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, C 3 H 8 Butane, C 4 H 10

7. Structural formulae

9. Molecular formulae  Formula to show the actual numbers of different atoms / elements in one molecule of a compound.  e.g., butane: C 4 H 10  Ethanol: C 2 H 6 O  Ethanoic acid: C 2 H 4 O 2

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 – CH 2 - 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 – CH 2 - 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: C n H 2n+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  CH 4 + Cl 2  CH 3 Cl + HCl  CH 3 Cl + Cl 2  CH 2 Cl 2 + HCl  CH 2 Cl 2 + Cl 2  CHCl 3 + HCl  CHCl 3 + Cl 2  CCl 4 + 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.  NH 3(g) + HCl (g)  NH 4 Cl (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: Cl 2 > Br 2 > I 2

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 one end such that the brances (substituents) bear the smallest numbers.  Name of alkyl groups branches) CH 3 - methyl group CH 3 CH 2 - ethyl group CH 3 CH 2 CH 2 - 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 C 4 H 10 butane methylpropane

30. Structural isomers  Draw all structual isomers of C 5 H 12.  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.