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Organic Chemistry

Topic 1 of 34

A formula should tell you exactly which atoms are connected

Move between molecular, displayed, condensed, skeletal and three-dimensional representations.

A-Level 9476 (2026-2027)

A formula should tell you exactly which atoms are connected

Move between molecular, displayed, condensed, skeletal and three-dimensional representations.

A molecular formula gives atom counts but does not uniquely identify an organic compound. A structural formula must make the connectivity unambiguous. For example, CH3CH(OH)COOH shows a three-carbon chain, an OH group on its middle carbon and a carboxylic acid group at its end. Its molecular formula is C3H6O3, and its empirical formula is CH2O.

Read each representation without changing the molecule
RepresentationWhat is shown
DisplayedEvery atom and bond, including all C-H bonds. A double bond is two shared pairs, not two different neighbouring atoms.
Condensed structuralGroups such as CH3, CH2 and CH(OH) preserve connectivity; parentheses identify branches.
SkeletalEach unlabelled line end or corner is carbon. Add enough implied H atoms for each carbon to have valency four. Heteroatoms and hydrogens bonded to them are shown.
StereochemicalA solid wedge points towards the viewer; a hashed wedge points away; ordinary lines lie in the page. These encode spatial arrangement.

Read an oxygen-containing skeletal structure

The left line end is CH3, the central junction is CH bearing OH, and the right junction is the carboxyl carbon with a double bond to O and a single bond to OH. This is CH3CH(OH)COOH.

The three carbon positions are implicit; every oxygen is explicit. Count bonds before adding hydrogens. Aromatic rings may be drawn as a hexagon with a circle rather than displaying all six carbon and hydrogen labels.

For naming, choose a parent chain containing the principal functional group and number it to give that group the required low locant, then locate double bonds and substituents. Examples include butan-2-ol, 2-bromopropane and propan-2-one. Do not choose a visually longest line if it excludes the functional group that defines the name.

Worked example

Turn an ester name into a displayed formula

Draw methyl propanoate, then give its condensed and molecular formulae.

  1. Propanoate identifies the three-carbon acid-derived part. Include the carbonyl carbon in that count: C-C-C(=O)-O-.
  2. Methyl identifies the one-carbon group attached to the single-bonded oxygen. Join it as C-C-C(=O)-O-C, not as a branch on the carbonyl carbon.
  3. Complete carbon valency four by adding three H to the first carbon, two to the second and three to the last. The carbonyl carbon already has four bonds; neither oxygen has an attached H in this ester.
  4. Show every C-H bond for the displayed formula below. Check that each carbon has four bonds and each oxygen two, counting a double bond twice.
Answer

CH3CH2C(=O)OCH3; molecular formula C4H8O2. A molecular formula alone would not distinguish this ester from its constitutional isomers.

Displayed methyl propanoate: every atom and bond

The chain is H3C-CH2-C(=O)-O-CH3. Each of the eight hydrogen atoms has an explicitly drawn bond to carbon. The carbonyl carbon has no hydrogen, and the bridging oxygen bonds to two different carbons.

The acid-derived three-carbon part is on the left; the alcohol-derived methyl group is on the right. Removing C and attached-H labels while retaining every heteroatom converts this into a skeletal drawing.