12.1, 12.2, Alkanes & Radical Reactions
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What is a sigma bond?
A single covalent bond
What is the alternative name for a single covalent bond?
A sigma bond
When is a sigma bond formed?
When p-orbitals from alkanes directly overlap
What decreases as the amount of branching in a molecule increases?
The boiling point.
Why does boiling point decrease as the amount of branching increases?
• Because there are fewer surface points of contact between molecules within the branched alkane, resulting in fewer London forces. • Additionally, because of their shape, the branches of the molecules get in the way, and prevent the branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further.
What happens as the chain length of a molecule increases?
The molecules have a larger surface area, so more surface contact is possible between molecules.
Why do longer chain length molecules have higher boiling points?
Because they have a larger surface area, so more surface contact. Therefore, the London forces between the molecules will be greater, so more energy is required to overcome the forces.
How is crude oil separated into separate molecules?
By fractional distillation, in a distillation tower
What are alkanes?
A component of natural gas and crude oil, amongst the most stable organic compounds
Are alkanes saturated or unsaturated?
Saturated
Are alkanes reactive or unreactive?
Unreactive
What is lack of reactivity in alkanes a result of?
• C-H and C-C sigma bonds that are strong • Non-polar C-C bonds • The electronegativity of Carbon and hydrogen is so similar that the C-H bond can be considered non-polar
Why are alkanes used as fuel?
• Because their combustion reactions give out heat • They are readily available • They are easy to transport • They can burn in a plentiful supply of oxygen without releasing toxic products
What is produced in incomplete combustion?
Carbon Monoxide and Water
What is produced in complete combustion?
Carbon Dioxide and Water
What causes incomplete combustion?
A limited oxygen supply
What causes alkanes to react with halogens?
The presence of sunlight (because of the UV radiation)
What is a radical?
A chemical species with one or more unpaired electrons, that are very reactive
What are the three steps that radical substitution takes place in?
Initiation, propagation and termination
What happens in INITIATION?
The conversion of atoms into radicals (eg in homolytic fission)
What happens in PROPAGATION?
The reaction propagates through 2 propagation steps, causing a chain reaction (a radical reacts with a non-radical)
When does propagation stop?
When two radicals collide
When should propagation stop in theory?
When all the reactants have been used up.
What is TERMINATION?
When two radicals collide and react, forming a molecule with all electrons paired.