Alkanes-organic chemistry Flashcards
Fractional distillation, cracking, combustion, chlorination
Define alkanes
Alkanes are saturated hydrocarbons, meaning they:
Contain only single covalent bonds between carbon atoms.
Have the general formula CₙH₂ₙ₊₂.
Are unreactive except for combustion and substitution.
What is petroleum?
Petroleum (crude oil) is a complex mixture of hydrocarbons, mainly alkanes, extracted from the Earth.
How is petroleum separated into useful fractions?
By fractional distillation:
Crude oil is heated until vaporised.
Vapours enter a fractionating column which is hotter at the bottom, cooler at the top.
Different hydrocarbons condense at different heights based on boiling points.
This separates the mixture into fractions with similar chain lengths.
Why can alkanes in crude oil be separated by fractional distillation?
Alkanes have different boiling points due to differences in chain length and intermolecular forces (van der Waals).
Shorter chains = lower boiling point = condense higher up the column.
What is cracking in organic chemistry?
Cracking is the process of breaking C–C bonds in large alkane molecules to produce shorter-chain hydrocarbons (alkanes and alkenes).
What are the two types of cracking?
Thermal cracking
High pressure (up to 70 atm)
High temperature (900°C)
Produces high % of alkenes
Catalytic cracking
Slight pressure
High temperature (~450°C)
Uses a zeolite catalyst
Produces motor fuels and aromatic hydrocarbons
Why is cracking economically important?
Converts less useful long-chain alkanes into more valuable shorter-chain hydrocarbons
Produces motor fuels (high demand)
Produces alkenes used as feedstock for polymers and other chemicals
Why are alkanes used as fuels?
Alkanes release large amounts of energy when burned due to their high C–H bond enthalpies, making them excellent fuels for domestic heating, transport, and electricity generation.
What is the difference between complete and incomplete combustion?
Complete combustion:
Alkane+𝑂2→𝐶𝑂2+𝐻2𝑂
Occurs with plenty of oxygen.
Incomplete combustion:
Alkane+O2(𝑙𝑖𝑚𝑖𝑡𝑒𝑑)→𝐶𝑂 𝑎𝑛𝑑/𝑜𝑟 𝐶+𝐻2𝑂
Produces carbon monoxide (CO) and soot (C), which are harmful.
What pollutants are produced by internal combustion engines?
CO – poisonous, binds to haemoglobin
NO and NO₂ (NOₓ) – cause acid rain and smog
C (soot) – respiratory problems, global dimming
Unburned hydrocarbons – contribute to smog formation
How do catalytic converters reduce pollution?
Catalytic converters use a platinum or rhodium catalyst to convert harmful gases:
2𝐶𝑂+2𝑁𝑂→2𝐶O2+𝑁2
Unburned hydrocarbons → CO₂ + H₂O
How is sulfur dioxide (SO₂) formed and removed from flue gases?
Formed by burning hydrocarbons containing sulfur.
Causes acid rain.
Removed using calcium oxide (CaO) or calcium carbonate (CaCO₃):
CaO+SO₂→CaSO₃
CaO+SO₂→CaSO₃
CaCO₃+SO₂→CaSO₃+CO₂
CaCO₃+SO₂→CaSO₃+CO2
What is the overall equation for the reaction between methane and chlorine in UV light?
CH₄+Cl₂→CH₃Cl+HCl
This is a free-radical substitution reaction requiring UV light to initiate.
What type of mechanism occurs in the reaction between methane and chlorine?
Free-radical substitution, which proceeds via three steps:
Initiation
Propagation
Termination
What happens in the initiation step?
UV light causes homolytic fission of the chlorine molecule:
Cl₂→2Cl*
Cl₂→2Cl*
Each chlorine atom becomes a free radical (Cl*), which is highly reactive.
What are the two propagation steps in this reaction?
Cl+CH₄→CH₃+HCl
Cl+CH₄→CH₃+HCl
CH₃+Cl₂→CH₃Cl+Cl
CH₃+Cl₂→CH₃Cl+Cl
These steps repeat, maintaining the chain reaction.
What are possible termination steps in this reaction?
Free radicals combine to end the chain:
Cl+Cl→Cl₂
Cl+Cl→Cl₂
CH₃+CH₃→C₂H₆
CH₃+CH₃→C₂H₆
CH₃+Cl→CH₃Cl
CH₃+Cl→CH₃Cl
Free radicals combine to end the chain:
Cl+Cl→Cl₂
Cl+Cl→Cl₂
CH₃+CH₃→C₂H₆
CH₃+CH₃→C₂H₆
CH₃+Cl→CH₃Cl
CH₃+Cl→CH₃Cl
Can lead to multiple substitutions, e.g., CH₂Cl₂
Mixture of products, lack of selectivity
Not useful for producing pure compounds
