Alkanes
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Give the definition of a functional group
Part of a compound responsible for the chemical reactions
What is the definition for alkanes?
Naturally occurring hydrocarbons formed from crude oil
What reaction is most common for alkanes due to their unreactivity?
Combustion reactions
Give the definition of a sigma bond
A bond formed by the lengthways overlap of orbitals directly between two bonding atoms
Is a sigma bond covalent?
Yes
What are the strongest type of covalent bond?
C-C and C-H bonds, have a high bond enthalphy, are sigma bonds
What is the equation for complete combustion of alkanes?
Alkane + Oxygen —> Carbon Dioxide + Water When there is an excess supply of oxygen
What is the equation for incomplete combustion of alkanes?
Alkane + Oxygen —> Carbon Monoxide + Water OR Alkane + Oxygen —> Carbon + Hydrogen Both when there is a limited supply of oxygen
Is it possible to see CO2 and C or CO in an incomplete combustion equation?
Yes - mostly seen in MCQs
Why is CO a toxic gas?
CO bonds to haemoglobin which reduces blood oxygen concentration
What can covalent bonds be broken by?
Homolytic fission or heterolytic fission
What is the definition of homolytic fission?
Each bonding atom receives one electron from the covalent bond, forming two radicals
What is a radical?
A species with an unpaired electron
What is the overall general equation for free radical substitution?
Halogen + alkane —> Haloalkane + hydrogen halide
What type of reaction is a free radical substitution reaction?
A substitution reaction
Is it possible for free radical substitution reactions to continue until all hydrogen atoms are replaced by halogens?
Yes
What condition will there need to be for free radical substitution?
UV light
What is the reagent for a free radical substitution reaction?
Halogen molecule
What is the initiation step of free radical substitution?
Stage 1 - Halogen molecule broken into two free radicals
What is the propagation step for free radical substitution?
Stage 2 - 1st propagation - Alkane + Halogen radical —> alkyl radical + hydrogen halide 2nd propagation - alkyl radical + halogen molecule —> Haloalkane + halogen radical
What is the termination step for free radical substitution?
Stage 3 - Two radicals collide to form a neutral molecule; two halogen radicals, a halogen radical and an alkyl radical, or two alkyl radicals
What are the issues with free radical substitution?
Yield is low because; further substitution causes a mixture of products/ different termination steps form a mixture of products and substitution can occur in different positions meaning position isomers are formed
Why would further substitution occur in a free radical substitution reaction and how could we reduce it?
• Excess of halogen - chance of further substitution reduced by an excess of alkanes
What type of reaction is the initiation step?
Photochemical - absorbs UV light to break covalent bonds