Chapter 12.2 - Chemical reactions of alkanes Flashcards
Why do alkanes not react with most common reagents?
- The sigma bonds between C-C and C-H are strong
- C-C bonds are non-polar
- The electronegativity of carbon and hydrogen are so similar that the C-H bond can be considered as non-polar
Give the word equation of the complete combustion of alkanes
alkane + oxygen -> carbon dioxide + water
Give examples of what short chain alkanes are used for
- Domestic heating and cooking
- Fuels in barbecues
- Patio heaters
- Portable kitchen appliances
Give the word equation of the incomplete combustion of alkanes
alkane + oxygen -> carbon + carbon monoxide + water
What is the problem of carbon monoxide production during incomplete combustion?
It is a colourless, odourless, poisonous gas
- Prevents the haemoglobin present within RBCs to bind with oxygen
- Cells and tissues become oxygen starved
- Can be fatal
What is cracking?
A process in which long chained alkanes are broken down into smaller chained alkanes and alkenes which are much more useful
What are the two types of cracking?
- Thermal - heating alkanes to a high temperature under high pressure
- Catalytic - heating alkanes to a lower temperature under lower pressure with the presence of a zeolite catalyst
Why are short chain hydrocarbons in high demand? Give examples
Short chain alkanes are used as fuels
Short chain alkenes are used in polymer production
What is the reaction mechanism for the reaction of alkanes and halogens?
Free radical substitution
What are the conditions of free radical substitution?
- In the presence of UV radiation
- Temperature of about 300°C
What are the three steps that take place during free radical substitution?
- Initiation
- Propagation
- Termination
What happens at the initiation stage?
A covalent bond is broken by homolytic fission ,in the presence of UV light, producing two radicals
What happens at the propagation stage?
A radical reacts with a reactant molecule to form a new molecule and another radical, causing a chain reaction (substitution)
What happens at the termination stage?
Two radicals combine to form a molecule
What is a limitation of free radical substitution?
A mixture of products will be formed and it is hard to control
How do you reduce the chance of by-product formation?
To have an excess of the alkane as there will be a greater chance of the radical reacting with an alkane molecule rather than a substituted molecule
