Chapter 12 - Alkanes Flashcards
What are alkanes?
Alkanes are the main components of natural gas and crude oil
How stable are alkanes?
They are among the most stable organic compounds, and their lack of reactivity has allowed crude oil deposits to remain in the earth for millions of years
What are alkanes mainly used as?
Fuels
What is the general formula of alkanes?
CnH2n+2
What type of hydrocarbons are alkanes?
Saturated hydrocarbons, as they only contain single covalent bonds
What type of covalent bond is present in alkanes?
Sigma bonds
What is a covalent bond defined as?
A shared pair of electrons
What is a sigma bond?
The result of the overlap of two orbitals, one from each bonding atom
How many electrons are in a sigma bond?
Each overlapping orbital contains 1 electron, so the sigma bond has two electrons (that are shared between the bonding atoms)
Where is a sigma bond positioned?
On a line directly between bonding atoms
How many sigma bonds does each carbon atom in an alkane have?
Four
What is the shape of alkanes?
- each carbon atom is surrounded by four electron pairs in four sigma bonds
- repulsion in these bonds results in a 3D tetrahedral arrangement with a bond angle of 109.5
Are the shapes of alkanes rigid?
The sigma bonds act as axes around which the atoms can rotate freely, so the shapes are not rigid
How is crude oil separated into alkanes?
- the crude oil is separated into fractions by fractional distillation in a distillation tower
- each fraction contains a range of alkanes
- alkanes have different boiling points, increasing as their chain length increases
Why do the boiling points of alkanes increase as they get larger?
- there are more electrons/larger surface area in larger alkanes
- strength of London forces increases as number of electrons/surface area increases
- because London forces are stronger more energy is needed to break them
- so boiling point is higher
What is the effect of branching on boiling point?
- there are fewer surface points of contact between molecules of the branched alkanes
- so there are fewer London forces
- the branches also prevent the branched molecules getting as close together as straight-chain molecules, decreasing the London forces further
Why are alkanes relatively unreactive?
- C-C and C-H sigma bonds are strong
- C-C bonds are non-polar
- the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
What happens when alkanes react with oxygen?
- all alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water
- this is called combustion
- this gives out heat
Why are alkanes used as fuels?
- they are readily available
- they are easy to transport
- they burn in a plentiful supply of oxygen without releasing toxic product
What is the chemical equation for the complete combustion of methane?
CH4 (g) + 2O2 (g) -> CO2 (g) + H2O (l)
What is the algebraic chemical equation for the complete combustion of any alkane?
CxHy + (x + y/4)O2 -> xCO2 + y/2H2O
What happens when there is a limited supply of oxygen reacting with the alkane?
- incomplete combustion occurs
- carbon monoxide or carbon can be formed
What are the conditions needed for alkanes to react with halogens?
UV light
What happens during the reaction of alkanes with halogens?
- a substitution reaction occurs
- a hydrogen atom in the alkane has been substituted by a halogen atom
What are the three steps of radical substitution of alkanes?
- Initiation
- Propagation
- Termination
What happens during the initiation stage during radical substitution of alkanes?
- the covalent bond in a bromine (example) molecule is broken by homolytic fission
- each bromine (example) atom takes on electron from the pair, forming two highly reactive radicals
- the energy for this bond fission is provided by UV radiation
What is a radical?
A highly reactive species with an unpaired electron
How is a radical shown when drawing?
The radical is shown with a single dot to represent the electron
What happens during the propagation stage during radical substitution of alkanes? (Using bromine as an example)
- the reaction propagates through two propagation steps (a chain reaction):
1. In the first propagation step, a bromine radical reacts with a C-H bond in the methane, forming a methyl radical and a molecule of hydrogen bromide
2. In the second propagation step, each methyl radical reacts with another bromine molecule, forming bromomethane (CH3Br), together with a new bromine radical - the steps continue as the new bromine radical reacts with another methane
What happens during the termination stage during radical substitution of alkanes?
Two radicals collide, forming a molecule with all electrons paired
What are the limitations of radical substitution in organic synthesis?
- further substitution
- substitution at different positions in a carbon chain
How does further substitution limit radical substitution in organic synthesis?
- another bromine radical could collide with a bromomethane (CH3Br) molecule, substituting a further hydrogen atom to form dibromomethane (CH2Br2)
- further substitution can continue until all hydrogen atoms have been substituted
- the result is a mixture of compounds
How does substitution at different positions in a carbon chain limit radical substitution in organic synthesis?
- for methane, all four hydrogen atoms are bonded to the same carbon atom, so only one monobromo compound is possible
- if the carbon chain is longer, we will get a mixture of mono-substituted isomers by substitution at different positions in the carbon chain
