Cracking, Alkenes, Addition & Condensation Polymerisation
Long-chain alkanes from fractional distillation are in low demand because they are viscous and poorly flammable, so oil refineries break them down by thermal decomposition (cracking) into smaller, more useful short-chain alkanes (for fuels) and unsaturated alkenes (for making polymers). Alkenes belong to a homologous series with the general formula CₙH₂ₙ containing a reactive C=C double covalent bond, which turns orange bromine water colourless. Examiners regularly test balancing cracking equations, drawing repeating units for addition polymers, and (for Triple Science) comparing addition and condensation polymerisation.
- Cracking (Catalytic & Steam Cracking)
- A thermal decomposition reaction that breaks covalent C-C bonds in long-chain saturated alkanes to produce a shorter-chain alkane (high demand as petrol/fuel) plus one or more alkenes (used to make polymers/plastics and chemicals). Catalytic cracking uses a hot zeolite/Al₂O₃ catalyst (~550°C); steam cracking mixes hydrocarbon vapour with steam at very high temperatures (~800°C).
- Alkenes Homologous Series (CₙH₂ₙ)
- Unsaturated hydrocarbons containing at least one carbon-carbon double covalent bond (C=C functional group). General formula: CₙH₂ₙ (for alkenes with one double bond). First four members: Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈), Pentene (C₅H₁₀). Note: methene does not exist because you need at least 2 carbons for a C=C bond.
- Bromine Water Test & Addition Reactions (Triple)
- Alkenes are more reactive than alkanes because the C=C double bond can open up to add two new atoms across the bond (addition reactions). 1. With bromine water: orange to colourless (forms dibromoalkane, e.g., C₂H₄ + Br₂ -> C₂H₄Br₂). 2. With hydrogen (hydrogenation at 60°C with a nickel catalyst): converts unsaturated alkene to saturated alkane (e.g., C₂H₄ + H₂ -> C₂H₆). 3. With steam (hydration at 300°C, 60-70 atm with phosphoric acid catalyst): forms an alcohol (C₂H₄ + H₂O(g) ⇌ C₂H₅OH).
Curly-Arrow Organic Mechanisms (Nucleophilic Substitution & Addition)
3 Golden Rules for Curly Arrows:
1. Start at electrons: Every curly arrow MUST start at a lone pair of electrons or the middle of a covalent bond (never start an arrow on a δ+ or positive charge).
2. Show dipoles: Always label δ+ and δ− on polar bonds like C–Br or H–Br.
3. Balance charges: If a neutral molecule loses a leaving ion (:Br⁻), the intermediate carbocation must carry a formal + charge.
Key Concepts & Exam Definitions (6)
- Cracking (Catalytic & Steam Cracking)
- A thermal decomposition reaction that breaks covalent C-C bonds in long-chain saturated alkanes to produce a shorter-chain alkane (high demand as petrol/fuel) plus one or more alkenes (used to make polymers/plastics and chemicals). Catalytic cracking uses a hot zeolite/Al₂O₃ catalyst (~550°C); steam cracking mixes hydrocarbon vapour with steam at very high temperatures (~800°C).
- Alkenes Homologous Series (CₙH₂ₙ)
- Unsaturated hydrocarbons containing at least one carbon-carbon double covalent bond (C=C functional group). General formula: CₙH₂ₙ (for alkenes with one double bond). First four members: Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈), Pentene (C₅H₁₀). Note: methene does not exist because you need at least 2 carbons for a C=C bond.
- Bromine Water Test & Addition Reactions (Triple)
- Alkenes are more reactive than alkanes because the C=C double bond can open up to add two new atoms across the bond (addition reactions). 1. With bromine water: orange to colourless (forms dibromoalkane, e.g., C₂H₄ + Br₂ -> C₂H₄Br₂). 2. With hydrogen (hydrogenation at 60°C with a nickel catalyst): converts unsaturated alkene to saturated alkane (e.g., C₂H₄ + H₂ -> C₂H₆). 3. With steam (hydration at 300°C, 60-70 atm with phosphoric acid catalyst): forms an alcohol (C₂H₄ + H₂O(g) ⇌ C₂H₅OH).
- Combustion of Alkenes (Triple)
- Alkenes burn in air just like alkanes, but they typically burn with a smoky yellow flame due to incomplete combustion. This happens because alkenes (CₙH₂ₙ) have a higher percentage of carbon to hydrogen by mass than alkanes (CₙH₂ₙ₊₂), requiring more oxygen per mole to oxidise all the carbon.
- Addition Polymerisation
- Many small unsaturated alkene molecules (monomers) open up their C=C double bonds and join together end-to-end to form a very large, long-chain saturated molecule (addition polymer), with no other product formed. For example, ethene monomers polymerise to form poly(ethene), and propene monomers form poly(propene).
- Condensation Polymerisation (Triple Higher Tier)
- Involves monomers that have two functional groups at opposite ends of the molecule. For example, a diol (HO-R-OH, with two -OH alcohol groups) reacts with a dicarboxylic acid (HOOC-R'-COOH, with two -COOH carboxylic acid groups) to form a polyester and eliminate a small molecule of water (H₂O) for each ester link formed: n HO-□-OH + n HOOC-■-COOH -> -[-O-□-O-CO-■-CO-]ₙ- + 2n H₂O.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Why is cracking of long-chain crude oil fractions economically essential?
Complete the balanced cracking equation: C₁₂H₂₆ -> C₈H₁₈ + 2 ______.
Describe the chemical test and result to distinguish between hexane and hexene.
State the industrial conditions for reacting ethene with steam to manufacture ethanol (Triple).
Why does poly(ethene) not decolourise bromine water even though it is made from ethene?
State two key differences between addition polymerisation and condensation polymerisation (Triple).
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Hexadecane (C₁₆H₃₄) is a long-chain alkane found in the diesel/fuel oil fraction of crude oil. Explain why oil refineries crack hexadecane, describe how catalytic cracking is carried out, and write a balanced symbol equation for cracking C₁₆H₃₄ into octane (C₈H₁₈), propene (C₃H₆), and one other alkene. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Long-chain alkanes like C₁₆H₃₄ are viscous, have low flammability, and are in low demand compared to high-demand short-chain alkanes (like octane used in petrol) and alkenes (used as feedstock for making polymers) [1 mark]. 2. In catalytic cracking, the liquid C₁₆H₃₄ is heated until it vaporises [1 mark]. 3. The hydrocarbon vapour is passed over a hot powdered aluminium oxide (or zeolite) catalyst at around 550°C [1 mark]. 4. Thermal decomposition breaks C-C covalent bonds in the alkane chain [1 mark]. 5. Balanced equation: C₁₆H₃₄ -> C₈H₁₈ + C₃H₆ + C₅H₁₀ (pentene) [1 mark].
- Q2
Propene (CH₂=CHCH₃) is an unsaturated hydrocarbon. Explain what happens when propene is shaken with bromine water, and describe how propene molecules form the addition polymer poly(propene). (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Propene contains a reactive C=C double covalent bond (it is unsaturated) [1 mark]. 2. When shaken with orange bromine water, an addition reaction occurs where the C=C double bond opens up and two bromine atoms bond to the two carbon atoms (forming colourless 1,2-dibromopropane, C₃H₆Br₂), turning the solution from orange to colourless [1 mark]. 3. Under high pressure and with a catalyst, many propene monomer molecules undergo addition polymerisation [1 mark]. 4. The C=C double bond in each propene monomer opens up to form single C-C covalent bonds with neighbouring monomers, forming a long saturated polymer chain of poly(propene) with no other product formed [1 mark].
- Q3
Compare the reactions of ethene (C₂H₄) with hydrogen gas and with steam, stating the reagents, conditions, and products for both reactions. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Reaction with hydrogen (hydrogenation): Ethene reacts with H₂ gas at ~60°C in the presence of a nickel catalyst [1 mark]. 2. Hydrogen atoms add across the C=C double bond to form the saturated alkane ethane (C₂H₄ + H₂ -> C₂H₆) [1 mark]. 3. Reaction with steam (hydration): Ethene vapour reacts reversibly with steam (H₂O(g)) at ~300°C and 60-70 atmospheres pressure in the presence of a phosphoric acid catalyst [1 mark]. 4. Water adds across the C=C double bond to form the alcohol ethanol (C₂H₄ + H₂O ⇌ C₂H₅OH) [1 mark].
- Q4
Ethanediol (HO-CH₂CH₂-OH) reacts with hexanedioic acid (HOOC-C₄H₈-COOH) to form a polyester. Explain why this reaction is classified as condensation polymerisation rather than addition polymerisation. (Triple Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The monomers do not contain C=C double bonds; instead, each monomer possesses two reactive functional groups at opposite ends of the molecule [1 mark]. 2. Ethanediol is a diol containing two alcohol (-OH) functional groups, and hexanedioic acid is a dicarboxylic acid containing two carboxylic acid (-COOH) functional groups [1 mark]. 3. Each time an -OH group on the diol reacts with a -COOH group on the dicarboxylic acid to form an ester link (-COO-), a small molecule of water (H₂O) is eliminated (lost) [1 mark]. 4. Because two different products are formed (the polyester chain plus water molecules), it is a condensation polymerisation [1 mark].
Common Exam Questions (4)
What are the two methods of cracking and their conditions?
1. Catalytic cracking: Vaporise the long-chain alkane and pass it over a hot powdered aluminium oxide (or zeolite/porous pot) catalyst at ~550°C. 2. Steam cracking: Vaporise the long-chain alkane, mix it with steam, and heat it to a very high temperature (~800°C) with no catalyst.
How do you test for an alkene (an unsaturated hydrocarbon)?
Add orange bromine water and shake. An alkene (unsaturated, contains a C=C double bond) will decolourise the bromine water from orange to colourless (because the Br₂ adds across the double bond to form a dibromoalkane). An alkane (saturated) will have no reaction, and the bromine water stays orange.
How do you draw the repeating unit of an addition polymer from its alkene monomer?
1. Draw the alkene monomer in an 'H-shape' with the C=C double bond in the middle and the four groups pointing up and down. 2. Change the C=C double bond into a C-C single bond. 3. Draw brackets around the two carbons with extension (trailing) single bonds sticking straight out through both brackets, and write a subscript 'n' outside the right bracket.
What is the difference between addition polymerisation and condensation polymerisation? (Triple Science)
Addition polymerisation uses one type of monomer containing a C=C double bond, and forms only one product (the polymer). Condensation polymerisation uses monomers with two reactive functional groups (e.g., a diol + a dicarboxylic acid), and every time two monomers join, a small molecule (such as water, H₂O) is lost, forming two products (the polymer + water).
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