4.1.2 Alkanes
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what are alkanes mainly used for and why
• mainly used as fuels as they lack reactivity > combine with O2 to generate heat
what are alkanes the main components of
• natural gas + crude oil > their lack of reactivity has allowed crude oil deposits to remain on earth for millions of years
what type of hydrocarbon is an alakne
• saturated > contains single carbon to carbon bonds
what is the type of covalent bond that allows carbon to join to 4 other atoms by single bonds
• sigma bond
what is a sigma bond
• is the result of the overlap of 2 orbitals • each overlapping orbital contains one electron so sigma bond has 2 electrons shared between bonding atoms
what is the shape + angles of an alkane
• repulsion between electron pairs results in tetrahedral shape • 109.5
what is the general formula for an alkane
CnH2n+2
describe the variations in boiling points of alkanes in terms of chain length
• as chain length increases so do bp of alkanes > molecules have larger SA so more surface contact available between molecules > greater London forces between molecules so more energy required to overcome forces
crude oil has no use in its raw for, how can it be useful
• crude oil contains many diff alkanes • fractional distillation can be used due to the diff boiling points of alkanes as they vary with chain length
describe the effect of branching of alkanes on boiling point
• branched isomers of alkanes with same molecular formula have lower boiling point than straight chained alkanes > with branched isomers, there are less surface points of contact so fewer London forces > also branches get in the way + prevent molecules form getting as close together as straight chain molecules so weaker IMF
why are alkanes not reactive
• C-C and C-H sigma bonds are strong • C-C bonds are non-polar • electronegativity of C and H2 is so similiar that the C-H bond can be considered non-polar
alkanes can react with ….. to produce …. the reaction is called combustion
• a plentiful supply of oxygen • CO2 + H2O
why are alkanes used as fuels
• combustion gives out heat • alkanes used as they’re readily available, easy to transport and burn in plentiful supply of O2 without releasing toxic products
what happens in incomplete combustion of alkanes
• limited supply of O2 so you produce: > H2 atoms always oxidise to water H2O > incomplete combustion of C can form CO or C as soot
why is CO considered toxic
• colourless, odorless highly toxic gas > combines with haemoglobin preventing it from transporting oxygen around the body
what is cracking
• the thermal decomposition of alkanes > shorter chained alkanes more useful
what is the reaction of alkanes with halogens called
• free radical substitution
what conditions are required for alkanes to undergo free radical substitution
• UV light (from sunlight) providing initial energy for reaction to take place
what are the 3 stages of free radical substitution + describe what happens in them
• initiation - covalent bond in halogen molecule broken (homolytic fission) forming 2 halogen radicals (highly reactive) • propagation - halogen radical reacts with alkane forming alkane radical + HX (halogen) alkane radical reacts with halogen molecule forming haloalkane + halogen radical • termination - 2 radicals collide forming a molecule with all electron paired
what are some limitations of radical substitution in organic synthesis
• although we can make haloalkanes, the reaction has problems such as: > further substitution such as a halogen radical reacting with the haloalkane formed + now forms a dihaloalkane > substitution at diff positions in a carbon chain leading to many isomers
what is a radical
• a very reactive species with an unpaired electron > radical shown with a single dot to represent the electron