Alkenes — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

Section 1 of 23Structure of alkenes

Structure of alkenes

Alkenes

• Alkenes unsaturated hydrocarbons containing at least one C=C double bond
• Unsaturated means the molecule has a carbon–carbon bond that is not a single bond.

General formula

C_nH_2n

Bonding

• Bonding covalent bonds between non-metal atoms

Section 2 of 23First members of the alkene series

Alkene	Molecular formula
Ethene	C2H4
Propene	C3H6
But-1-ene	C4H8
Pent-1-ene	C5H10

• Smallest alkene
  • ethene, because two carbon atoms are needed to form a C=C bond

Section 3 of 23Why alkenes are reactive

Double bond

• Double bond the C=C bond can open up
• Incoming atoms then form a new single bond with each carbon atom.

Result

• Result each carbon again forms four single bonds, which makes alkenes more reactive than alkanes

Section 4 of 23Manufacture of alkenes

1Cracking

• Cracking long-chain alkanes are broken into shorter alkanes and alkenes
• It uses a high temperature and a catalyst.

2Sequence

Sequence long alkane → cracking → shorter alkane + alkene

Section 5 of 23Distinguishing alkenes from alkanes

Bromine water

• Bromine water an orange solution used as the test reagent

Alkane

• Alkane the solution stays orange

Alkene

• Alkene the solution turns colourless
• Bromine adds across the C=C bond in an addition reaction.

Section 6 of 23More from the specification

1

Refining fossil fuels to carboxylic acids

• Refining fossil fuels to carboxylic acids thermal cracking of crude fossil fuels yields reactive alkenes that undergo oxidation to form carboxylic acids.

2

Substitution reaction

• A reaction in which one atom or group is replaced by another.

3

Ultraviolet radiation

• Radiation that provides the activation energy for alkane–halogen reactions.

4

Alkenes functional group

• Unsaturated hydrocarbons containing a C=C double bond with the general molecular formula CnH2n.

5

Hydration of ethene

• Ethene reacts with steam in the presence of a phosphoric acid (H₃PO₄) catalyst at 300 deg C and 60 atm to produce ethanol.

6

Oxidation of alcohols

• Ethanol is oxidised by fermentation or acidified potassium manganate(VII) to form ethanoic acid, a carboxylic acid.

7

Testing unsaturation

• Alkenes rapidly decolourise bromine water from orange-brown to colourless via addition across the double bond.

8

Distillation

• Separating a liquid from a solution by boiling and condensing the vapour.

9

Mixture

• Two or more substances mixed but not chemically combined.

10

Carboxylic acid

• A compound containing –COOH with a name ending in -oic acid.

11

Carbon dioxide

• A gas that turns limewater cloudy.

12

Fractional distillation

• Separating a mixture of liquids by their boiling points.

13

Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

14

Displayed formula

• A formula showing all atoms and all bonds.

15

Homologous series

• A family of compounds with the same functional group and general formula.

16

Fraction

• A group of hydrocarbons with similar chain length and boiling points.

17

Alcohol

• A compound containing –OH with a name ending in -ol.

18

Boiling

• Liquid to gas throughout the liquid at the boiling point.

19

Combustion

• Burning in oxygen

Section 7 of 23Addition reactions of alkenes

• Addition reaction
  • atoms from a small molecule add across the C=C double bond
  • The C=C bond breaks and new single bonds form.
  • Only one product is formed, and it is saturated.

Section 8 of 23Addition of bromine

Reagent

• Reagent bromine water, which is orange

Reaction

C₂H₄ + Br₂ → CH₂BrCH₂Br, giving 1,2-dibromoethane

• Bromine atoms add across the C=C bond.

Observation

• Observation the orange colour changes to colourless, which is the test for alkenes

Section 9 of 23Addition of hydrogen

Conditions

• Conditions a nickel catalyst

Reaction

C₂H₄ + H₂ → C₂H₆

• An alkane is formed.
• Propene + hydrogen gives propane.

Section 10 of 23Addition of steam (hydration)

Conditions

• Conditions steam with a phosphoric acid catalyst at high temperature and pressure

Reaction

C₂H₄ + H₂O → C₂H₅OH

• An alcohol is formed.
• Propene + steam gives propanol.

Drawing products

• Drawing products the structural and displayed formulae of the products with bromine, hydrogen and steam can be drawn, with C=C replaced by a single bond

Section 11 of 23Alcohols made from alkenes

Alcohols

• Alcohols a homologous series containing the hydroxyl group –OH
• The general formula is C_nH_2n+1OH.
• The first three are methanol, ethanol and propanol.

Ethanol

• Ethanol used as a fuel for cars and as a solvent
• It burns completely in excess oxygenC₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Section 12 of 23Hydration of ethene compared with fermentation

	Hydration of ethene	Fermentation of glucose
Reaction	C2H4 + H2O → C2H5OH	C6H12O6 → 2C2H5OH + 2CO2
Conditions	Steam, phosphoric acid catalyst, about 300 °C, high pressure	Yeast, 25–35 °C, no oxygen
Type of process	Continuous	Batch

• Why fermentation is a batch process
  • yeast are killed at about 15% alcohol
  • The vessel is emptied and the process restarted.

Section 13 of 23Carboxylic acid salts

Reaction with carbonates

• Reaction with carbonates a carboxylic acid forms a salt, water and carbon dioxide

Salt names

• Salt names they end in -anoate
• 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.

Section 14 of 23More from the specification

More from the specification

Fractional Distillation of Hydrocarbons in a Fractionating Column — In a fractionating column, crude oil is separated into distinct fractions based on boiling points;

• as carbon chain length increases, boiling point and viscosity increase due to stronger intermolecular forces.

Cracking Long-Chain Hydrocarbons to Produce Alkenes

• Heavy petroleum fractions containing long-chain alkanes are thermally cracked in the presence of a catalyst to produce valuable shorter-chain alkanes for fuels and unsaturated alkenes for polymers.

Complete vs. Incomplete Combustion of Alkenes

• Like all hydrocarbons, alkenes undergo exothermic combustion with oxygen;
  • complete combustion yields carbon dioxide and water,
• whereas limited oxygen results in smoky flames containing carbon monoxide and carbon soot.

Addition Polymerisation and Synthetic Derivatives

• Unsaturated alkenes polymerise across double bonds to produce synthetic polyalkenes,
  • (called monomers)
• or can be oxidised to form organic acids and other derivatives under specific reaction conditions.

Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

Covalent bond

• A shared pair of electrons between two atoms, giving noble gas configurations.

Substitution reaction

• A reaction in which one atom or group is replaced by another.

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.

Hydrocarbon

• A compound containing hydrogen and carbon only.

Fraction

• A group of hydrocarbons with similar chain length and boiling points.

Boiling

• Liquid to gas throughout the liquid at the boiling point.

Combustion

• Burning in oxygen

Distillation

• Separating a liquid from a solution by boiling and condensing the vapour.

Mixture

• Two or more substances mixed but not chemically combined.

Section 15 of 23Pathway starting from an alkene

Pathway starting from an alkene

Ethene, made by cracking, is hydrated with steam to give ethanol.

Ethanol is oxidised to ethanoic acid.

Ethanoic acid reacts with an alcohol to form an ester and water.

• Functional group sequence
  • C=C, then –OH, then –COOH, then –COO–

Section 16 of 23Oxidation of ethanol to ethanoic acid

Microbial oxidation

• Microbial oxidation acetobacter bacteria use oxygen from the air to oxidise ethanol
• It makes a weak solution of ethanoic acid called vinegar.
• This happens when a bottle of wine is opened.

Chemical oxidation

• Chemical oxidation ethanol is heated under reflux with acidified potassium manganate(VII)
• C₂H₅OH + 2[O] → CH₃COOH + H₂O.
• The purple colour disappears as the manganate(VII) is reduced.

Section 17 of 23Esterification

Esterification

• Esterification an alcohol reacts with a carboxylic acid to form an ester and water
• Esters contain the group R–COO–R and are sweet-smelling, oily liquids.
• They are used in food flavourings and perfumes.

Catalyst

• Catalyst concentrated sulfuric acid is required
• CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.
• The reaction is reversible.

Section 18 of 23Naming the ester

1First part

• First part from the alcohol, ending in -yl

2Second part

• Second part from the acid, ending in -oate

3Example

Example ethanol and ethanoic acid give ethyl ethanoate, CH₃COOC₂H₅

Section 19 of 23Functional groups along the pathway

Compound	Functional group	Test or property
Ethene	C=C	Decolourises bromine water
Ethanol	–OH	Burns; neutral solution
Ethanoic acid	–COOH	Fizzes with carbonates; vinegar smell
Ethyl ethanoate	–COO–	Sweet fruity smell; no reaction with carbonates

Section 20 of 23Safety and practical points

Corrosive acid

• Corrosive acid concentrated sulfuric acid needs eye protection

Flammable substances

• Flammable substances ethanol and esters are heated without a naked flame, using a water bath or electric heater

Reflux

• Reflux a condenser stops the volatile ethanol and ethanoic acid escaping

Section 21 of 23More from the specification

1Petrochemical cracked fuels and organic acids

• Petrochemical cracked fuels and organic acids catalytic cracking of crude fossil fuels yields alkenes converted via hydration into alcohols and carboxylic acids.

2Saturated alkanes vs unsaturated alkenes

• Alkanes are saturated hydrocarbons containing only single C-C bonds,
  • (general formula CnH2n+2)
• whereas alkenes are unsaturated hydrocarbons containing a reactive C=C double bond.
  • (general formula CnH2n)

3Electrophilic addition across double bonds

• Alkenes undergo addition reactions where the pi-bond breaks to attach new atoms;
  • for instance, reacting with hydrogen gas (hydrogenation) in the presence of a nickel catalyst converts alkenes into alkanes.

4Fractional distillation of petroleum

• Crude oil is separated into hydrocarbon fractions of varying carbon chain lengths in a fractionating column based on differences in boiling point.

5Catalytic cracking of long-chain hydrocarbons

• Long-chain alkane molecules obtained from heavy fractions are cracked using heat and catalysts into smaller, more valuable alkanes and unsaturated alkene monomers.

6Saturated alkanes vs unsaturated alkenes

• Alkanes are saturated hydrocarbons containing only single C-C bonds,
  • (general formula CnH2n+2)
• whereas alkenes are unsaturated hydrocarbons containing a reactive C=C double bond.
  • (general formula CnH2n)

7Electrophilic addition across double bonds

• Alkenes undergo addition reactions where the pi-bond breaks to attach new atoms;
  • for instance, reacting with hydrogen gas (hydrogenation) in the presence of a nickel catalyst converts alkenes into alkanes.

8Fractional distillation of petroleum

• Crude oil is separated into hydrocarbon fractions of varying carbon chain lengths in a fractionating column based on differences in boiling point.

9Catalytic cracking of long-chain hydrocarbons

• Long-chain alkane molecules obtained from heavy fractions are cracked using heat and catalysts into smaller, more valuable alkanes and unsaturated alkene monomers.

10General formula

• A formula giving the composition of every member of a homologous series.

11Homologous series

• A family of compounds with the same functional group and general formula.

12Substitution reaction

• A reaction in which one atom or group is replaced by another.

13Ultraviolet radiation

• Radiation that provides the activation energy for alkane–halogen reactions.

14Combustion

• Burning in oxygen

15Distillation

• Separating a liquid from a solution by boiling and condensing the vapour.

16Mixture

• Two or more substances mixed but not chemically combined.

17Carbon dioxide

• A gas that turns limewater cloudy.

18Fractional distillation

• Separating a mixture of liquids by their boiling points.

19Addition reaction

• A reaction in which atoms add across a C=C bond to give one product.

20Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

21Displayed formula

• A formula showing all atoms and all bonds.

22Hydrogen

• A gas that gives a squeaky pop with a lighted splint.

23Alkane

• A saturated hydrocarbon with a name ending in -ane.

24Molecule

• Two or more atoms held together by covalent bonds.

25Formula

• Symbols and numbers showing the atoms in a compound.

26Hydrocarbon

• A compound containing hydrogen and carbon only.

27Fraction

• A group of hydrocarbons with similar chain length and boiling points.

28Boiling

• Liquid to gas throughout the liquid at the boiling point.

Section 22 of 23Key terms

Term	Meaning
Alkene	An unsaturated hydrocarbon containing a C=C double bond.
Unsaturated	Containing a carbon–carbon bond that is not a single bond.
Addition reaction	A reaction in which atoms add across a double bond to give one product.
Cracking	Breaking long alkanes into shorter alkanes and alkenes.
Bromine water test	Orange bromine water turns colourless with alkenes.

Term	Meaning
Addition reaction	A reaction in which a small molecule adds across a C=C bond.
Hydration	Addition of water (steam) to an alkene.
Displayed formula	A formula showing all atoms and all bonds.
Unsaturated	Having a C=C double bond.
Catalyst	A substance that speeds up a reaction without being used up.
Fermentation	Conversion of glucose to ethanol and carbon dioxide by yeast.

Term	Meaning
Pathway	A sequence of reactions converting one compound into another.
Esterification	Alcohol + carboxylic acid → ester + water.
Ethyl ethanoate	The ester CH₃COOC₂H₅.
Oxidising agent	A substance that oxidises another, such as acidified potassium manganate(VII).
Reflux	Heating with a condenser so that vapours return to the flask.

Section 23 of 23Exam technique

1

Using C_nH₂n+2 for alkenes — that is the formula for alkanes

2

Describing the bromine water result as clear — it becomes colourless

3

Saying alkenes are saturated — they are unsaturated

4

Starting the alkene series at methene — ethene is the first alkene

1

Describing the bromine test result as clear — it becomes colourless

2

Thinking atoms are lost in an addition reaction — all atoms of the small molecule add across the C=C bond

3

Saying hydration of ethene needs yeast — it uses steam and a phosphoric acid catalyst

4

Drawing C=C in the product of an addition reaction — the product has only single bonds

1

Forgetting to oxidise ethanol to the acid before making the ester

2

Confusing vinegar (an acid) with an ester

3

Naming esters in the wrong order — the alcohol part comes first

4

Omitting the acid catalyst, or forgetting that water is also formed