Unit 23

1. Unit 23 Alkanes

2. What are organic compounds ?

3. Organic Compounds • Derived from living organisms. • Name some organic compounds. • Is carbon dioxide an organic compound? • ??? • Examples: proteins, fats, carbohydrates, petroleum, etc. ???

4. Carbon as a unique element • Uses up its four outermost shell electrons to form 4 covalent bonds.. • Electronic diagram of methane: ?

5. Carbon as a unique element • Able to form long straight chain of carbon atoms, • long carbon chain with branches, and cyclic compounds, • and able to form multiple bonds, such as carbon –carbon double and triple bonds

6. Carbon as a unique element • long carbon chain with branches, and cyclic compounds, Propane, C3H8 Butane, C4H10

7. Structural formulae

8. Structural formulae

9. Molecular formulae • Formula to show the actual numbers of different atoms / elements in one molecule of a compound. • e.g., butane: C4H10 • Ethanol: C2H6O • Ethanoic acid: C2H4O2

10. Carbon as a unique element • Straight carbon chain with branches • Start to have branches when there are 4 carbon atoms.

11. Carbon as a unique element • Cyclic compounds

12. Carbon as a unique element • Able to form multiple bonds

13. Homologous Series • A group of organic compounds with the same general formula and with successive members differing by a –CH2- unit. • e.g., Alkanes, Alkenes, Alkanols and alkamoic acid

14. Characteristics of homologous series • Have the same general formula. • With similar chemical properties. • With a gradual change / increase in physical properties such as boiling and melting points. • Have the same functional group. • Each successive member differs by a –CH2- unit. • Can be prepared by similar methods.

15. Functional group • An organic compound can be divided into 2 parts – the unreactive saturated hydrocarbon part and the reactive functional group.

16. Functional Group

17. Alkanes • General formula: CnH2n+2 • Saturated hydrocarbons with no functional group • Unreactive, with no reaction with common oxidizing agents, dilute acids and alkalis.

18. Physical Properties of alklanes • Insoluble in water, but soluble in organic solvents. • The first four homologues (methane to butane) are gases at room temperature and pressure. Why ? • The heat of combustion increases roughly by the same amount between successive members. Why? • The heat of combustion if the heated released when 1 mole of the compound is burnt completely in air. • There is a grdaulal increase in m.p. or b.p. as the molecular sizes (masses) increase. Why?

19. Chemical properties of alkanes • Saturated hydrocarbons – unreactive • Without any carbon-carbon multiple bond(s). • Have no reaction with oxidzing agents, dilute acids and alkalis.

20. Reaction with halogens • No reaction with halogens in dark. • Reacts with halogens under diffused sunlight. • The hydrogen atoms are substituted (replaced) by halogen atoms. • The reaction is known as substitution. • A mixture of substituted products are formed when a mixture of methane and chlorine is exposed to diffused sunlight.

21. Reaction of methane with chlorine • CH4 + Cl2  CH3Cl + HCl • CH3Cl + Cl2  CH2Cl2 + HCl • CH2Cl2 + Cl2  CHCl3 + HCl • CHCl3 + Cl2  CCl4 + HCl • Hydrogen chloride gas forms a misty white fume in moist air. • Put a glass rod wetted with concentrated ammonia solution near the the mouth of the gas jar. A dense white fume of ammonium chloride is formed. • NH3(g) + HCl(g)  NH4Cl(s)

22. Reaction with halogens • Bromine reacts similarly with methane, but with a slower reaction rate. • Iodine - no reaction with methane. • Relative reactivity of halogens: Cl2 > Br2 > I2

23. Combustion of alkanes • Complete combustion – with excess oxygen (air) or small alkanes • Burns with a blue flame with yellow tip. • Products of combustion ? • Carbon dioxide and steam (water vapour)

24. Combustion of alkanes • Incomplete combustion – with limited supply of oxygen / large alkanes • Burns with a yellow (luminous) flame. • Carbon soots give a yellow colour to the flame. • Products of incomplete combustion: carbon particles, carbon monoxide and water vapour

25. Systematic Naming of alkanes (IUPAC Naming System) • Choose the longest carbon chain as the parent chain. • Number the parent chain from oneend such that the brances (substituents) bear the smallest numbers. • Name of alkyl groups branches) • CH3- methyl group • CH3CH2- ethyl group • CH3CH2CH2- propyl group

26. Systematic Naming of alkanes (IUPAC Naming System) • Construct the name by using the prefix to indicate the number of carbon atoms in the parent chain. • The parent chain name ends with –ane. • Use mono-, di-, tri- and tetra- to indicate the frequency of the substitutent • Use 1, 2, 3, …. To indicate the positions of the substituents.

27. IUPAC Naming 2-methylbutane

28. Structural isomerism • Organic compounds exhibit structural isomerism when they have the same molecular formulae, but with different structural formulae (structures). • Starting from butane (4 carbon atoms), there are structural isomers.

29. Structural isomers of C4H10 butane methylpropane

30. Structural isomers • Draw all structual isomers of C5H12. • Pentane • 2-methylbutane • Dimethypropane • Ex. P. 29 Bk 2

31. Functional Groups • An organic compound – with a unreactive hydrocarbon part (tail) and the reactive funtional group. • Functional group – an atom / groups of atoms that determines most of the properties of the compound. • Compounds with the same functional group are put into the same homologous series.