Chapter 12 Flashcards
What is an alkane?
Saturated hydrocarbons containing single C-C and C-H bonds as sigma bonds
What is a sigma bond?
A type of covalent bond formed from the head on overlap of two orbitals - one from each bonding atom
What is a special feature of sigma bonds?
They allow free rotation of the atoms around the bond
What is the effect of chain length on the boiling point of an alkane and why?
Increased chain length leads to an increased boiling point. London forces act between molecules that are in close surface contact, therefore as the chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules. The London forces between the molecules will be greater and so more energy is required to overcome the forces.
What is the effect of branching on the boiling point of an alkane and why?
Increased branching leads to a decreased boiling point. This is because there are fewer surface points of contact between molecules of the branched alkanes giving fewer London forces.
Why are alkanes typically unreactive?
The C-C and C-H bonds are strong
The 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
Why are alkanes used as fuels?
Because they are readily available, easy to transport and burn in plentiful supply
What are the products for complete combustion?
Carbon dioxide and water
What are the products for incomplete combustion?
Carbon monoxide and water or carbon and water
When does complete combustion occur?
In a plentiful supply of oxygen
When does incomplete combustion occur?
In a limited supply of oxygen
Complete combustion of pentane:
C5H12 + 8O2 –> 5CO2 + 6H2O
Incomplete combustion of hexane:
C6H14 + 6.5O2 –> 6CO + 7H2O
What is needed for alkanes to be able to react with halogens?
UV light (sunlight)
What is the mechanism called for the halogenation of an alkane?
Radical substitution
What happens in the initiation step in radical substitution?
The reaction is started when the covalent bond in the halogen molecule is broken by homolytic fission. Each halogen atom takes one electron from the pair, forming two highly reactive halogen radicals.
What happens in the propagation step in radical substitution?
In the propagation step, the reaction propagates through two propagation steps, a chain reaction.
In the first step, a halogen radical reacts with a C-H bond in the alkane, forming an alkyl radical and a molecule of a hydrogen halide.
In the second step, each alkyl radical reacts with another halogen molecule, forming the organic haloalkane alongside a new halogen radical.
The new radical formed then reacts with another alkane molecule, and the two steps continue to cycle as a chain reaction.
What happens in the termination step in radical substitution?
In the termination step, two radicals collide, forming a molecule with all electrons paired. There are a number of possible termination steps with different radicals in the reaction mixture.
