B3.5 - Halogenoalkanes
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Define the term nucleophile
a nucleophile is a species that can donate a lone pair of electrons
What is the trend in boiling points of halogenoalkanes in terms of mass/going down group 7?
boiling point increases as mass increases increase strength of London forces as molecular mass and size of the halogen increase - more electrons electron cloud is more polarizable
What is the trend in boiling points of halogenoalkanes in terms of chain length?
increased chain length greater SA of contact so stronger IMF boiling point increases
What is the trend in boiling points of halogenoalkanes in terms of branching?
the more branching the less SA of contact so weaker IMF and the lower the boiling point
Are halogenoalkanes soluble in organic solvents?
yes
Are halogenoalkanes soluble in water?
no - cannot form H-bonds with water immiscible with water layers would form
Define the term electronegativity.
a measure of the attraction between a bonded atom and the pair of electrons in a covalent bond
What kind of IMF’s exist between halogenoalkane molecules?
London forces permanent dipole-dipole interactions
Explain the polarity of halogenoalkanes.
C-X bond is polar (X=halogen) if the molecule is asymmetrical then the molecule is polar if the molecule is symmetrical then the molecule is non-polar
What is the trend in electronegativity/polarity of the halogens down the group?
electronegativity decreases resulting in decrease in polarity of the carbon-halogen bond
What are the 4 examples of nucleophiles?
Hydroxide ions - :OH- Water - H2O- Nitrite ions - :CN- Ammonia - -NH3
What can halogenoalkanes make when hydrolysed?
alcohols
What type of reaction is halogenoalkane –> alcohol?
substitution reaction where X is replaces by OH
What is the definition of a substitution reaction?
in a substitution reaction is a reaction in which one atom is replaced by another atom or group
What are the reactants for the hydrolysis of halogenoalkanes?
hot aqueous (dilute) potassium hydroxide or sodium hydroxide
What are the conditions for the hydrolysis of halogenoalkanes?
heat under reflux
What is the mechanism for the hydrolysis of halogenoalkanes called?
nucleophilic substitution
Write down the mechanism with a primary halogenoalkane - 1-bromobutane.
:OH- lone pair onto carbon atom bond between C-Br onto Br atom products - alcohol and Br-
What type of bond fission occurs during nucleophilic substitution and why?
heterolytic bond fission both electrons move onto the halogen, forming a halide ion
Reaction equations with different nucleophiles: OH-
forms alcohol and halide ion
Reaction equations with different nucleophiles: H2O-
forms alcohol and hydrogen-halogen
Explain the different in the reaction rate when using OH- or H2O-.
slower when using H2O- H2O- is a much weaker nucleophile than OH-
Reaction equations with different nucleophiles: excess ammonia/ethanol heated in a sealed tube
forms primary amide and ammonium-halogen
Reaction equations with different nucleophiles: CN-/ethanol
forms nitrite (triple bond) and halide ion