Everything from the 10 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A molecular formula gives the number and type of each atom (C2H6O); a structural formula shows how the atoms are arranged without drawing bonds (CH3CH2OH); a displayed formula shows every atom and every bond. A functional group is the atom or group of atoms that decides the chemical properties of a compound: C=C in alkenes, −OH in alcohols, −COOH in carboxylic acids. A homologous series is a family of compounds with the same general formula and same functional group, so they have similar chemical properties and a trend in physical properties; neighbouring members differ by CH2. A saturated compound has only C−C single bonds; an unsaturated compound contains at least one C=C double (or C≡C triple) bond.
A homologous series has similar chemical properties but a trend in physical properties: do not swap the two words. "Saturated" means only single carbon-carbon bonds, not "contains as much hydrogen as possible dissolved in it". And the −OH in an alcohol is a covalently bonded group, not a hydroxide ion: alcohols are not alkalis.
The stem counts the carbon atoms in the chain: meth- 1, eth- 2, prop- 3, but- 4. The ending names the homologous series: -ane for alkanes, -ene for alkenes, -ol for alcohols and -oic acid for carboxylic acids. A number shows where the functional group sits when it could be in more than one place, as in propan-1-ol or but-2-ene. (Extended) An ester is named alcohol part first (ending -yl) then acid part (ending -oate): ethanol + ethanoic acid gives ethyl ethanoate. (Extended) Structural isomers have the same molecular formula but different structural formulae, such as butane and methylpropane (both C4H10).
A chain drawn with a bend is the same molecule, not an isomer: follow the bonds, not the shape on the page. Name the longest unbroken carbon chain, so a "bent butane" is still butane. In esters the alcohol gives the first word (ethyl) and the acid gives the second (ethanoate), the reverse of the order in the word equation.
The fossil fuels are coal, natural gas and petroleum (crude oil). The main constituent of natural gas is methane, CH4. Petroleum is a mixture of hydrocarbons (compounds of hydrogen and carbon only) that is separated by fractional distillation: the oil is vaporised, the vapour rises up a column that is hot at the bottom and cooler at the top, and each fraction condenses at the level where the temperature falls below its boiling point. Going down the column the molecules have longer chains, higher boiling points, lower volatility and higher viscosity.
Fractional distillation is a physical separation: no bonds inside molecules are broken, so it is not cracking. Do not say the fractions are separated by melting point or by density; they condense at different temperatures because they have different boiling points. A volatile fraction evaporates easily, so high volatility goes with a low boiling point.
Alkanes are saturated hydrocarbons with the general formula CnH2n+2; every bond is a single covalent bond. They are generally unreactive, except for combustion and substitution by chlorine. In complete combustion an alkane burns in plenty of oxygen to give carbon dioxide and water, releasing a lot of energy, which is why alkanes are used as fuels. In substitution a hydrogen atom is replaced by a chlorine atom: CH4 + Cl2 → CH3Cl + HCl. (Extended) This is a photochemical reaction: ultraviolet light provides the activation energy.
Substitution makes two products: do not forget the HCl. Alkanes do not undergo addition, because they have no C=C bond to open. The condition is ultraviolet light, not heat with a catalyst, and the chlorine replaces a hydrogen, never a carbon.
Alkenes are unsaturated hydrocarbons with the general formula CnH2n and a C=C double bond as the functional group. To tell an alkene from an alkane, shake it with aqueous bromine: an alkene turns it from orange to colourless, an alkane leaves it orange. (Extended) Alkenes undergo addition reactions, in which the double bond opens and two atoms or groups add on, giving a single product: with bromine (ethene → 1,2-dibromoethane), with hydrogen using a nickel catalyst (ethene → ethane), and with steam using an acid catalyst (ethene → ethanol).
An addition reaction gives one product only: there is no HBr or H2O left over, unlike substitution. Learn the catalysts as pairs: hydrogen with nickel, steam with an acid (phosphoric acid). And the bromine test result is orange to colourless, never "clear": clear means transparent, and orange bromine water is already clear.
Cracking is the thermal decomposition of large alkane molecules into smaller alkanes and alkenes, and sometimes hydrogen. It needs a high temperature (about 500–600 °C) and a catalyst (such as silica or alumina, often in the form of zeolites). Cracking equations must balance: C10H22 → C8H18 + C2H4. It is done because crude oil contains more long-chain fractions than are needed and not enough short-chain fractions such as petrol, and because the alkenes made are the starting materials for polymers and other chemicals such as ethanol.
Cracking is thermal decomposition, not combustion: no oxygen is involved and nothing burns. The products always include an alkene (unless the only other product is hydrogen), because there are not enough hydrogen atoms for every product to be saturated. Always check that both carbon and hydrogen balance.
Alcohols contain the –OH functional group; ethanol is C2H5OH. Ethanol is made by fermentation: aqueous glucose with yeast at 25–35 °C in the absence of oxygen: C6H12O6 → 2C2H5OH + 2CO2. It is also made by the catalytic addition of steam to ethene at about 300 °C and 6000 kPa with an acid catalyst (phosphoric acid): C2H4 + H2O → C2H5OH. Extended: fermentation uses a renewable raw material and little energy but is slow and gives impure, dilute ethanol; hydration is fast, continuous and gives pure ethanol but uses non-renewable crude oil and much energy. Ethanol burns completely to carbon dioxide and water and is used as a fuel and a solvent.
Fermentation needs the absence of oxygen, but combustion of ethanol needs oxygen: do not mix them up. The fermentation temperature is 25–35 °C, not a high temperature: heating kills (denatures the enzymes in) the yeast. In the hydration equation water is steam, and the catalyst is an acid (phosphoric acid), not yeast.
Carboxylic acids contain the –COOH functional group; ethanoic acid is CH3COOH. Ethanoic acid is a weak 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. Its salts are ethanoates. Extended: ethanoic acid is made by oxidising ethanol, either with acidified aqueous potassium manganate(VII) or by bacterial oxidation during vinegar production. A carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water: ethanoic acid + ethanol ⇌ ethyl ethanoate + water.
In an ester name the alcohol part comes first (ethyl, methyl) and the acid part second (ethanoate, propanoate), even though the formula CH3COOC2H5 is written acid part first. Methanol + ethanoic acid gives methyl ethanoate, not ethyl methanoate. Ethanoic acid is weak because it is only partly dissociated, not because it is dilute: weak and dilute mean different things.
Polymers are large molecules built up from many small molecules called monomers. Many plastics are polymers made from chemicals obtained from crude oil. In addition polymerisation, alkene monomers join when their C=C double bonds open; the polymer is the only product. Ethene forms poly(ethene), whose repeat unit is –CH2–CH2–. Extended: draw the polymer from a given alkene, and deduce the monomer from a section of polymer. Plastics are hard to dispose of: they are non-biodegradable and fill landfill sites, they accumulate in oceans, and burning them can release toxic gases. Recycling reduces these problems.
The repeat unit has single bonds only: the double bond belongs to the monomer, not the polymer. Do not include the side group in the backbone (poly(propene) has a two-carbon repeat unit with a CH3 branch, not three carbons in a row). Plastics are made from crude oil, so they are not renewable; "non-biodegradable" means microorganisms cannot break them down.
Extended. In condensation polymerisation two different functional groups react to form a link and a small molecule, usually water, is released each time. Nylon is a polyamide made from a dicarboxylic acid and a diamine; the monomers join by amide linkages, –CONH–. PET is a polyester made from a dicarboxylic acid and a diol; the monomers join by ester linkages, –COO–. PET can be converted back into its monomers and re-polymerised, so it can be recycled. Proteins are natural polyamides made from amino acids, H2N–CHR–COOH, joined by the same amide link; hydrolysis breaks them back into amino acids.
The amide link is –CO–NH– and the ester link is –CO–O–: check whether the atom after the C=O is N or O. In a repeat unit, the water has already gone, so do not leave –OH or –H on the ends of the bracketed unit. Hydrolysis is the reverse of condensation: water is used, not released.