increased electron number which increases van der Waals forces
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Melting point is lowered if the haloalkanes are branched
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Haloalkanes have higher BP to alkanes due to higher molecular masses and being polar
How Haloalkanes react: the reactivity of the C-X bond
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Factors that determnine how readily the C-X bond reacts:
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the C-X bond polarity and the C-X bond enthalpy
Bond Polarity
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Carbon on the C-X bond is partial positive in charge due to the halogen being more electronegative so it is electron deficient
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It can be attacked by reagents called nucleophiles
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Nucleophile: an electron pair donor
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The more positive the charge of C the more easily attacked by a nucleophile
Bond Enthalpies
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Bond is weaker down the group due to the shared electrons in the C-X bond being further away from the Halogen nucleus
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Experiments show reactivity increases as we go down the group so bond enthalpy is more important than bond polarity
Nucleophilic Substitution
Nucleophiles:
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Reagents that attack and form bonds with positively charged carbon atoms
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It is a negatively charged ion or atom with a delta minus charge
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It has a lone pair which it can use to form a covalent bond
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It is a species that can donate its lone pair to an electron deficient carbon atom
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Common: Hydroxide Ion, Ammonia, Cyanide Ion
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When they replace a halogen in a haloalkane this in nucleophilic substitution
The Nucleophilic Substitution
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A curly arrow is drawn from the lone pair on the nucleophile to the partially positive carbon
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Another curly arrow is draw from the carbon-halogen bond to the halogen forming a halide ion (This is the leaving group)
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Going down the halogen group the rate of reaction increases as the C-Halogen bond strength decreases
Haloalkanes with Hydroxide Ions
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Conditions: Room temperature, Ethanol solvent in which the haloalkane and ions mix
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This reaction is called a hydrolysis reaction
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It forms an alcohol
Haloalkanes with Cyanide Ions
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Product: Nitrile
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On a nitrile there is a triple bond between the carbon and the…
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