alkanes- chapter 12 Flashcards
what are alkanes
- the main components of natural gas and crude oil
- mainly used as fuels, exploiting their reaction with oxygen to generate heat
what is the general formula of alkanes
CnH2n+2
what type of covalent bond are in alkanes
a sigma bond
what is a sigma bond the result of
- two orbitals, one from each bonding atom
- each sigma bond has two electrons that are shared between the bonding atoms
- its positioned on a line directly between bonding atoms
what is the shape of alkanes
4 sigma bonds
repulsion between the electron pairs results in a 3D tetrahedral arrangement
each bond angle is 109.5
can alkanes rotate freely or not?
yes, as the sigma bonds act as axes which they can freely rotate around and are not rigid
does boiling point increase or decrease in alkanes? and why or why not?
increases as..
there are weak intermolecular forces called London forces
these forces hold molecules together in solids and liquids but once broken, the molecules move apart from each other and the alkane becomes a gas
the greater the intermolecular forces, the higher the boiling point
how is crude oil separated and why
into fractions by fractional distillation in a distillation tower, each fraction contains a range of alkanes
- can do separation like this because the boiling points of alkanes are different, increasing as their chain length increase
what is the effect of chain length on boiling points in alkanes
as the chain length increases, the molecules have a larger surface area, so more surface contact is possible between the molecules
the london forces between the molecules will be greater and so more energy is required to overcome the forces
what is the effect of branching on boiling points in alkanes
branched isomers compared to straight chain isomers have lower boiling points
this is because there are fewer surface points of contact between molecules of the branched alkanes, giving fewer london forces
another factor is because of the shape of the molecules, the branches get in the way and prevent the branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further
why don’t alkanes react with most common reagents
C-C and C-H sigma bonds are strong
C-H bonds are non polar
the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar
why are alkanes used as fuels
as they are readily available, easy to transport and burn into a plentiful supply of oxygen without releasing toxic products
what is given off in a complete combustion reaction
carbon dioxide and water
what is given off in incomplete combustion of alkanes
carbon monoxide or soot and water
when do alkanes react with halogens
in the presence of sunlight, the UV present provides the initial reaction to take place
what is the mechanism for brommination of alkanes and the 3 steps
radical substitution
initiation, propagation and termination
explain the initiation stage in bromination of alkanes
the reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission
each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals
the energy for this bond fission is provided by UV radiation
explain propagation in bromination of alkanes
the reaction propagates through two propagation steps, a chain reaction
- in the first step, a bromine radical reacts with a C-H bond in the methane, forming a metal radical and a molecule of hydrogen bromide
- in the second step, each methyl radical reacts with another bromine molecules, forming an organic product bromomethane, together with a new bromine radical
the new bromine radical then reacts with another CH4 molecule as in the first step, and the two steps can continue to cycle
explain termination in bromination of alkanes
the two radicals collide, forming a molecule with all electron paired
when two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction
what is further substitution of bromination of alkanes
another bromine radical can collide with a broom ethane molecules, substitution a further hydrogen atom to form dibromoethane
further substituting can continue until all hydrogen atoms have been substituted
what happens when the carbon chain is longer (substitution)
can get mixture of mono substituted isomers by substitution at different positions in the carbon chain
