13. Molecular and empirical formulae The molecular formula of a compound shows the number of each type of atom present in one molecule of the compound. The empirical formula of a compound shows the simplest ratio of the atoms present. CH 2 OC2H4O2C2H4O2 C 6 H 12 O 6 CH 3 C2H6C2H6 Empirical formula Molecular formula Neither the molecular nor empirical formula gives information about the structure of a molecule. There are many ways of representing organic compounds by using different formulae.

14. The displayed formula of a compound shows the arrangement of atoms in a molecule, as well as all the bonds. Single bonds are represented by a single line, double bonds with two lines and triple bonds by three lines. Displayed formula of organic compounds The displayed formula can show the different structures of compounds with the same molecular formulae. ethanol (C 2 H 6 O)methoxymethane (C 2 H 6 O)

15. Structural formula of organic compounds The structural formula of a compound shows how the atoms are arranged in a molecule and, in particular, shows which functional groups are present. Unlike displayed formulae, structural formulae do not show single bonds, although double/triple bonds may be shown. CH 3 CHClCH 3 2-chloropropane H 2 C=CH 2 ethene CH 3 C≡N ethanenitrile

16. Displayed and structural formula activity

17. Types of formulae

18. Functional groups and homologous series A functional group is an atom or group of atoms responsible for the typical chemical reactions of a molecule. A homologous series is a group of molecules with the same functional group but a different number of –CH 2 groups. Functional groups determine the pattern of reactivity of a homologous series, whereas the carbon chain length determines physical properties such as melting/boiling points. propanoic acid (CH 3 CH 2 COOH) ethanoic acid (CH 3 COOH) methanoic acid (HCOOH)

19. Functional groups

20. Homologous series and general formulae The general formula of a homologous series can be used to calculate the molecular formula of any member of the series by substituting n for the number of carbon atoms. Example: what is the molecular formula of chloroethane? For example, the general formula of a halogenoalkane is C n H 2n+1 X, where X is a halogen. 1. Write down the general formula:C n H 2n+1 X 3. Substitute n into the general formula:C 2 H 5 Cl 2. Write down the value of n: n = 2

21. Homologous series

35. What is isomerism? Isomers are molecules with the same molecular formula (i.e. the same number and type of atoms) but in which the atoms are arranged in a different way. There are two main categories of isomerism: structural isomerism and stereoisomerism. Structural isomers have different structural formulae. Three types of structural isomerism are chain isomerism, positional isomerism and functional group isomerism. Stereoisomers have the same structural formula, but the 3D arrangement of atoms is different. Two types are cis–trans isomerism and optical isomerism.

37. Chain isomerism in alkanes In chain isomers, the carbon chain is arranged differently. For example, hexane has several chain isomers, all with the molecular formula C 6 H 14 : hexane 3-methylpentane 2,3-dimethylbutane

38. Positional isomerism In positional isomers, the functional group is attached to a different carbon atom. For example, chloropentane has several positional isomers, all with the molecular formula C 5 H 11 Cl: 3-chloropentane 2-chloropentane 1-chloropentane

39. Positional isomerism in alkenes Positional isomerism also exists in alkenes with four or more carbon atoms. For example, hexene has several positional isomers, all with the molecular formula C 6 H 12 : hex-1-ene hex-2-ene hex-3-ene

40. Functional group isomerism Functional group isomers contain different functional groups and so are members of different homologous series. propanol (C 3 H 8 O) an alcohol methoxyethane (C 3 H 8 O) an ether For example, both alcohols and ethers have the general formula C n H 2n+2 O so they may be functional group isomers:

41. Structural isomers activity

44. What’s the keyword?

45. Multiple-choice quiz