Module 4 Chapter 12 Flashcards
What is a property of alkanes and cycloalkanes
They are saturated hydrocarbons
What is the bonding in alkanes
They only have a carbon - carbon single covalent bonds
Each carbon is joined to 4 other atoms by a single covalent bond
what type of covalent bond are these bonds
They are sigma bonds
What is a sigma bonds
The front on overlap of two orbitals one from each bonding atom, each overlapping orbital contains one electron so the sigma bond has two electrons that are shared between bonding atoms.
Where is the sigma bond
it is in line directly between the bonding atoms
What is each carbon surrounded by in an alkane
By 4 sigma bonds
What does repulsion between the electron pairs mean
It results in a 3D tetrahedral arrangement around each carbon atom.
What does a sigma bond act as
It acts as an exes around which the atoms can freely rotate so the alkanes are not a rigid shape.
What is the reactivity of alkanes and what is their general formula
they are extremely unreactive
CnH2n+2
Where are alkanes found and what are they used for
They are found in crude oil
And are used as fuels (combustion)
What happens to the boiling point of alkanes as n increases and why
The boiling point of the alkanes increases with n, as the number of electrons in a molecule increases, and therefore the instantaneous dipoles that are formed are stronger and as a result the induced-induced dipoles interactions are stronger and require more energy to break. So the boiling point increases as n increases.
What happens as the chain length increases
As chain length increases, there is a larger surface area, more contact between molecules resulting in stronger induced-induced dipoles interactions, more energy being required to break them and a higher boiling point.
How is crude oil separated
By fractional distillation
What is crude oil
Crude oil is a mixture of different hydrocarbons
How is crude oil separated into different fractions
Through fractional distillation
What is the process of fractional distillation (1)
Crude oil is vaporised and then fed into the fractionating column at the bottom. the vapours will rise and condense at their different boiling points. The column is hotter at the bottom than the top (temperature differential).
What is the process of fractional distillation (2)
Therefore hydrocarbons with short carbon chains that have low boiling points will rise higher up the column and then condense where they will then be collected at the top of the column. Whereas hydrocarbons with long carbon chains and high boiling points condense at the bottom so are collected at the bottom of the fractionating column.
What is the reactivity of alkanes
The alkanes are unreactive
Why are alkanes unreactive
The C-C and C-H sigma bonds are stronger. The C-C and C-H bonds are also non polar.
What type of reaction is a combustion reaction
It is an exothermic reaction
What do alkanes react with a large supply of oxygen to produce
They undergo complete combustion to produce carbon dioxide and water
Why are alkanes used as fuels
They are readily available, easy to transport and they also burn easily without releasing toxic chemicals.
What can alkanes do with a oxygen if not enough oxygen is present
They can undergo incomplete combustion if not enough oxygen is present. This produces either CO and H2O or C + H2O
What is carbon monoxide
It a colourless and odourless toxic gas with the formula CO
Why is carbon monoxide toxic
It irreversibly binds with Haemoglobin which carries O2 in our blood. When it binds it forms carboxyhaemoglobin and stops haemoglobin from bonding with oxygen.
How can CO be detected
An odour is put into the gases so we can smell it.
CO detectors are also important in homes.
What is required for alkanes to react with halogens
UV must be present as this provides the initial energy for the reaction to take place.
What is the equation for the reaction of methane and bromine
CH4 (g) + Br2 (l) —> CH3Br (g) + HBr (g)
What type of reaction is this
This is a substitution reaction as the halogen has swapped with H
What are the halogens that alkanes react with
They only react with Cl and Br as F is too reactive, small and explosive, and iodine is not reactive enough.
What does the mechanism for the bromination or chlorination of alkanes show
It is represented in a series of steps, showing how electrons move during a reaction.
What is the mechanism for the bromination or chlorination of an alkane an example of
Radical substitution
What are the 3 different stages of the mechanism
Initiation, propagation and bromination
What is step 1 and what is required what type of reaction is it and what is produced
.UV light is required for the reaction to take place
.It is an example of homolytic fission
.Radicals are produced (two bromine or chlorine radicals)
Draw the mechanism for step 1 (initiation)
Br2 —> *Br + *Br
DRAW THE CURLY ARROW ONE
What are the features of step 2 probation
It works int so steps
And it works in a relay/chain reaction
The radical reacts and a new radical is generated
What are the mechanisms for the two different steps in propogation
*Br + CH4 —> *CH3 + HBr
*CH3 + Br2 —> CH3Br + *BR
What happens to the radicals produced in propogation
They feed into termination
What occurs in step 3 termination
Two radicals react together
There are NO radicals produced
What are the products of termination in the reaction between methane and bromine
*Br + *Br —> Br2
*CH3 + *CH3 —> C2H6
*CH3 + *Br —> CH3Br
What are the two features of termination
For every termination reaction that occurs, there may be around 80 propagations
And the termination products are insignificant
What is a feature of the bromination or chlorination of alkanes
It is not the best way to make halogenalkanes. As further substitution takes place and therefore a mixture of products form.
What is further substitution
During the 2nd propagation step CH3BR was made, but if another radical collides with it, further substitution can take place and the desired product will not be produced.
What happens if longer alkanes are used
We can get a mixture of mono substituted isomers by substitution at different positions in the carbon chain.
