IUPAC naming and classification

Learning objectives

Objectives and contextOverview

By the end of this chapter you should be able to:

• identify the functional group in a molecule and classify it as an alkane, alkene, alcohol, carboxylic acid or ester A
• state the IUPAC name of a molecule with up to six carbon atoms in its main chain, including position numbers where needed A
• draw the full structural (displayed) formula of a molecule from its name, showing every bond A
• deduce molecular formulas using general formulas and the conservation of atoms A
• relate the properties and uses of fuels, plastics, solvents and flavourings to the families they belong to D

In the seventeenth century a naturalist distilled a sharp-smelling acidic liquid from crushed ants and called it formic acid, from the Latin word for ant. Names like this record where a substance was first found, but they say nothing about its structure. Today more than ten million carbon compounds are known, and chemists need a naming system that works in the opposite direction: from the name you should be able to draw the molecule, and from the molecule you should be able to write the name. The International Union of Pure and Applied Chemistry (IUPAC) provides that system. Formic acid is methanoic acid: “meth-” says the molecule has one carbon atom, and “-oic acid” says it contains the carboxylic acid group.

02 / Why carbon

Carbon, chains and familiesClassification

Carbon has four electrons in its outer shell. It completes its outer shell by forming four covalent bonds, sharing one electron pair in each. Two features make carbon unusual. First, carbon atoms bond strongly to each other, so they can form chains of almost any length, branched chains and rings. Second, the same carbon skeleton can carry different groups of atoms — hydrogen, oxygen, nitrogen, halogens — so the variety of possible molecules is enormous. Compounds built on carbon skeletons are called organic compounds; simple carbon compounds such as carbon dioxide and carbonates are normally treated as inorganic.

A hydrocarbon is a compound of carbon and hydrogen only. Alkanes and alkenes are hydrocarbons. Alcohols, carboxylic acids and esters also contain oxygen, so they are not hydrocarbons, even though their skeletons are hydrocarbon chains.

The family decides the chemistry. Every alkane burns and is otherwise fairly unreactive; every alkene reacts by adding atoms across its double bond; every alcohol has an –OH group that makes the small ones mix with water; every carboxylic acid is a weak acid; every ester has a characteristic, often fruity, smell. Learning five families is far easier than learning ten million compounds.

03 / Representing molecules

Formulas that show structureStructural formulas

Different formulas give different amounts of information. For ethanol:

• the molecular formula, C₂H₆O, gives only the number of atoms of each element;
• the condensed structural formula, CH₃CH₂OH, lists the atoms carbon by carbon, so the order of attachment can be read;
• the full structural formula (also called the displayed formula) shows every atom and every bond as a line.

A full structural formula is the one to use whenever a question says “showing all of the bonds”. It is also the best way to check your own work, because each atom must have its correct number of bonds: C four, O two, H one. If a carbon in your drawing has three or five lines, the drawing is wrong.

04 / The naming system

How an IUPAC name is builtIUPAC naming

An IUPAC name for these families has up to three parts. The stem gives the number of carbon atoms in the longest chain that contains the functional group. The ending gives the family. A position number tells you which carbon carries the functional group, when there is more than one possibility.

The rules for position numbers are the same for every family:

1. Find the longest continuous chain of carbon atoms that contains the functional group.
2. Number the chain from the end that gives the functional group the lowest possible number.
3. Place the number immediately before the part of the name it refers to, separated by hyphens: but-1-ene, propan-2-ol.
4. Leave the number out when there is only one possible position: ethene, propene, ethanol, methanoic acid. A carboxylic acid group is always on carbon 1, so it never needs a number.

05 / Alkanes

AlkanesAlkanes

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2. They are the main compounds in natural gas (methane) and crude oil, and they are used as fuels: methane in gas cookers, propane and butane in bottled gas and portable heaters, and longer alkanes in petrol, diesel and candle wax. Because they contain only strong C–C and C–H single bonds, alkanes are unreactive except in combustion. With enough oxygen they burn completely to carbon dioxide and water:

C₅H₁₂(l) + 8O₂(g) → 5CO₂(g) + 6H₂O(g)

The balancing follows directly from the formula: five carbons give 5CO₂, twelve hydrogens give 6H₂O, and the oxygen atoms on the right (10 + 6 = 16) fix 8O₂ on the left. Topic 6 develops this method.

The general formula lets you identify an alkane from its molecular formula alone. Candle wax contains C₂₅H₅₂: 2 × 25 + 2 = 52, so it is an alkane. C₅H₁₂ fits the same pattern (2 × 5 + 2 = 12), so it belongs to the same family and is called pentane.

As the chain gets longer, the molecules are larger and the forces between them are stronger, so boiling points rise steadily along the series. Methane to butane are gases at room temperature, pentane to about C₁₆ are liquids, and the long-chain alkanes in wax are solids. This gradual trend is what makes fractional distillation of crude oil possible (Topic 5).

06 / Alkenes

AlkenesAlkenes

Alkenes are unsaturated hydrocarbons containing one C=C double bond. Their general formula is C_nH_2n: each has two hydrogen atoms fewer than the alkane with the same number of carbons, because two electrons from those carbon atoms are now used in the second bond between them. The smallest alkene has two carbon atoms (ethene, C₂H₄) — a double bond needs two carbons, so there is no “methene”.

The double bond is the functional group, and it is the reactive part of the molecule. Atoms can add across it, turning C=C into C–C. This is why alkenes are the starting materials for many plastics: thousands of ethene molecules join together to form poly(ethene), and propene molecules join to form poly(propene).

Alkenes are made from long-chain alkanes by cracking: the large molecules are heated, often with a catalyst, and break into smaller alkanes and alkenes. Atoms are conserved, so a missing product can be deduced by subtraction:

C₁₀H₂₂ → C₂H₄ + C₃H₆ + C₅H₁₂

Carbon: 10 − 2 − 3 = 5. Hydrogen: 22 − 4 − 6 = 12. The product C₅H₁₂ fits C_nH_2n+2, so it is an alkane (pentane). Cracking always produces at least one alkene, because there are not enough hydrogen atoms for every product to be saturated.

07 / Alcohols

AlcoholsAlcohols

An alcohol contains the hydroxyl group, –OH, bonded to a carbon atom that has only single bonds. The name ending is “-ol”, added to the alkane stem after removing the final “e”: methane → methanol, ethane → ethanol. From three carbons upwards the –OH can be on different carbons, so a number is needed: propan-1-ol has the –OH on an end carbon, propan-2-ol on the middle carbon.

The –OH group changes the properties considerably compared with the alkane of similar size. The small alcohols mix completely with water, and they have much higher boiling points than alkanes of similar mass, because the –OH groups attract neighbouring molecules strongly. Ethanol is a fuel (it burns to carbon dioxide and water, as alkanes do), a solvent in perfumes and medicines, and the alcohol in drinks. Methanol is a fuel and industrial solvent but is toxic.

08 / Carboxylic acids

Carboxylic acidsCarboxylic acids

A carboxylic acid contains the carboxyl group, –COOH: a carbon atom that carries both a double-bonded oxygen (C=O) and an –OH group. The carbon of the –COOH group is counted as part of the chain, and it is always carbon 1. The name ending is “-oic acid”: methanoic acid, HCOOH (one carbon); ethanoic acid, CH₃COOH (two carbons, the acid in vinegar); propanoic acid, CH₃CH₂COOH.

Carboxylic acids are weak acids. In water only a small fraction of the molecules release the hydrogen of the –OH group as a hydrogen ion, and the process is reversible:

HCOOH(aq) ⇌ H⁺(aq) + HCOO⁻(aq)

They show the typical reactions of acids — with bases, carbonates and reactive metals — and they are neutralised by adding a base or an alkali (Topic 7).

09 / Esters

EstersEsters

Esters form when a carboxylic acid reacts with an alcohol, usually with a little concentrated sulfuric acid as a catalyst. Water is also produced, and the reaction is reversible:

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
ethanoic acid + ethanol ⇌ ethyl ethanoate + water

The ester link, –COO–, joins the two parts: the C=O comes from the acid and the single-bonded O leads on to the carbon chain that came from the alcohol.

An ester's name has two words, and the order catches many students out. The first word comes from the alcohol and ends in “-yl” (methyl, ethyl, propyl). The second word comes from the acid and ends in “-oate” (methanoate, ethanoate). So ethanol + propanoic acid gives ethyl propanoate, CH₃CH₂COOCH₂CH₃.

Esters are volatile liquids with sweet, fruity smells. Many natural flavours and scents are esters or mixtures containing them, and ethyl ethanoate is a common solvent in glues and nail-varnish remover. Some fabrics are made of polyesters, long molecules held together by many ester links.

10 / Practice

Examination practicePast examination tasks

These tasks are adapted from past on-screen examinations; the chemistry, data and marks are unchanged. Attempt each one before opening the marking guidance.

A Criterion A · Knowing and understanding

B Criterion B · Inquiring and designing

None of the available examination tasks assessed criterion B through this topic.

C Criterion C · Processing and evaluating

None of the available examination tasks assessed criterion C through this topic.

D Criterion D · Reflecting on the impacts of science

11 / Examination feedback

Examiner's overall observationEvidence from examination feedback

12 / Summary

Summary and knowledge organiserRevision

Essential knowledge

• Carbon forms four covalent bonds and makes chains, so it forms a huge number of compounds, organised into homologous series by functional group.
• Alkanes C_nH_2n+2 (saturated, -ane); alkenes C_nH_2n (C=C, -ene); alcohols –OH (-ol); carboxylic acids –COOH (-oic acid); esters –COO– (-yl …-oate).
• Name = stem (chain length) + ending (family) + position number (lowest possible, only when needed).
• Full structural formula: every bond drawn; C four bonds, O two, H one.
• Ester names: alcohol part first (-yl), acid part second (-oate).