Chemical Ideas 12 Organic chemistry frameworks

1. Chemical Ideas 12Organic chemistry frameworks 12.1 Alkanes

2. Many carbon compounds are found in living organisms, this why their study is named organic chemistry

3. Carbon is unique About 7 million compounds containing carbon and hydrogen are known to chemists This is far more than the number of compounds from all the other elements put together

4. Electron structure makes it the first member of Gp 4 in the centre of the periodic table – this is responsible for its special properties Carbon atom has 4 electrons in its outer shell – too many to lose or gain – the ions would have +4 or -4 (too highly charged) All carbon compounds are covalent rather than ionic Methane(CH4) C shares 4 electrons with 4 H atoms Carbon forms strong covalent bonds with itself to form rings and chains this is called catenation Each C atom can form 4 covalent bonds- chains may be straight or branched and can have other atoms or groups substituted on to them Why carbon? H   H  C  H   H

5. Hydrocarbons • Only contain carbon and hydrogen • General formula CXHY Methane –CH4 is an alkane Ethene C2H4 is an alkene Benzene C6H6 is an arene H l H - C - H l H

6. Alkanes • Saturated hydrocarbons • Contain C and H only • Contain single bonds C-C • Have 4 bonds to every carbon (C) atom • Are non polar • General formula CnH2n+2 • Physical properties such as m.pt,b.pt and density change as the number of carbon atoms in the molecule increase

7. Alkanes Names of all alkanes end in -ane shortened Name # carbons Structural Formula Methane 1 CH4 Ethane 2 CH3CH3 Propane 3 CH3CH2CH3 Butane 4 CH3CH2CH2CH3 Pentane 5 CH3CH2CH2CH2CH3

8. Alkanes Names of all alkanes end in -ane shortened Name # carbons Structural Formula Hexane 6 CH3CH2CH2CH2CH2CH3 Heptane 7 CH3CH2CH2CH2CH2CH2CH3 Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3 Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3 Decane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 A series of compounds which are related in this way are called ahomologous series

9. Finding the formula of alkanes Molecular formula = m x empirical formula Where m is 1,2,3,…… • For hydrocarbons , composition of mass is easily found by burning a known mass in oxygen, measuring the amounts of CO2 and H2O this is called combustion analysis

10. Finding the formula of alkanes Example 0.100g of hydrocarbon X on complete combustion gave 0.309g CO2 and H2O 0.142g calculate the empirical formula of X Answer 1st calc the masses of C and H in 0.100g 44g CO2 contains 12g of C mass of C in 0.100g X = (12/44) x 0.309g = 0.0843g 18g H2O contains 2g H mass of H in 0.100g X = (2/18) x 0.142g = 0.0158g

11. Finding the formula of alkanes C H Ratio by mass 0.0843 : 0.0158 Ratio by moles 0.00703 : 0.0158 Simplest by ratio(÷ by smaller) 1 : 2.25 Whole number ratio 4 : 9 Empirical formula = C4H9

12. Finding the formula of alkanes Example Relative molecular mass of X was found to be 114 by using a mass spectrometer Answer Empirical formula of X is C4H9 but MrC4H9 = 57 this is ½ of 114. So the molecular formula of X must be (C4H9)2 = C8H18

13. Structure of alkanes • Full structural formula of methane Shows all atoms and bonds • Propane • Shortened structural formula for propane • Further shortened to

14. Structure of alkanes

15. Branched alkanes -Isomers • Same molecular formula • Same number and types of atoms • Different arrangement of atoms

16. Alkyl Groups Branches on carbon chains H H C CH3 methyl H H H H C C CH3CH2 ethyl H H

17. Naming Branched Alkanes CH3 methyl branch CH3CH2CH2CHCH2CH3 6 5 4 3 2 1 Count 3-Methylhexane on third C CH3 six carbon chain group

18. Naming Summary 1. Count the C’s in the longest chain 2. Name each attached group 3 Count the longest carbon chain to give the first attached group the smallest number 4. Name and locate each group

19. Cycloalkanes • The is a group of alkanes that have a cyclic structure. • These cycloalkanes contain a carbon chain that is in a ring. • Each cycloalkane has a formula that is 2C less than the corresponding alkane. • For example, propane is C3H8 whereas cyclopropane ic C3H6. Butane is C4H10 and cyclobutane is C4H10. • The names of the cyclic structures use the prefix cyclo in from of the alkane name for the carbon chain Cyclopropane CH2 CH2 CH2 Cyclobutane CH2 CH2 CH2 CH2

20. More Cycloalkanes Cyclopentane CH2 CH2 CH2 CH2 CH2 Cyclohexane CH2 CH2 CH2 CH2 CH2 CH2

21. Shapes of alkanes Pairs of electrons in the bonds repel each other so in all covalent compounds the bonds are as far away from each other as possible. The bond angle for H-C-H is 109 represents a bond in the plane of the paper represents a bond in a direction behind the plane of the paper represents a bond in a direction in front of the plane of the paper

22. Shapes of alkanes Ethane Each carbon atom is at the centre at of a tetrahedral arrangement a simpler way of drawing ethane which shows the shape less accurately

23. Shapes of alkanes Hydrocarbon chains are not really straight but a zig-zag of carbon atoms. All bond angles are 109◦

24. Reaction of alkanes • Oxidation are v. unreactive unaffected by acids and alkalis and oxidising agents • When they do react it is usually in the gas phase and energy needs to be supplied to get the reaction started

25. Reaction of alkanes Combustion alkane + O2 CO2 + H2O + heat C6H14 + 9 1/2 O2 6CO2 + 7H2O + heat • If air supply is limited combustion in incomplete and products include CO and C (soot) along with partially oxidised hydrocarbons

26. Action of heat on alkanes • When alkanes fractions are heated under different conditions,3 different reactions can occur, isomerisation, reforming and cracking