Chapter 12: Alkanes (12.1-12.2)
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general formula of alkanes
CnH2n+2
alkanes are saturated hydrocarbons, what does this mean
alkanes have single covalent bonds and contain single carbon-hydrogen bonds only (sigma bonds)
properties of sigma bonds
formed from the overlap of orbitals directly between the bonding atoms can rotate freely
shape and bond angle of alkane
tetrahedral 109.5
boiling point trends in straight-chain alkanes
as the carbon chain length increases, the boiling point increases more electrons and more points of contact between molecules means strength of London forces between molecules increases. More energy is needed to break IMFs
boiling point trends in branched-chain alkanes
as branching in isomers increases, the boiling point decreases fewer points of contact between molecules means strength of London forces decreases less energy needed to break IMFs
solubility of alkanes
alkanes are non-polar molecules not soluble in polar solvents (e.g. water). soluble in non-polar solvents (e.g. oil)
reactivity of alkanes
low reactivity sigma bonds (C-C and C-H) are have high enthalpy (strong, hard to break) C-C and H-H bonds are non-polar (making them less reactive)
what is a fuel
fuels are substances that can be burnt in oxygen to release heat energy
how does an alkane act as a fuel
although unreactive, many alkanes react exothermically with oxygen to make good fuels
alkane + oxygen –> excess oxygen, complete combustion
CO2 (g) + H2O (l)
alkane + oxygen –> excess alkane, incomplete combustion
CO (g) + H2O (l) carbon only undergoes partial oxidation, unburnt carbon atoms can be released as carbon particles (soot)
dangers of carbon monoxide
toxic gas that binds with haemoglobin in red blood cells, preventing haemoglobin from carrying oxygen difficult to detect: colourless and odourless
reactions of alkanes with halogens
alkane + halogen –> haloalkane + hydrogen-hailide radical substitution, needs UV light to start
steps of radical substitution (alkane + halogen)
1) initiation. UV radiation provides energy for homolytic fission of halogen molecule to form 2 radicals 2) propagation. 2 reactions of radicals reacting with molecules to form a molecule and a new radical (radical will be on carbon a molecule or a lone halogen) 3) termination. 3 reactions of 2 radicals reacting to form a molecule
limitations of radical substitution for organic synthesis (production of organic molecules on an industrial level)
a large mixture of products can be formed: Further substitution: the main product will still contain hydrogen atoms (easily replaced) Structural isomers: for alkanes with a chain of 3 or more C atoms, substitution can take place at different points along the chain giving different structural isomers