3.3.3 Halogenoalkanes
Sammer Sheikh
Nucleophilic substitution:
contain polar bonds
OH⁻
CN⁻
NH₃
carbon-halogen bond enthalpy affects rate of reaction: bond gets weaker down the group as the shared electrons in the C-X bond are further away from the from the halogen nucleus; weaker bond means more reactive halogen, and faster rate of reaction
Elimination:
OH⁻ as a base: removes H⁺ ion from the halogenoalkane
Ozone depletion:
Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation.
Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break.
Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.
Appreciate that results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants. Chemists have now developed alternative chlorine-free compounds e.g. HCFCs
Cl• + O₃ → ClO• + O₂
ClO• + O₃ → 2O₂ + Cl•
Properties:
contain polar Cδ+ --- Xδ- bond as halogens are more electronegative than carbon; bond polarity decreases down the group; C is electron deficient
insoluble in water: Cδ+ --- Xδ- bond is not polar enough
main intermolecular forces of attraction are dipole-diploe interactions and van der Waal forces
boiling point:
increase with chain length
increase down the halogen group
decrease with branching
effects caused by increased vdw forces; larger molecules means more electrons
higher than alkanes with similar chain lengths; Mr is larger; more polar
nucleophile: electron pair donor; can be attacked by electron rich reagents; negatively charged ion or has an atom with a δ- charge; has a lone pair of electrons which can be used to form a covalent bond; lone pair is on an electronegative atom
uses: diagram from textbook
a hydrogen halide is eliminated from the molecule, leaving a double bond, forming an alkene
conditions:
sodium/potassium hydroxide dissolved in ethanol
no water
heated
mechanism
conditions:
aqueous
room temperature
(primary and secondary halogenoalkanes)
(tertiary and secondary halogenoalkanes)
CFCs
unreactive under normal conditions
short chain ones are gaseous; used as refrigerants, aerosol
longer chain ones used as dry cleaning an de-greasing solvents
end up in atmosphere and decompose to give chlorine atoms