Chemical reactions of the alkanes Flashcards
Are alkanes reactive?
No - don’t react with most common reagents
What are the three reasons for why alkanes lack reactivity?
1) C-C and C-H sigma bonds are strong
2) C-C bonds are non polar
3) The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar
What type of reaction do alkanes undergo?
Combustion
How do alkanes combust?
They react with a plentiful supply of oxygen to produce carbon dioxide and water
What do combustion reactions give out?
Heat
Why are alkanes used as fuels? (3 reasons)
1) Readily available
2) Easy to transport
3) Burn in a plentiful supply of oxygen without releasing toxic products
What do alkanes do in a plentiful supply of oxygen?
They burn completely to produce carbon dioxide and water - complete combustion
What do alkanes do in a limited supply of oxygen?
Hydrogen atoms are oxidised to water, but combustion of carbon may be incomplete, forming carbon monoxide and sometimes carbon itself as soot - incomplete combustion
Why do alkanes not react fully in incomplete combustion?
There isn’t enough oxygen for complete combustion
How many additional oxygen molecules are required for complete combustion as you descend down the homologous series?
1.5 oxygen molecules
What is the equation for complete combustion of heptane?
C7H16 (l) + 11O2 (g) –> 7CO2 (g) + 8H2O (l)
What is the equation for incomplete combustion of heptane when CO is formed?
C7H16 (l) + 7.5O2 (g) –> 7CO (g) + 8H2O (l)
What is the equation for incomplete combustion of heptane when C is formed?
C7H16 (l) + 4O2 (g) –> 7C (s) + 8H2O (l)
What do alkanes react with in the presence of sunlight?
The halogens
Why do alkanes react with halogens only in the presence of sunlight?
The high-energy UV radiation present in sunlight provides the initial energy for a reaction to take place
What is a reaction mechanism?
A series of steps showing how electrons are thought to move during the reaction
What is the mechanism for bromination of methane an example of?
Radical substitution
What are the three steps of bromination of methane?
1) Initiation
2) Propagation
3) Termination
What happens in the initiation stage?
The reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission - each bromine takes one electron from the pair, forming two highly reactive bromine radicals
Where is the energy for initiation provided from?
UV radiation
What is a radical?
A very reactive species with an unpaired electron
How is a radical shown?
With a single dot to represent the electron
What happens in the propagation stage?
The reaction propagates through two propagation steps, a chain reaction
What happens in the first propagation step?
A bromine radical reacts with a C-H bond in methane, forming a methyl radical, and a molecule of hydrogen bromide
What happens in the second propagation step?
Each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, together with a new bromine radical
What does the new bromine radical do?
Reacts with another CH4 molecule as in the first propagation step
Why is propagation a chain reaction?
The two steps cycle through in a chain reaction
When is propagation terminated?
When two radicals collide
What happens in the termination stage?
Two radicals collide, forming a molecule with all electrons paired
Why does termination stop the reaction?
When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction
What are the two limitations of radical substitution in organic synthesis?
1) Further substitution
2) Substitution at different positions in a carbon chain
What is further substitution?
Another bromine radical can collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane - further substitution can continue until all hydrogen atoms have been substituted, resulting in a mixture
When does substitution at different positions in the carbon chain occur?
If the carbon chain is longer - there will be a mixture of monosubstituted isomers by substitution at different positions in the carbon chain
