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Halogenoalkanes (13.3: Elimination Reactions in Halogenoalkanes (A…
Halogenoalkanes
13.3: Elimination Reactions in Halogenoalkanes
A hydrogen halide is eliminated from the molecule, leaving a double bond so an alkene can form.
OH- ion acting as a base: Removes H+ ion from halogenoalkane (elimination). E.g. bromoethane with potassium hydroxide to form ethane. HBr is eliminated then they react. Reaction produces ethene, potassium bromide and water.
Condition of reaction: Potassium hydroxide dissolved in ethanol and mixed with halogenoalkane.
NO WATER PRESENT
Mixture is heated and produces ethene which decolorises bromine
Mechanism of Elimination: OH- ion gives lone pair to bond with hydrogen atom which are slightly d+
Electron pair from C--H becomes part of double bond
Bromine takes electrons and leaves as Br
This is useful for making C=C
Substitution or elimination? OH- ion reacts with HAs as a nucleophile OR a base, there is competition between sub and el. OH- ions at room temp dissolved in water favour sub. OH- ions at high temp dissolved in ethanol favour el. Primary HAs react sub and tertiary react el. Secondary do both.
CFCs: Unreactive in normal conditions. Short chain are gases and long chain are used in dry cleaning. They end up in the atmosphere and decompose ozone causing a hole in the layer. CFCs are being replaced by HCFCs hydrofluorochlorocarbons which are safer
13.1: Halogenoalkanes Intro
General formula: CnH2n+1X where X is halogenoalkane, or R--X
Bond Polarity: C--X bond in which C is d+ and X is d- as halogens are more electronegative than C. As you go down the group the bonds get less polar.
Physical Properties
Solubility:
-Polar bonds not polar enough to make halogenoalkanes soluble in water
-Main IM forces are dipole-dipole and van der waal forces
-Mix with HCs in dry cleanings fluids to remove oily stains (oil is a mixture of HCs)
Boiling Point:
-Increases with increased chain length
-Increases going down the halogen group
Both caused by increased van der waals forces
Branching lowers melting point
-Halogenoalkanes have higher BP's than alkanes with same chain length as they have higher Mr's and are more polar
How they React - Reactivity of C--X Bond
C--X bond almost always breaks in a reaction
There are 2 factors which determine this; Cd+--Xd-bond polarity and C---X bond enthalpy
Bond Polarity:
C has a partial positive charge so it is electron deficient. This can be attacked by electron rich reagents called
nucleophiles
, electron pair donors.
C--F bond is most polar (C has most positive charge) so it is most easily attacked by nucleophile. This makes C--I bond least reactive as it is least polar
Bond Enthalpies:
Bonds get weaker down the group.
F is the smallest atom and the shared electrons in the C--F bond are strongly attracted to the F nucleus which makes a strong bond.
Going down the group the shared electrons get further away from the X nucleus so the bond gets weaker.
This predicts iodo-compounds with weakest bonds are most recative and fluoro-compounds with strongest bonds are least reactive
Reactivity increases down group so bond enthalpy is more important than bond polarity
13.2: Nucleophilic Substitution in Halogenoalkanes
Nucleophiles
Reagents that attack and form bonds with positively or partially positively charged C atoms
Either negative charged ion or has an atoms with d- charge
Has a lone pair of electrons which it can use to form a covalent bond (dative)
Lone pair is on an electronegative atom
Common nucleophiles: -:OH hydroxide ion
:NH3 ammonia
-:CN cyanide ion
They replace halogens in halogenoalkanes (nucleophilic substitution)
Nucleophilic Substitution
Curly arrows used to show how electron pairs move. Lone pairs are attracted to partially positive C atoms. Arrow starts at lone pair and moves towards Cd+
The halide ion is the leaving group
Rate of substitution depends on halogen. Rate of reaction increases down the group as strength of C--X bond decreases.
Examples (look in book)
Uses: Introduce new functional groups into compounds. Halogenoalkanes can be converted into alcohols, amines and nitriles which can be converted into other functional groups.