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Organic chemistry · Lesson 1

Same atoms, different molecules

Carbon builds chains, branches and rings, and a single formula can describe many different compounds. The IUPAC naming rules, the functional groups and the kinds of isomers.

In the 1820s two young German chemists, Justus von Liebig and Friedrich Wöhler, were studying two silver compounds. Liebig’s silver fulminate exploded at the slightest touch; Wöhler’s silver cyanate was perfectly stable. When they analysed them, each assumed the other had made a mistake: the two compounds had exactly the same composition.

Neither had made a mistake. The atoms were the same; the way they were joined was different. In 1830 Jöns Jacob Berzelius named the phenomenon isomerism, from the Greek for “equal parts”.

Carbon compounds are full of isomers. C2H6O is ethanol, the liquid in wine, and also methoxymethane, a gas used as an aerosol propellant. With millions of possible compounds, chemists need a naming system in which every name describes exactly one structure. This lesson teaches that system.

Carbon chains and homologous series

Carbon forms four covalent bonds and bonds strongly to itself, so it builds chains, branches and rings. A homologous series is a family of compounds with the same functional group and general formula, each member differing by CH2: alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n−2.

The same molecule can be written as a molecular formula (C4H10), a condensed structural formula (CH3CH2CH2CH3), a full structural formula showing every bond, or a skeletal formula, where each corner or line end is a carbon atom and hydrogens on carbon are not drawn.

Naming: the IUPAC rules

  1. Find the longest carbon chain containing the functional group (and any multiple bond). Its length gives the stem: meth, eth, prop, but, pent, hex, hept, oct.
  2. Number the chain so that the functional group (or multiple bond) gets the lowest number; then give substituents the lowest numbers.
  3. Name the branches (methyl, ethyl) and halogens (chloro, bromo) as prefixes with their positions, in alphabetical order, using di-, tri- for repeats.
  4. Punctuation: commas between numbers, hyphens between numbers and words: 2,3-dimethylpentane, but-2-ene, propan-1-ol.

For example, CH3CH2CH(CH3)CH(CH3)CH3: the longest chain has five carbons, numbering from the right gives the methyl groups positions 2 and 3, so it is 2,3-dimethylpentane.

Functional groups

FamilyGroupName endingExample
AlkaneC–C only-aneCH3CH2CH3, propane
AlkeneC=C-eneCH2=CHCH3, propene
AlkyneC≡C-yneCH3C≡CH, propyne
Haloalkane–Xchloro-, bromo- (prefix)CH3CHClCH3, 2-chloropropane
Alcohol–OH-olCH3CH(OH)CH3, propan-2-ol
Aldehyde–CHO-alCH3CH2CHO, propanal
Ketone–CO–-oneCH3COCH3, propanone
Carboxylic acid–COOH-oic acidCH3CH2COOH, propanoic acid
Ester–COO–alkyl -oateCH3COOCH2CH3, ethyl ethanoate
Amine–NH2-amineCH3CH2NH2, ethanamine
Amide–CONH2-amideCH3CONH2, ethanamide

Alcohols are primary, secondary or tertiary according to how many carbon atoms are bonded to the carbon carrying the –OH: one (propan-1-ol), two (propan-2-ol) or three (2-methylpropan-2-ol). The classification matters for their reactions (Lesson 3).

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Lab: six bottles, three pairs of isomers

Run tests on each unlabelled bottle, record the observations, then identify all six compounds. Each pair shares a molecular formula.

Isomers

Structural isomers have the same molecular formula but atoms joined in a different order:

  • chain isomers have different carbon skeletons (butane and methylpropane);
  • position isomers have the same group in a different position (propan-1-ol and propan-2-ol);
  • functional group isomers have different functional groups (propanal and propanone; ethanoic acid and methyl methanoate).

Cis/trans (geometric) isomers arise because there is no rotation about a C=C double bond. They exist only when each carbon of the double bond carries two different groups. In cis isomers the matching groups are on the same side; in trans isomers, on opposite sides.

Common traps

  • A chain drawn bent is still the same chain: always count the longest continuous path.
  • But-1-ene has no cis/trans isomers: one carbon of its double bond holds two hydrogens.
  • In esters, the alkyl part comes from the alcohol and is named first: CH3COOCH2CH3 is ethyl ethanoate.
Now put it to workFive worked examples, twenty practice questions and ten harder problems on this lesson.
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