11.3Organic Chemistry
Alkanes — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
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11.3 Alkanes — COMBINED SCIENCE (CHEMISTRY) notes: State that alkanes are saturated hydrocarbons with single covalent bonds, and describe their properties and combustion.Alkanes — saturated hydrocarbonsSaturated means they contain only single carbon–carbon bonds and no double bon…
- General formula
- Appearance
- Reactions they do undergo
- Complete combustion
- Alkanes — saturated hydrocarbonsSaturated means they contain only single carbon–carbon bonds and no double bonds
- Each carbon atom forms four single bonds
Section 1 of 19Structure and bonding of alkanes
Structure and bonding of alkanes
Alkanes
- Alkanes saturated hydrocarbons
- Saturated means they contain only single carbon–carbon bonds and no double bonds.
- Each carbon atom forms four single bonds.
Bonding
- Bonding single covalent bonds between non-metal atoms
General formula
CnH2n+2, where n is the number of carbon atoms
Section 2 of 19First members of the alkane series
| Alkane | Molecular formula |
|---|---|
| Methane | CH4 |
| Ethane | C2H6 |
| Propane | C3H8 |
| Butane | C4H10 |
| Pentane | C5H12 |
Section 3 of 19Physical properties of alkanes
Appearance
- Appearance colourless compounds
Trend
- Trend physical properties change gradually as the number of carbon atoms increases
- Boiling point and viscosity increase with chain length.
Section 4 of 19Chemical properties of alkanes
1
Alkanes are generally unreactive
- The strong single bonds make them resistant to most reagents.
2Reactions they do undergo
- Reactions they do undergo combustion, cracking and substitution with halogens in the presence of light
Section 5 of 19Combustion of alkanes
Complete combustion
- Complete combustion alkane + oxygen → carbon dioxide + water
Methane
CH4 + 2O2 → CO2 + 2H2O
Ethane
2C2H6 + 7O2 → 4CO2 + 6H2O
Propane
C3H8 + 5O2 → 3CO2 + 4H2O
Butane
2C4H10 + 13O2 → 8CO2 + 10H2O
Incomplete combustion
- Incomplete combustion occurs in limited oxygen
- It produces carbon monoxide, water and sometimes carbon (soot).
Section 6 of 19More from the specification
More from the specification
Cracking fossil fuels and acid synthesis
- Cracking fossil fuels and acid synthesis refining hydrocarbon fossil fuels produces short-chain precursors reacted with steam and oxidised into organic carboxylic acids.
Homologous series
- A family of compounds with the same functional group and general formula.
Ultraviolet radiation
- Radiation that provides the activation energy for alkane–halogen reactions.
Functional group
- The atom or group of atoms that determines the reactions of a series.
General formula of alkanes
- Saturated hydrocarbons following the general formula CnH2n+2 containing unreactive single covalent bonds.
Cracking of long-chain alkanes
- Thermal or catalytic decomposition breaking long fractions into shorter useful alkanes and reactive alkenes (e.g. ethene).
Substitution reactions of alkanes
- Alkanes react with halogens in the presence of ultraviolet (UV) light, substituting hydrogen atoms with halogen atoms.
Petroleum fractions gradation
- Viscosity, boiling point, and flammability vary systematically with increasing carbon chain length in petroleum fractions.
Distillation
- Separating a liquid from a solution by boiling and condensing the vapour.
Mixture
- Two or more substances mixed but not chemically combined.
Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
Fractional distillation
- Separating a mixture of liquids by their boiling points.
Bromine water
- An orange solution that is decolourised by alkenes.
Addition reaction
- A reaction in which atoms add across a C=C bond to give one product.
Fossil fuel
- A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
Displayed formula
- A formula showing all atoms and all bonds.
Alkene
- A hydrocarbon with a C=C bond and a name ending in -ene.
Fraction
- A group of hydrocarbons with similar chain length and boiling points.
Alcohol
- A compound containing –OH with a name ending in -ol.
Catalyst
- A substance that speeds up a reaction and is not used up.
Section 7 of 19Why alkanes are cracked
Demand mismatch
- Demand mismatch fractional distillation gives far more long-chain hydrocarbons than are needed
Purpose of cracking
- Purpose of cracking long, less useful molecules are broken into shorter, more useful ones
Section 8 of 19Catalytic cracking
Conditions
- Conditions vaporised hydrocarbons at about 600–700 °C pass over a hot alumina or silica catalyst
- The catalyst surface breaks covalent bonds, which is thermal decomposition.
Products
- Products a mixture of shorter alkanes and alkenes, and sometimes hydrogen
- Bonds break at random, so cracking always produces at least one alkene.
- Higher temperature and pressure give more hydrogen and a higher proportion of alkenes.
Example
C10H22 → C8H18 + C2H4
- Octane is used in petrol.
- Ethene is used to make ethanol.
Section 9 of 19Uses of the products of cracking
Kerosene and diesel
- Kerosene and diesel are often cracked to give petrol and other alkenes
Alkenes
- Alkenes are used to make polymers
Hydrogen
- Hydrogen is used to make ammonia
Section 10 of 19Alkanes compared with alkenes
| Feature | Alkanes | Alkenes |
|---|---|---|
| Bonding | Single C–C bonds only | Contain a C=C double bond |
| Type | Saturated | Unsaturated |
| General formula | CnH2n+2 | CnH2n |
| Reactivity | Generally unreactive | More reactive |
| Bromine water | Stays orange | Turns colourless |
Section 11 of 19Bromine water test
Reagent
- Reagent bromine water, an orange solution
Alkane result
- Alkane result shaking with an alkane leaves the solution orange
Alkene result
- Alkene result shaking with an alkene turns the solution colourless
- Bromine atoms add across the C=C double bond, so the bromine is no longer in solution.
- The result is described as colourless, not clear.
Section 12 of 19More from the specification
1
Catalytic steam cracking of fossil fuels
- Catalytic steam cracking of fossil fuels heating crude petrochemical fractions with high-temperature steam converts heavy fossil fuels into lighter industrial alkenes.
2
Fractional distillation of crude oil
- Crude oil is separated into fractions in a fractionating column based on boiling point;
- shorter-chain alkanes with lower boiling points condense at the cooler top of the column.
3
Saturated alkanes vs addition reactions
- Alkanes are saturated hydrocarbons containing only single C-C bonds and do not undergo addition reactions, unlike unsaturated alkenes which decolourise bromine water in an addition process called halogenation.
4
Free-radical substitution of alkanes
- In the presence of ultraviolet (UV) light, alkanes react with halogens via a photochemical free-radical substitution mechanism,
- forming halogenoalkanes and hydrogen halides.
5
Oxidation pathways to carboxylic acids
- Halogenoalkanes produced from alkanes can be hydrolysed to alcohols, which may then undergo further oxidation to form carboxylic acids.
6
Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
7
Carbon dioxide
- A gas that turns limewater cloudy.
8
Addition reaction
- A reaction in which atoms add across a C=C bond to give one product.
9
Fossil fuel
- A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
10
Displayed formula
- A formula showing all atoms and all bonds.
11
Homologous series
- A family of compounds with the same functional group and general formula.
12
Substitution reaction
- A reaction in which one atom or group is replaced by another.
13
Ultraviolet radiation
- Radiation that provides the activation energy for alkane–halogen reactions.
14
Alcohol
- A compound containing –OH with a name ending in -ol.
15
Boiling
- Liquid to gas throughout the liquid at the boiling point.
16
Combustion
- Burning in oxygen
Section 13 of 19Substitution reaction with halogens
Substitution reaction with halogens
Substitution
- Substitution a hydrogen atom of the alkane is replaced by a halogen atom
- Extended tier content.
Conditions
- Conditions ultraviolet radiation is needed
- Sunlight is a source of ultraviolet radiation.
- The reaction is called a photochemical reaction.
Role of ultraviolet light
- Role of ultraviolet light it provides the activation energy for the reaction
Section 14 of 19Methane with chlorine
Equation
CH4 + Cl2 → CH3Cl + HCl, giving chloromethane and hydrogen chloride
Monosubstitution
- Monosubstitution only one hydrogen atom is replaced by one chlorine atom
- The displayed formula of chloromethane shows chlorine bonded to carbon in place of one hydrogen.
Further substitution
- Further substitution more hydrogen atoms can be replaced if more ultraviolet radiation is present
- This gives products such as dichloromethane.
Section 15 of 19Combustion compared with substitution
| Feature | Combustion | Substitution with halogens |
|---|---|---|
| Reagent | Oxygen | Chlorine or bromine |
| Condition | Heat or flame | Ultraviolet light |
| Products | Carbon dioxide and water | Halogenoalkane and hydrogen halide |
| Bonds | All bonds break | One C–H bond replaced by C–halogen |
Section 16 of 19Why alkanes are generally unreactive
Bonding
- Bonding strong single covalent C–C and C–H bonds
Result
- Result alkanes react only with oxygen (combustion) and with halogens in ultraviolet light
Section 17 of 19More from the specification
More from the specification
Combustion of conventional fossil fuels
- Combustion of conventional fossil fuels burning coal, petroleum,
- and natural gas fossil fuels releases stored chemical potential energy and greenhouse gases.
Saturated vs Unsaturated Hydrocarbons
- Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds,
- whereas alkenes (like ethene) contain a reactive carbon-carbon double bond (C=C).
Addition Reactions of Alkenes
- Alkenes undergo electrophilic addition reactions across the double bond, reacting with bromine water to turn the orange solution colourless.
Hydration to Produce Alcohols
- Ethene reacts with steam in the presence of a phosphoric acid catalyst to form ethanol, which can be further oxidised into carboxylic acids.
Fractionation of Petroleum
- Fractional distillation separates crude oil into hydrocarbon fractions based on carbon chain length, with larger molecules having stronger intermolecular forces.
Functional group
- The atom or group of atoms that determines the reactions of a series.
Catalyst
- A substance that speeds up a reaction and is not used up.
Distillation
- Separating a liquid from a solution by boiling and condensing the vapour.
Mixture
- Two or more substances mixed but not chemically combined.
Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
Fractional distillation
- Separating a mixture of liquids by their boiling points.
Addition reaction
- A reaction in which atoms add across a C=C bond to give one product.
Fossil fuel
- A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
General formula
- A formula giving the composition of every member of a homologous series.
Alkene
- A hydrocarbon with a C=C bond and a name ending in -ene.
Molecule
- Two or more atoms held together by covalent bonds.
Hydrocarbon
- A compound containing hydrogen and carbon only.
Fraction
- A group of hydrocarbons with similar chain length and boiling points.
Alcohol
- A compound containing –OH with a name ending in -ol.
Boiling
- Liquid to gas throughout the liquid at the boiling point.
Section 18 of 19Key terms
| Term | Meaning |
|---|---|
| Alkane | A saturated hydrocarbon with only single carbon–carbon bonds. |
| Saturated | Containing only single carbon–carbon bonds. |
| Hydrocarbon | A compound containing only hydrogen and carbon. |
| Complete combustion | Burning in excess oxygen to give carbon dioxide and water. |
| General formula | Formula giving the composition of every member of a homologous series. |
| Term | Meaning |
|---|---|
| Alkane | A saturated hydrocarbon with only single bonds. |
| Alkene | An unsaturated hydrocarbon with a C=C double bond. |
| Catalytic cracking | Breaking long alkanes into shorter alkanes and alkenes using heat and a catalyst. |
| Saturated | Containing only single carbon–carbon bonds. |
| Unsaturated | Containing at least one C=C double bond. |
| Thermal decomposition | Breaking down of a compound by heat. |
| Term | Meaning |
|---|---|
| Substitution reaction | A reaction in which one atom or group is replaced by another. |
| Photochemical reaction | A reaction started by light. |
| Ultraviolet radiation | Radiation that provides the activation energy for alkane–halogen reactions. |
| Monosubstitution | Replacement of one hydrogen atom by one halogen atom. |
| Halogenoalkane | An alkane in which one or more hydrogens are replaced by halogen atoms. |
Section 19 of 19Exam technique
1
Saying alkanes contain double bonds — they have only single C–C bonds
2
Using CnH2n for alkanes — that is the general formula of alkenes
3
Calling alkanes very reactive — they are generally unreactive apart from combustion and substitution
4
Forgetting water as a product of combustion
1
Saying alkanes decolourise bromine water — only alkenes do
2
Forgetting that cracking needs a catalyst and a high temperature
3
Using CnH2n+2 for alkenes — that is the general formula for alkanes
4
Writing clear instead of colourless for the bromine water result
1
Saying the reaction with chlorine needs no light — ultraviolet light is required
2
Calling the reaction addition — alkanes are saturated, so hydrogen atoms are substituted
3
Forgetting the hydrogen halide product — HCl forms with chloromethane
4
Writing a double bond in the product — substitution leaves only single bonds
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