Alkanes Flashcards
What is a hydrocarbon
Compounds which only contain carbon and hydrogen
What is a saturated organic compound
One which contains only single bonds between carbon atoms
Are alkanes saturated or unsaturated
Saturated (single bonds)
What is the prefix for a carbon chain with 1 carbon
Meth-
What is the prefix for a carbon chain with 2 carbons
Eth-
What is the prefix for the carbon chain with 3 carbons
Prop-
What is the prefix for the carbon chain with 4 carbons
But-
What is the prefix for the carbon chain with 10 carbons
Dec-
What is the general formula for alkanes
CnH2n+2
What is a functional group
an atom or group of atoms that gives a compound its characteristic chemical reactivity
Do alkanes have a functional group
No
Define a homologous series
A series of compounds that have the same functional group and the same general formula, and in which successive members of the series differ by an extra- CH2- unit
Do alkanes for a homologous series
Yes
What are the types of covalent bonds
- Sigma
- Pi
What is a covalent bond
A shard pair of electrons
What are the 2 bonds in an alkane that are sigma bonds
C-C and C-H
What is a sigma bond
The result of the overlap of 2 orbitals, one from each bonding atom. This bond has 2 shared electrons
How many sigma bonds does each carbon have in an alkane
4 sigma bonds
In an alkane what is the bond angle around each carbon
109.5 because there are 4 sigma bonds so it’s tetrahedral
What process splits crude oil into different fractions
Fractional distillation
How is fractional distillation able to separate alkanes into different fractions
Since the alkanes have different boiling points, it increases as their chain length increases
Why does the boiling point of alkanes increase
Due to the increased number of London forces, the more London forces the higher the boiling point
Why do longer alkanes have a higher boiling point
The molecules have a larger surface area, so more surface contact is possible therefore there are more London forces which then require more energy to overcome
Which alkane has a higher boiling point, pentane or 2-methylbutane, and why
Pentane because there is no branching
Why do branched alkanes have a lower boiling point than straight chains
Because there are fewer surface points of contact between the molecules, giving fewer London forces
What are the 3 reasons that alkanes lack reactivity
- C-C and C-H sigma bonds are strong
- C-C bonds are non-polar
- The electronegativity of carbon ad hydrogen is so similar that the C-H bond can be considered to be non-polar
What are the 2 productions of complete combustion of alkaned
- Carbon dioxide
- Water
What reaction occurs when there is a limited supply of oxygen when reacting alkanes
Incomplete combustion
During incomplete combustion of alkanes what are the 2 possible products other than water
- Carbon monoxide
- Carbon (soot)
Why is carbon monoxide so toxic
Because it more readily binds to haemoglobin in blood which starves cells of oxygen
What type of reaction, reacts alkanes with halogens
Substitution reaction, because a one or hydrogens are substituted for a halogen
What is the condition that reacts halogens with alkanes
Sunlight / UV light
Name this haloalkane, CH3BR
Bromomethane
What are the 3 stages of a radical substitution
- Initiation
- Propagation
- Termination
In a radical substitution, what happens at the initiation stage (use bromine as your example)
The reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission
What is homolytic fission
When a covalent is broken and each atom takes of the electrons from the shared pair
What happens to bromine during the initiation step in a radical substitution
2 bromine radicals are formed Br.
What is a radical
An atom or group of atoms which has an unpaired electron
What is the 2nd step in an radical substitution reaction
Propagation
What happens during propagation in a radical substitution reaction (Bromine and methane)
- CH4 + Br. -> CH3. + HBr
(The bromine radical reacts with the C-H bond in methane forming a methyl radical and a molecule of hydrogen bromide) - CH3. + Br2 -> Ch3Br + Br.
(Methyl radical reacts with a bromine molecule forming bromomethane and a new bromine radical)
What makes propagation a chain reaction in a radical substitution reaction
Become it reactions continue to occur in a cycle
What is the 3rd step in the radical substitution reaction
Termination
What happens during termination of radical substitution
2 radicals collide, forming a molecule with all electrons paired
What happens when 2 radicals collide
Both radicals are removed from the reaction mixture, stopping the reaction
What are the 2 limitation of radical substitution in organic synthesis
- Further substitution
- Substitution at different positions in a carbon chain
Why is further substitution a limitation to radical substitution
Because the reaction is continuous, more hydrogen can be substituted with halogens so would eventually produce a mixture of different haloalkanes or a compound with no hydrogen in
e.g. CH4 -> CH3Br -> CH2Br2 -> CHBr3 -> CBr4
Why is substitution at different positions in a carbon chain a limitation to radical substitution
Different products would be formed due to them being isomers
