Organic chemistry

Worked solution

Substitute the number of carbon atoms into each general formula. For n = 5, CnH2n+2 gives C₅H₁₂. Option A has two hydrogen atoms fewer than this, which is the pattern for alkenes (CnH2n). Options B and C contain oxygen, so they cannot be members of a series whose general formula contains only C and H; they are an alcohol and a carboxylic acid.

Worked solution

"Unsaturated" means that one or more carbon–carbon bonds are not single bonds. Look only at the bonds between carbon atoms. In propene, CH₂=CHCH₃, the "=" is between two carbon atoms, so propene is unsaturated. A common mistake is to choose ethanoic acid, CH₃COOH, because it contains a double bond. That double bond is between carbon and oxygen (C=O), not between two carbon atoms, so it does not make the compound unsaturated.

Worked solution

Structural isomers need the same molecular formula but a different structural formula. Count atoms in each pair first. In A, both compounds contain 5 C and 12 H, and one has a branch (a CH₃ group on the second carbon) while the other is a straight chain, so they are isomers. In B, CH₃CH₂CH=CH₂ read backwards is CH₂=CHCH₂CH₃: it is the same molecule, with the double bond at the end of the chain in both. In C the hydrogen counts differ (10 and 8). In D the compounds differ by a CH₂ unit, which makes them homologues, not isomers.

Worked solution

(a) A homologous series is a family of similar compounds which have similar chemical properties because they all contain the same functional group.

(b) The functional group of the alcohols is –OH. It is covalently bonded to carbon, so do not call it a hydroxide ion.

(c) Alkenes have the general formula CnH2n. With n = 5, the number of hydrogen atoms is 2 × 5 = 10, giving C₅H₁₀.

(d) Alcohols have the general formula CnH2n+1OH. With n = 4, 2n + 1 = 9, giving C₄H₉OH.

Worked solution

(a) Reading down the table, each alcohol has one more carbon atom than the one before, and the boiling point rises each time. So boiling point increases as chain length increases.

(b) The rises are 13 °C, 19 °C and 20 °C. Adding roughly another 20 °C to 117 °C gives about 137 °C. Any sensible value between 130 °C and 145 °C is acceptable.

(c) Members of a homologous series have the same functional group, the same general formula, differ from one member to the next by –CH₂– and have similar chemical properties. Any two of these score.

(d)(i) Butan-1-ol has an unbranched chain of four carbon atoms with the –OH on the first carbon: CH₃CH₂CH₂CH₂OH.

(d)(ii) A molecular formula only tells you how many of each atom there are. Butan-2-ol, CH₃CH(OH)CH₂CH₃, also has the molecular formula C₄H₉OH. A structural formula is an unambiguous description of how the atoms are arranged, so it shows which carbon the –OH group is on.

Worked solution

(a) Structural isomers are compounds with the same molecular formula but different structural formulae.

(b) The alkene general formula is CnH2n. For four carbon atoms, the number of hydrogen atoms is 2 × 4 = 8, which matches C₄H₈.

(c) With an unbranched chain of four carbon atoms, the C=C double bond can be between carbons 1 and 2 (but-1-ene, CH₂=CHCH₂CH₃) or between carbons 2 and 3 (but-2-ene, CH₃CH=CHCH₃). Putting the double bond between carbons 3 and 4 just gives but-1-ene again, written backwards.

(d) Picture the displayed formula of CH₂=CH–CH₂–CH₃. The hydrogen atoms are 2 + 1 + 2 + 3 = 8, and each hydrogen atom has one bond to carbon, so there are 8 C–H bonds. Between the carbon atoms there is one C=C double bond (C1–C2) and two C–C single bonds (C2–C3 and C3–C4).

Worked solution

Split the name into two parts. "Prop-" means three carbon atoms. "-ene" means the compound is an alkene, with a C=C double bond. Alkenes have the general formula CnH2n, so three carbon atoms need 2 × 3 = 6 hydrogen atoms: C₃H₆. C₃H₈ is the formula of propane, an alkane (ending -ane), so option B mixes the alkene name with an alkane formula.

Worked solution

An ester is named in two parts: first the part from the alcohol (ending -yl), then the part from the acid (ending -oate). In a structural formula written as R–COO–R', the acid part is R–COO (the C=O carbon belongs to the acid) and the alcohol part is the group attached to the single-bonded oxygen. Here the acid part is CH₃CH₂COO: three carbons, so propanoate. The alcohol part is CH₂CH₂CH₂CH₃: four carbons, so butyl. The name is butyl propanoate. A common error is to name the parts in the order they are written, which gives the wrong answer "propyl butanoate".

Worked solution

(a)(i) Four carbon atoms, only single bonds: butane.

(a)(ii) Four carbon atoms with a C=C between the second and third carbon: but-2-ene. The number is needed because but-1-ene is a different compound.

(a)(iii) Three carbon atoms with the –OH on the middle (second) carbon: propan-2-ol.

(a)(iv) Count every carbon atom, including the one in –COOH: CH₃, CH₂, CH₂ and COOH make four, so this is butanoic acid.

(b) Butan-2-ol has a chain of four carbon atoms with the –OH on the second carbon: CH₃CH(OH)CH₂CH₃.

Worked solution

Petroleum (crude oil) is a fossil fuel made of many different hydrocarbons mixed together, which is why it can be separated into fractions by fractional distillation. A hydrocarbon is a compound containing hydrogen and carbon only. Do not confuse petroleum with natural gas, which is mainly methane.

Worked solution

The column is hottest at the bottom and coolest at the top. Only substances with low boiling points stay as vapour all the way to the top, and low boiling points go with short chains (small molecules). Short-chain fractions also flow easily, so their viscosity is low. All three properties therefore change together: going up the column, chain length decreases, boiling point decreases and viscosity decreases.

Worked solution

(a) The petroleum is heated until most of it turns into vapour and it enters the bottom of the fractionating column. The column is hot at the bottom and gets cooler towards the top. As the vapours rise, each substance condenses when it reaches a level that is cooler than its boiling point. Substances with high boiling points condense low down; substances with low boiling points rise further before condensing. Each fraction is collected at a different height.

(b) Kerosene (paraffin) is used for jet fuel, bitumen is used for making roads, and naphtha is used as a chemical feedstock for making other chemicals.

Worked solution

(a)(i) The fraction with the lowest boiling range, P (below 40 °C), rises furthest up the column, so it is collected at the top.

(a)(ii) Viscosity increases as chain length increases. Put the fractions in order of increasing number of carbon atoms: P (1–4), Q (10–16), S (14–20), R (more than 35).

(b) Volatility means how easily a substance turns into a vapour. The fraction with the lowest boiling range and the shortest chains, P, is the most volatile.

(c) Alkanes have the general formula CnH2n+2. With n = 12, 2 × 12 + 2 = 26, so the formula is C₁₂H₂₆.

(d) The three fossil fuels are coal, natural gas and petroleum, so the missing one is coal. Natural gas consists mainly of methane, CH₄.

Worked solution

In a substitution reaction, one atom or group of atoms is replaced by another. Here a chlorine atom replaces one hydrogen atom in ethane: C₂H₆ + Cl₂ → C₂H₅Cl + HCl. The hydrogen atom that is removed joins with the other chlorine atom to form HCl, not H₂. Energy from the ultraviolet light is needed to start the reaction; it provides the activation energy.

Worked solution

(a) "Hydrocarbon" means the compound contains hydrogen and carbon only. "Saturated" means all of its carbon–carbon bonds are single bonds. Propane, CH₃CH₂CH₃, fits both descriptions.

(b) Alkanes are generally unreactive, but they do burn (combustion) and they undergo substitution with chlorine.

(c)(i) One H atom is swapped for one Cl atom, and the removed H joins the other Cl atom: C₃H₈ + Cl₂ → C₃H₇Cl + HCl. Check: 3 C, 8 H and 2 Cl on each side.

(c)(ii) Propane has two kinds of carbon atom: the two end carbons and the middle carbon. Replacing an H on an end carbon gives CH₃CH₂CH₂Cl (1-chloropropane). Replacing an H on the middle carbon gives CH₃CHClCH₃ (2-chloropropane). Replacing an H on the other end carbon gives the same compound as the first one, so there are only two products.

(c)(iii) This is a photochemical reaction. The ultraviolet light provides the activation energy. It is not a catalyst.

Worked solution

Aqueous bromine is orange (it is sometimes described as yellow or brown). An alkene has a C=C double bond, and bromine adds across it, forming a colourless product, so the mixture becomes colourless. An alkane has only single bonds and cannot undergo this addition reaction, so no change is seen and the mixture stays orange. This is the test that distinguishes saturated from unsaturated hydrocarbons.

Worked solution

Butane has two more hydrogen atoms than but-2-ene (C₄H₁₀ compared with C₄H₈), so the reagent needed is hydrogen. The addition of hydrogen to an alkene needs a nickel catalyst: CH₃CH=CHCH₃ + H₂ → CH₃CH₂CH₂CH₃. The other addition reactions add Br₂ or H₂O instead, so they produce compounds containing bromine or oxygen.

Worked solution

(a) Cracking breaks large alkane molecules into smaller ones using a high temperature and a catalyst.

(b) Balance the atoms. Carbon: 10 − 8 = 2. Hydrogen: 22 − 18 = 4. The missing product is C₂H₄ (ethene), which is an alkene.

(c) Carbon: 4 + (2 × 3) = 10, so all the carbon is used. Hydrogen: 8 + (2 × 6) = 20, so 22 − 20 = 2 hydrogen atoms are left, which form H₂. Cracking can make hydrogen as well as alkenes.

(d) Fractional distillation produces more long-chain alkanes than are needed and less petrol than people want. Cracking converts the less useful long chains into shorter, more useful fuels such as petrol. It also produces alkenes, which are used to make polymers and other chemicals, and it can produce hydrogen.

Worked solution

(a) In an addition reaction, the reactants join together to form only one product.

(b) In each addition, the C=C double bond becomes a single bond and one new atom or group is added to each of the two carbon atoms that were joined by the double bond (carbons 2 and 3).

(i) Bromine adds one Br to each: CH₃CHBrCHBrCH₃.

(ii) Hydrogen adds one H to each: CH₃CH₂CH₂CH₃, butane.

(iii) Steam adds H to one carbon and OH to the other: CH₃CH(OH)CH₂CH₃.

(iv) Because the double bond is in the middle of a symmetrical molecule, the –OH always ends up on the second carbon atom, so the product is butan-2-ol.

(c) The C=C double bond in but-2-ene lets bromine add to it. Butane has only single bonds (it is saturated), so it cannot take part in addition reactions; it only reacts with chlorine or bromine by substitution in ultraviolet light.

Worked solution

Yeast contains enzymes that convert glucose to ethanol and carbon dioxide. The temperature must be warm but not hot (25–35 °C), because at high temperatures such as 80 °C the yeast dies. Oxygen must be kept out (anaerobic conditions); otherwise the ethanol is not produced and any ethanol can be oxidised. The conditions 300 °C, 6000 kPa and an acid catalyst are for the catalytic addition of steam to ethene, a different method.

Worked solution

(a) Ethanol is used as a solvent and as a fuel.

(b) Complete combustion of any compound of carbon, hydrogen and oxygen gives carbon dioxide and water. Balance carbon first: 2 C in ethanol gives 2CO₂. Then hydrogen: 6 H in ethanol gives 3H₂O. Then count oxygen on the right: (2 × 2) + 3 = 7. Ethanol already provides 1 O, so 6 more O atoms are needed, which is 3O₂. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Check: C 2 = 2; H 6 = 6; O 7 = 7.

(c) With too little oxygen, combustion is incomplete and carbon monoxide can form. Carbon monoxide is toxic.

(d) Yeast converts glucose into ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Worked solution

(a) In method 2, ethene and steam are passed over an acid catalyst at about 300 °C and 6000 kPa (60 atm).

(b) One molecule of steam adds across the C=C double bond: C₂H₄ + H₂O → C₂H₅OH. Only one product is formed, so this is an addition reaction.

(c) Fermentation uses sugar from a crop, which is renewable, and it needs only a warm temperature (25–35 °C), so it uses much less energy than method 2. Method 2 depends on ethene from petroleum, which is non-renewable. However, fermentation is slow and is done in batches, and it gives a dilute, impure solution of ethanol that has to be fractionally distilled. Method 2 is fast, continuous and gives much purer ethanol.

Worked solution

Ethanoic acid reacts with carbonates like other acids: acid + carbonate → salt + water + carbon dioxide. The salt is calcium ethanoate. To write its formula, balance the charges: Ca²⁺ needs two CH₃COO⁻ ions, giving (CH₃COO)₂Ca (also written Ca(CH₃COO)₂). Hydrogen gas is given off when an acid reacts with a reactive metal such as magnesium, not with a carbonate.

Worked solution

(a) Acidified potassium manganate(VII) is purple. It is an oxidising agent: as it oxidises ethanol, it is reduced and the purple colour disappears, leaving a colourless solution.

(b) In vinegar production, bacteria oxidise ethanol to ethanoic acid using oxygen from the air.

(c) Ethanoic acid reacts with a reactive metal like other acids: acid + metal → salt + hydrogen. Here zinc + ethanoic acid → zinc ethanoate + hydrogen.

(d) A carboxylic acid and an alcohol react (with an acid catalyst) to form an ester and water. Name the alcohol part first (methanol gives "methyl") and the acid part second (ethanoic acid gives "ethanoate"): methyl ethanoate. The structural formula is CH₃COOCH₃: the CH₃COO part comes from the acid, and the CH₃ attached to the single-bonded oxygen comes from the alcohol. CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O.

Worked solution

"Poly" means many. A polymer is a very large molecule made when many small molecules, the monomers, join together. For example, many ethene molecules join to form poly(ethene). Plastics are made from polymers, so their monomers have already been joined.

Worked solution

In addition polymerisation, the C=C double bond of each monomer opens up and the monomers join into a long chain. Working backwards, the repeat unit –[CH₂–CHCl]– has two carbon atoms in the main chain: one with two H atoms, and one with one H and one Cl. The monomer has exactly the same atoms on each carbon, joined by a double bond: CH₂=CHCl (chloroethene). Always check that the groups on each carbon in the monomer match the groups in the repeat unit.

Worked solution

In addition polymerisation, monomers with a C=C double bond join together and nothing else is made, so the polymer is the only product. In condensation polymerisation, two different functional groups react (for example –COOH with –OH or with –NH₂) and each time they join, a small molecule such as water is removed. The linkages formed are ester linkages (in polyesters) or amide linkages (in polyamides).

Worked solution

(a) Poly(ethene) is made by addition polymerisation. The C=C double bond in each ethene molecule opens, and very many ethene molecules join together into a long chain. No other product is formed.

(b) Because poly(ethene) is unreactive and non-biodegradable, it does not break down when buried. It stays in landfill for a very long time, so landfill sites fill up and more land has to be used.

(c) Plastic waste builds up in the oceans. Animals can swallow it or get trapped in it, and it breaks into tiny pieces that can enter food chains.

(d) When plastics are burned, toxic gases such as carbon monoxide (and, from some plastics, hydrogen chloride) can be produced, which harm people and the environment.

Worked solution

(a) Each monomer has two functional groups (two –COOH groups or two –OH groups). When a –COOH group reacts with an –OH group, an ester linkage forms and a water molecule is removed. This is condensation polymerisation.

(b) Remove H from the –OH of the diol and OH from the –COOH of the acid, then join them. The acid part becomes –CO–CH₂–CH₂–CO– and the diol part becomes –O–CH₂–CH₂–O–. Joining one of each gives the repeat unit –[CO–CH₂–CH₂–CO–O–CH₂–CH₂–O]–. Each end bond links to the next repeat unit. One repeat unit contains one acid part and one diol part.

(c) The –CO–O– group joining the parts is an ester linkage, so this polymer is a polyester.

(d) With a diamine instead of a diol, the –COOH group reacts with –NH₂ and water is again removed. The linkage formed is the amide linkage, –CO–NH–, so the polymer is a polyamide.

Worked solution

(a) In propene, CH₂=CH–CH₃, only the two carbon atoms joined by the double bond become part of the main chain. The CH₃ group becomes a side branch. When the double bond opens, the repeat unit is –[CH₂–CH(CH₃)]–, with a bond at each end linking to the next unit. A common error is to put all three carbon atoms in the chain.

(b) Work backwards: put a double bond between the two carbon atoms of the repeat unit, keeping the same atoms on each carbon. The monomer is CF₂=CF₂ (tetrafluoroethene).

(c) PET is a condensation polymer. It can be converted back into its monomers, and these monomers can then be re-polymerised to make new PET.