12.2 Chemical reactions of the alkanes
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Do alkanes react with common reagents
They do not react with most
What are the reasons for the lack of reactivity of alkanes
Their C-C and C-H σ- 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
Despite their low reactivity, all alkenes react with a plentiful supply of ______ to produce ______ ______ and ______. This reaction is called combustion
Oxygen Carbon dioxide Water
Why are alkanes used as fuels
They are readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxic products
Methane info
Greenhouse gas with global warming potential of 22 Vast quantity stored in ice-saturated arctic tundra of Siberia and N. Canada Formed by decomposition of organic material
What do alkanes burn completely to produce in a plentiful supply of oxygen
Carbon dioxide and water
What happens if oxygen is limited during the combustion of alkanes
The hydrogen atoms in the alkane are always oxidised to water, but combustion of the carbon may be incomplete, forming carbon monoxide, or carbon itself as soot
Carbon monoxide info
Colourless, odourless and highly toxic gas. Combines irreversibly with haemoglobin in red blood cells to form cherry-pink compound called carboxyhaemoglobin which prevents haemoglobin from transporting oxygen around body
When do alkanes react with halogens
In the presence of sunlight High energy UV radiation present in sunlight provides initial energy for a reaction to take place
What is an example of radical substitution
The mechanism for the bromination of methane
Three stages of the mechanism for the bromination of methane (radical substitution)
Initiation, propagation and termination
What happens in the initiation stage (radical substitution)
STEP 1 Reaction is started when covalent bond in bromine molecule is broken by homolytic fission Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals Energy for this bond is provided by UV radiation Br-Br->Br•+•Br
What is a radical
A very reactive species with an unpaired electron Shown with a single dot to represent the electron
What happens in the propagation stage (radical substitution)
STEP 2 Reation propagates through two steps, a chain reaction CH4+Br•->•CH3+HBr • CH3+Br2->CH3Br+Br• In first, bromine radical Br• reacts with C-H bond in methane, forming methyl radical, •CH3 and molecyle of HBr In second, each methyl radical reacts with another bromine forming bromomethane with new bromine radical New bromine radical reacts with another CH4, continues as cycle Terminated when radicals collide
What happens in the termination stage (radical substitution)
STEP 3 Two radicals collide, forming molecule with all electrons paired, Number of possible termination steps with different radicals in reaction mixture Br•+•Br->Br2 • CH3+•CH3->C2H6 • CH3+VBr->CH3Br When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction
Limitations of radical substitution in organic synthesis
Another bromine radical may collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane, further substitutions may continue until all hydrogen atoms have been substituted resulting in a mixture of molecules For methane, only one monobromo compound is possible; if the carbon chain is longer, we will get a mixture of monosubstituted isomers