4.1.2 Alkanes Flashcards
(14 cards)
What type of bonds are present in alkanes?
Alkanes contain single C-C and C-H bonds which are σ-bonds formed by the direct overlap of orbitals between bonding atoms.
What allows free rotation in alkanes?
Free rotation occurs around σ-bonds in alkanes.
What is the shape and bond angle around each carbon atom in alkanes?
The shape is tetrahedral with a bond angle of 109.5° explained by electron pair repulsion theory.
How does carbon-chain length affect the boiling points of alkanes?
Longer carbon chains increase the boiling point due to stronger London forces caused by more surface area and electrons.
How does branching affect the boiling points of alkanes?
Increased branching lowers the boiling point as it reduces the surface area for London forces to act.
Why are alkanes generally unreactive?
Alkanes are unreactive due to their high bond enthalpy and the very low polarity of the σ-bonds.
What are the products of the complete combustion of alkanes?
Complete combustion of alkanes produces carbon dioxide (CO2) and water (H2O).
What are the dangers of incomplete combustion of alkanes?
Incomplete combustion produces carbon monoxide (CO) a toxic gas that can bind to haemoglobin and prevent oxygen transport.
What is required for alkanes to react with chlorine or bromine?
Alkanes react with chlorine or bromine via radical substitution requiring ultraviolet radiation to initiate the reaction.
What happens during the initiation step of radical substitution?
UV radiation causes homolytic fission of Cl2 or Br2 producing two radicals (e.g. Cl*).
What happens during the propagation steps of radical substitution?
Radicals react with alkanes to form new radicals and molecules (e.g. CH4 + Cl* → CH3* + HCl CH3* + Cl2 → CH3Cl + Cl*).
What happens during the termination step of radical substitution?
Radicals combine to form stable molecules (e.g. Cl* + Cl* → Cl2 or CH3* + CH3* → C2H6).
What are the limitations of radical substitution in synthesis?
Radical substitution produces a mixture of products due to further substitution and reactions at different positions in the carbon chain.
What must be included in equations for radical reactions?
Radicals must be shown with a single dot (*) representing the unpaired electron.
