Organic Chemistry

• Molecular formula counts. Structural formula arranges. Displayed formula draws. If you can point at every bond, it is displayed — and if you cannot, it is not.
• Same group, same general formula, differ by CH2, gradual physical trend, similar chemical behaviour. Five statements — and Cambridge can ask for any one of them on its own.
• Saturated means every carbon-to-carbon bond is single. Isomers means same molecular formula, different connectivity. Rotating the page changes neither.
• Stem from the carbon count. Suffix from the functional group. Number only when there is a real choice. For an ester: alkyl from the alcohol first, alkanoate from the acid second.
• Petroleum is a mixture, so a physical process can separate it. Hot at the bottom, cool at the top. Going up: chains shorter, boiling point lower, volatility higher, viscosity lower.
• Alkanes: all single bonds, limited reactivity, two reactions. Burn them, or replace one hydrogen with chlorine under ultraviolet light. Substitution always leaves a second product behind.
• Alkene means C=C. Cracking breaks big alkanes into shorter alkanes, alkenes and hydrogen, using a high temperature and a catalyst. Aqueous bromine goes from orange-brown to colourless — never “clear”.
• Addition: the double bond opens, one incoming atom or group joins each carbon, everything else stays put, and there is exactly one product. Bromine at room temperature; hydrogen with nickel; steam with an acid catalyst.

• Interpret a structural formula, and say exactly what it is unambiguous about.
• Draw a displayed formula showing every atom and every bond, including hidden O–H bonds.
• State and use the general formula of an alkane, CnH2n+2.
• State and use the general formula of an alkene, CnH2n.
• State and use the general formula of an alcohol, CnH2n+1OH.
• State and use the general formula of a carboxylic acid, CnH2n+1COOH.
• Deduce the structural isomers of C4H10 and of C4H8, and prove they are genuinely different.
• Define a functional group and locate one in an unfamiliar structure.
• State that members of a homologous series share the same functional group.
• State that members share the same general formula.
• Show that successive members differ by –CH2–.
• Describe the gradual trend in physical properties down a series.
• Explain why members have similar chemical properties.
• Define saturated, in terms of carbon–carbon bonds rather than hydrogen count.
• Define unsaturated, and identify the C=C that makes a molecule so.
• Name and draw unbranched alkanes up to four carbons.
• Name and draw unbranched alkenes up to four carbons, including but-1-ene and but-2-ene.
• Name and draw unbranched alcohols up to four carbons, including propan-1-ol, propan-2-ol, butan-1-ol and butan-2-ol.
• Name and draw unbranched carboxylic acids up to four carbons.
• Name and draw the organic products of the reactions in 11.4 to 11.7.
• Identify which family a compound belongs to from its name ending or its formula.
• Name and draw unbranched esters made from an alcohol and an acid each of up to four carbons.
• Name coal, natural gas and petroleum as the fossil fuels.
• State that methane is the main constituent of natural gas.
• Define a hydrocarbon as a compound of carbon and hydrogen only.
• State that petroleum is a mixture of hydrocarbons.
• Describe fractional distillation as separation into useful fractions by boiling point.
• Describe how chain length changes from the bottom of the column to the top.
• Describe how volatility changes from the bottom to the top.
• Describe how boiling point changes from the bottom to the top.
• Describe how viscosity changes from the bottom to the top.
• Name the use of refinery gas: heating and cooking.
• Name the use of gasoline (petrol): fuel for cars.
• Name the use of naphtha: chemical feedstock.
• Name the use of kerosene (paraffin): jet fuel.
• Name the use of diesel oil (gas oil): fuel for diesel engines.
• Name the use of fuel oil: fuel for ships and home heating systems.
• Name the use of lubricating oil: lubricants, waxes and polishes.
• Name the use of bitumen: making roads.
• Describe alkanes as saturated hydrocarbons with single covalent bonds only.
• Describe alkanes as generally of limited reactivity — while still explaining that they burn.
• Define a substitution reaction.
• Describe and draw the substitution of one hydrogen by chlorine, using ultraviolet light.
• Describe alkenes as unsaturated hydrocarbons containing a C=C double bond.
• Describe the manufacture of alkenes and hydrogen by cracking larger alkane molecules.
• Explain the reasons why cracking is carried out.
• Describe the aqueous bromine test, using the correct observation wording.
• Define an addition reaction, including the fact that only one product forms.
• Draw the product of the addition of bromine (and of aqueous bromine).
• Draw the product of the addition of hydrogen with a nickel catalyst.
• Draw the product of the addition of steam with an acid catalyst.
• State the conditions for making ethanol by fermentation.
• State the conditions for making ethanol by the catalytic addition of steam to ethene, and compare the two methods fairly.
• Describe the combustion of ethanol and write a balanced equation for it.
• State the use of ethanol as a solvent.
• State the use of ethanol as a fuel.
• Describe the reaction of an aqueous carboxylic acid with a metal, and name the salt.
• Describe the reaction with a base, and name the salt.
• Describe the reaction with a carbonate, and name the salt.
• Describe the oxidation of ethanol by acidified aqueous potassium manganate(VII).
• Describe the oxidation of ethanol by bacteria, as in vinegar production.
• Describe the formation of an ester from an alcohol and a carboxylic acid using an acid catalyst.
• Define a polymer and a monomer.
• Define a repeat unit and a linkage.
• Deduce the repeat unit of an addition polymer from its monomer.
• Deduce the monomer of an addition polymer from its repeat unit.
• Deduce a polyamide repeat unit from a dicarboxylic acid and a diamine.
• Deduce the monomers of a polyamide from its repeat unit.
• Deduce a polyester repeat unit from a dicarboxylic acid and a diol.
• Deduce the monomers of a polyester from its repeat unit.
• Compare addition and condensation polymerisation.
• Describe plastics as a major group of synthetic polymers.
• Describe the implications of disposing of plastics.
• Explain the problem of plastics in landfill.
• Explain the problem of plastics accumulating in the oceans.
• Explain the problem of toxic gases produced when plastics are burned.
• Draw the structure of nylon as a polyamide.
• Draw the structure of PET as a polyester.
• State that PET can be converted back into monomers and re-polymerised.
• Draw the general structure of an amino acid.
• Describe proteins as natural polyamides.
• Draw a section of a protein chain showing the amide linkages.

Why classify the errors rather than just re-reading? A recall gap is fixed by the Level 1 prompts. An application error is fixed by the reaction map laboratory, where the whole exercise is deciding which rule applies. A communication error is fixed by the mistake clinic, in about ten minutes. Rereading the chapter fixes none of them efficiently, because it spends the same time on material you already hold.

Condensation Polymerisation

The joining together of monomers each carrying two functional groups, in which a small molecule is removed every time a new linkage is formed, so that the polymer is not the only product.

Addition Reaction

A reaction in which a carbon-carbon double bond opens and atoms from another molecule attach to the two carbons that were sharing it, so that only one product is formed and nothing is displaced.

Homologous Series

A family of organic compounds that share the same functional group and the same general formula, whose successive members differ by a CH2 unit, which show a gradual trend in physical properties and which have similar chemical properties.

Esterification

The reaction between a carboxylic acid and an alcohol, using an acid catalyst, which forms an ester containing the -COO- linkage together with water.

Structural Formula

An unambiguous description of the way the atoms in a molecule are arranged, written in a condensed line of text such as CH3CH2OH, in which the connectivity is fully determined but individual bonds are not drawn.

Functional Group

An atom or group of atoms within a molecule that determines that molecule's characteristic chemical reactions, such as the C=C of an alkene, the -OH of an alcohol or the -COOH of a carboxylic acid.

Ester

An organic compound formed when a carboxylic acid reacts with an alcohol using an acid catalyst, containing the -COO- ester linkage, and named with the alkyl group from the alcohol first followed by the alkanoate stem from the acid.

Structural Isomer

One of two or more compounds that have the same molecular formula but different structural formulae, meaning their atoms are joined together in a different order or arrangement.

Saturated Compound

A compound in which every carbon-to-carbon bond is a single covalent bond; an unsaturated compound, by contrast, contains one or more carbon-to-carbon bonds that are not single, such as the C=C double bond of an alkene.

Addition Polymerisation

The joining together of many unsaturated monomer molecules, in which each carbon-carbon double bond opens to form single bonds to neighbouring units, so that the polymer is the only product and no small molecule is released.

Polyamide

A condensation polymer made from a dicarboxylic acid and a diamine, in which the monomer units are joined by amide linkages, -CO-NH-; nylon is a synthetic polyamide and proteins are natural ones.

Polymer

A large molecule built up from many smaller molecules called monomers, joined together in a long chain; the repeat unit is the smallest section of that chain which repeats along its length.

Carboxylic Acid

An organic compound containing the -COOH functional group, which behaves as an acid: it reacts with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide.

Cracking

An industrial process in which larger alkane molecules are broken down into smaller, more useful molecules using a high temperature and a catalyst, producing shorter alkanes, alkenes and hydrogen.

Substitution Reaction

A reaction in which one atom, or group of atoms, in a molecule is replaced by a different atom or group of atoms; the displaced atom leaves as part of a second product.

Displayed Formula

A representation of a molecule in which every atom is written and every covalent bond is drawn as a line, so that no bond is hidden inside a condensed group such as OH or COOH.

Hydrocarbon

A compound that contains the elements carbon and hydrogen only; alkanes and alkenes are hydrocarbons, whereas alcohols and carboxylic acids are not, because they also contain oxygen.

Fermentation

The process in which yeast converts aqueous glucose into ethanol and carbon dioxide, carried out at 25 to 35 degrees Celsius in the absence of oxygen.

Plastic

A synthetic polymer that can be shaped into useful objects; plastics are strong, light and chemically unreactive, which makes them useful and also makes their disposal difficult.

Polyester

A condensation polymer made from a dicarboxylic acid and a diol, in which the monomer units are joined by ester linkages, -CO-O-; PET is the polyester used for drinks bottles and clothing fibres.

• “The displayed formula of ethanol is CH3CH2OH.” why it fails That is a structural formula. It compresses C–H and O–H bonds into text instead of drawing them. correct A displayed formula shows every atom and every bond as a line, including the O–H bond that “OH” hides. Taught in 11.1A.
• “Here is butane drawn straight, and here it is drawn bent round a corner — two isomers.” why it fails Isomerism is about connectivity, not about the shape of the drawing. Bending the page does not move any atom onto a different neighbour. correct To make a real isomer of C4H10 you must detach a carbon from the end of the chain and attach it to a middle carbon, giving 2-methylpropane. Taught in 11.1C.
• “The alkene with four carbons is called butene.” why it fails With four carbons there is more than one place the double bond can sit, so the name is ambiguous. correct Give the locant: but-1-ene has the C=C between carbons 1 and 2; but-2-ene has it between carbons 2 and 3. Taught in 11.2.
• “Ethanol plus methanoic acid gives ethyl methanoic acid.” why it fails An ester name never keeps the word “acid”, and the two halves are easy to swap. correct Alkyl from the alcohol first, alkanoate from the acid second: ethanol gives “ethyl”, methanoic acid gives “methanoate”, so the ester is ethyl methanoate. Taught in 11.2.
• “Petroleum is a compound that is broken down in the fractionating column.” why it fails Two errors at once: petroleum is not a compound, and distillation does not break anything down. correct Petroleum is a mixture of hydrocarbons. Fractional distillation separates that mixture using differences in boiling point. No bonds inside the molecules are broken. Taught in 11.3.
• “Kerosene boils at 180 °C.” why it fails A fraction is itself a mixture of hydrocarbons of similar chain length, so it does not have one fixed boiling point, and the syllabus supplies no numbers. correct Fractions condense over a range of boiling points. Answer in terms of higher and lower, not in invented degrees. Taught in 11.3.
• “The top of the fractionating column is the hottest part.” why it fails It is the reverse. Vapours rise, cool as they go, and condense when they reach their own boiling range. correct The column is hottest at the bottom and coolest at the top, which is why the short, low-boiling molecules travel furthest up. Taught in 11.3.
• “The bromine water turns clear.” why it fails “Clear” means see-through. Orange bromine water is already clear — you can read print through it. Saying it turned clear describes no change at all. correct Write “the orange-brown colour is decolourised”, or “turns colourless”. Taught in 11.5A.
• “Ethene plus bromine gives 1,2-dibromoethane and hydrogen bromide.” why it fails That second product is imported from substitution. In an addition reaction there is nothing left over to form it. correct An addition reaction forms only one product. Every atom of both reactants ends up in that single molecule. Taught in 11.5B.
• “Ultraviolet light is the catalyst for the reaction between methane and chlorine.” why it fails A catalyst is a substance that is present at the end unchanged. Light is not a substance. correct Ultraviolet light supplies the activation energy the reaction needs to start. Call it a condition, never a catalyst. Taught in 11.4.
• “Air is bubbled through the fermentation mixture to keep the yeast alive.” why it fails With oxygen present, the process is no longer fermentation, and any ethanol that does form can be oxidised further. correct Fermentation requires the absence of oxygen, with yeast, aqueous glucose and a temperature of 25–35 °C. Taught in 11.6.
• “Ethene is hydrated at 300 °C and 60 kPa.” why it fails A unit slip by a factor of 100. 60 kPa is below atmospheric pressure — a partial vacuum, not a high-pressure plant. correct 6000 kPa, which is the same as 60 atm. Write one or the other, and check the unit before you leave the question. Taught in 11.6.
• “Ethanoic acid reacts with sodium carbonate to give hydrogen.” why it fails The “acid makes a gas, so the gas is hydrogen” shortcut. Which gas depends entirely on what the acid met. correct Metal → hydrogen. Base → water, no gas. Carbonate → carbon dioxide and water. Taught in 11.7A.
• “The repeat unit of poly(ethene) is [CH2CH2]n.” why it fails Written that way, the unit looks like a small closed molecule. A repeat unit is a section cut out of a much longer chain. correct Draw open continuation bonds passing through the brackets on both sides, to show that the chain carries on. Taught in 11.8A.
• “All polymerisation reactions release water.” why it fails This over-generalises from condensation polymers to every polymer. correct Addition polymerisation releases nothing at all — the polymer is the only product. Condensation polymerisation releases a small molecule, and you can only say which one by looking at the functional groups actually being joined. Taught in 11.8B.
• 1. “The displayed formula of ethanol is CH3CH2OH.” defect A structural formula supplied where a displayed one was demanded. why “Displayed” is a technical requirement, not a style. Every bond must appear as a line, and “OH” hides one. repair Draw the O joined to C by a line and the H joined to that O by another: –O–H. transfer Draw the displayed formula of propan-2-ol and count the bonds. You should reach 11.
• 2. “C4H10 has four isomers — straight, bent up, bent down and zig-zag.” defect Counting drawings instead of counting connectivities. why Molecules are not flat or rigid; the angle on the page carries no chemical information. repair For each structure, list how many carbons each carbon is bonded to. Identical lists means the same compound. transfer Show that C5H12 drawn as a straight chain and as an L-shape are the same compound.
• 3. “The four-carbon alkene is called butene.” defect A missing locant where more than one position is possible. why With four carbons the C=C can sit between carbons 1–2 or 2–3, giving two different compounds. The name is ambiguous. repair Add the number: but-1-ene or but-2-ene, choosing the numbering that gives the lower value. transfer Explain why propene needs no locant but propan-2-ol does.
• 4. “Ethanol plus methanoic acid gives ethyl methanoic acid.” defect The word “acid” retained in an ester name. why An ester is not an acid; it has no acidic hydrogen. The acid's ending becomes -oate. repair Delete “ic acid” and write “ate”: ethyl methanoate. transfer Name the ester from butan-1-ol and propanoic acid, then say which parent gave each word.
• 5. “Fractional distillation breaks petroleum down into its fractions.” defect The verb “breaks down”, which describes a chemical change. why Petroleum is a mixture, and distillation is physical. Every molecule survives intact; nothing is broken. repair Write separates. Reserve “breaks down” for cracking. transfer State one difference in the products of distillation and of cracking that proves one is chemical and the other is not.
• 6. “Naphtha is used as jet fuel.” defect Two adjacent fractions swapped. why Naphtha and kerosene sit next to each other in the column, so a list learned as a chant blurs them. repair Anchor naphtha by what makes it unique: it is the only fraction whose use is not a fuel. It is a chemical feedstock. Kerosene is the jet fuel. transfer Without looking, give the use of every fraction below kerosene.
• 7. “The hottest part of the fractionating column is the top.” defect The temperature gradient inverted. why If the top were hottest, nothing would ever condense there, and the column could not separate anything. repair Hot at the bottom, cool at the top. Vapour rises and cools until it reaches its own boiling range. transfer Explain, from that gradient alone, why bitumen never reaches the top of the column.
• 8. “The bromine water turns clear.” defect “Clear” used where “colourless” is meant. why Clear means transparent. Orange bromine water is already transparent, so this sentence reports no change at all. repair Write “the orange-brown colour is decolourised”, or “turns colourless”. transfer Write the observation for warming ethanol with acidified aqueous potassium manganate(VII), avoiding the same trap.
• 9. “Ethene + Br2 → 1,2-dibromoethane + HBr.” defect A second product invented for an addition reaction. why Substitution thinking applied to addition. Nothing was displaced from ethene, so there is nothing to build a second molecule from — and the equation no longer balances. repair Delete the second product. Check by adding the reactant formulae: C2H4 + Br2 = C2H4Br2 exactly. transfer Say how many products form when ethene reacts with steam, and why.
• 10. “Ultraviolet light catalyses the reaction of methane with chlorine.” defect A condition described as a catalyst. why A catalyst is a substance present unchanged at the end. Light is not a substance. repair Write “ultraviolet light supplies the activation energy”, and place it above the arrow as a condition. transfer Name a genuine catalyst from this chapter and say what it does that light cannot.
• 11. “Air is bubbled through the fermentation mixture to keep the yeast alive.” defect Oxygen introduced into a process defined by its absence. why Fermentation is what yeast does without oxygen. With oxygen present, any ethanol formed can itself be oxidised to ethanoic acid. repair Add four words: “in the absence of oxygen”. transfer Explain, using section 11.7B, exactly what would go wrong if air were admitted.
• 12. “Ethene is hydrated at 300 °C and 60 kPa.” defect A unit error of a factor of 100. why 60 kPa is below atmospheric pressure. The answer describes a partial vacuum rather than a high-pressure plant. repair Write 6000 kPa, or write 60 atm. The number 60 belongs only with “atm”. transfer State both conditions for hydration and both for fermentation, with correct units throughout.
• 13. “Ethanoic acid reacts with sodium carbonate to give hydrogen.” defect One memorised gas applied to all three acid reactions. why The second product is decided by the other reactant. Only a metal releases hydrogen. repair Metal → hydrogen. Base → water, no gas. Carbonate → carbon dioxide and water. transfer Give the observation and the confirming test for the gas in each of the two cases where one is produced.
• 14. “The repeat unit of poly(ethene) is [CH2CH2]n.” defect Missing continuation bonds. why Without bonds crossing the brackets, the unit reads as a small closed molecule rather than a section cut from a long chain. repair Draw a bond running out of each bracket: –[–CH2–CH2–]n–. transfer Draw the repeat unit of poly(chloroethene) and mark both continuation bonds.
• 15. “The repeat unit of poly(ethene) is –[–CH2=CH2–]n–.” defect The double bond kept after polymerisation. why Opening the C=C is the reaction. If it is still there, nothing has happened — and each carbon would need six bonds. repair Change the double line to a single line. Count the bonds on one backbone carbon: it must total four. transfer Take the repeat unit –[–CH2–CHCl–]n– back to its monomer and say what you had to add.
• 16. “The repeat unit of poly(propene) is –[–CH3–CH2–]n–.” defect A substituent moved onto the wrong backbone carbon. why In propene the CH3 hangs off the second double-bond carbon. Putting it into the backbone gives a carbon with five bonds. repair –[–CH2–CH(CH3)–]n–. Mark the two double-bond carbons before redrawing, and move nothing else. transfer Deduce the repeat unit of the polymer made from CH2=CHF, keeping the F on its own carbon.
• 17. “Nylon is made from ethanoic acid and an amine.” defect Condensation monomers given only one functional group each. why With one reactive end apiece, the two would join once and stop. You would get a small molecule, not a polymer. repair Use a dicarboxylic acid and a diamine: HOOC–□–COOH and H2N–▢–NH2. transfer Explain why an amino acid can build a protein on its own, despite there being only one kind of monomer.
• 18. “C2H5OH + 3.5O2 → 2CO2 + 3H2O.” defect The oxygen atom already inside the ethanol was not counted. why Alcohols differ from alkanes here: they bring one oxygen of their own to the left-hand side. repair Seven oxygen atoms are needed on the right; the ethanol supplies one, so O2 supplies six. The coefficient is 3. transfer Balance the complete combustion of methanol, CH3OH, remembering its own oxygen.
• 19. “Alkanes are unreactive, so petrol does not really burn well.” defect “Limited reactivity” read as “low vigour”. why The phrase restricts the number of types of reaction alkanes undergo, not how energetically those reactions proceed. repair Write: alkanes take part in few types of reaction — combustion, and substitution with chlorine under ultraviolet light — but combustion is vigorous and exothermic. transfer Explain why alkanes are still the basis of nearly every fuel in the fractionating column.
• 20. “Plastics should not be used because they are bad for the environment.” defect A verdict offered where a mechanism was asked for. why No property is named, no consequence is traced, and nothing in the sentence could be marked right or wrong. repair Name the property, then the consequence: “the C–C and C–H bonds are unreactive, so microorganisms cannot break them down, so the plastic persists in landfill”. transfer Build the same property-to-consequence chain for burning poly(chloroethene).

• Topic 11 is assessed across the whole qualification: multiple-choice items, structured theory questions, and practical or alternative-to-practical questions about tests and observations. What matters far more than which paper a question appears on is what output the command word is asking for. A perfect explanation scores nothing against a command word that asked you to draw.
• The same idea, six different demands. Take the aqueous bromine test. It can be asked as a name (“name a reagent that distinguishes ethane from ethene”), a describe (“describe the test and its result”), a draw (“draw the organic product”), an identify (“which sample is unsaturated, and how do you know?”), a compare (“compare what you would see with hexane and with hex-1-ene”), or a deduce (“deduce the structure of the product of the addition”). One piece of chemistry; six different pieces of writing. Practise all six.
• A drawing discipline worth building now. Whenever a question says draw, finish by running the same three-point audit every time: does every carbon have exactly four bonds; does every oxygen have exactly two; does every hydrogen have exactly one? It takes about eight seconds and it catches the great majority of avoidable losses in this topic.
• Not required by 5070 Topic 11. You may see organic structures drawn as bare zig-zag lines, with carbon and hydrogen atoms left implicit — a “skeletal formula”. It is the standard notation at A Level and beyond. It is not in this syllabus, and it is not a valid answer to a “draw the displayed formula” question, so this chapter draws every atom.