4.1.2 Alk_a_nes Flashcards
Properties of and chemical reactions of
What demonstrates that alkanes lack reactivity
Alkanes are the main component of natural gas and crude oil. They are amongst the most stable organic compounds and their lack of reactivity has allowed crude oil to remain in the earth for millions of years.
What are alkanes
alkanes are saturated hydrocarbons
What is the general formula for Alkanes
C nH 2n+2
where n is the number of carbon atoms in a molecule
What kind of bonds to saturated compounds form
Saturated compounds contain only single bonds. These are sigma bonds (π)
How are sigma (π-) bonds formed
π-bonds are formed by the βhead onβ overlap of orbitals
describe a sigma bond in an alkane
The sigma bond is a type of covalent bond.
A sigma bond is positioned on a line directly between bonding atoms.
Each carbon atom in an alkane has 4 sigma bonds either C-H or C-C.
The electron density of the bond is concentrated between the two atoms.
Explain the structure of an Alkane
Each carbon atom is surrounded by 4 electron pairs in 4 π-bonds. Repulsion between these electron pairs results in a tetrahedral arrangement around each carbon. Therefore each bond angle is 109.5
explain the structural movement of an alkane
The π-bond acts as an axes around which the atoms can rotate freely - so these shapes are not rigid.
eg. butane is shown as a zigzag but it can be rotated into a U shape
How do the boiling points of alkanes change along the homologous series
The b.p. of alkanes increase moving up the homologous series
BUT the presence of branching also impacts b.p.
the higher βnβ the further up the homologous series
What are the states for straight chain alkane isomers
CH4 to C4H10 = gases
C5 to C17 = liquids
C18+ = solids
Why does boiling point increase up a homologous series of alkanes
The b.p. increases because there are more electrons in larger molecules so the London forces are stronger, therefore more energy is required to break them.
How does branching in alkanes impact boiling point
In branching there are fewer surface points of contact between the molecules which gives fewer london forces so less energy is required to break them resulting in a lower b.p..
Additionally the branches get in the way of the molecules getting as close as possible so as a result decrease the london forces further
more branching β less surface area of contact β weaker london forces
Why do alkanes not react with most common reagents
(the reasons for alkanes lack of reactivity are)
- C-C and C-H π-bonds are strong
- C-C bonds are non-polar
- the electronegativity of C and H are so similar that the C-H bond can be considered to be non-polar
What does the complete combustion of alkanes produce
complete combustion produces COβ and HβO
What does the incomplete combustion of alkanes produce
Incomplete combustion produces CO and HβO
What dictates if the alkanes undergo complete or incomplete combustion
The availability of oxygen
If there is an excess of oxygen complete combustion occurs, if there is a lack of oxygen incomplete combustion occurs
What is the difference between incomplete and complete combustion
other than product and abundacy of oxygen
- Complete combustion is more exothermic (produces more energy) than incomplete combustion
- Incomplete combustion produces CO which is toxic and odourless- restricts the ability of blood to carry oxygen
what is the name of the reaction between alkanes and halogens
substitution reaction
What halogens react with alkanes
You need to look at chlorine and bromine primarily and fluorine (the most reactive) is too reactive and would likely explode. Additionally iodine is not reactive enough and barely reacts with alkanes (not to say it never does)
What is the mechanism for the bromination/ chlorination/ iodination of alkanes
radical substitution
Describe initiation as the first step of radical substitution
The reaction starts when the covalent bond in a halogen molecule is broken due to the presence of UV light causing homolytic fission of the bond.
This results in the covalent bond breaking which splits the halogen molecule into halogen atoms called free radicals
e.g. Br-Br β Brβ + Brβ
β = the dot representative of the eβ»
What are the steps of the mechanism of radical substitution
- initiation
- propagation
- termination
Describe propagation as the second step of radical substitution
This is the chain reaction.
A free radical reacts with a molecule to form a new molecule and a different free radical.
e.g. Clβ + CHβ β CHββ + HCl
The free radical produced can react with another molecule to form a new molecule and a different free radical.
e.g. CHββ + Clβ β CHβCl + Clβ
In each reaction a free radical reacts with a molecule to form a different molecule and a different free radical. This creates a chain reaction - each radical produces another. This will continue until termination
Describe termination as the third step of radical substitution
Sometimes free radicals react with each other to form a molecule. This removes free radicals from the reaction stopping the chain.
e.g. CHββ + Clβ β CHβCl
Making the overall reaction CHβ + Clβ β CHβCl + HCl
What is a limitation of radical substitution in organic synthesis
It produces a mixture of products
(especially as each product can then form a different one which can form another and so on )
This makes the yeild of the desired product low
The reaction require UV light so cannot happen in the dark - idk if this is a limitation but its a point
Where can radical substitution happen in alkanes
Radical substitution can happen at any position in the carbon chain of more complicated alkanes
