Types of Reaction in Organic Chemistry

1. Types of Reaction in Organic Chemistry

2. 8 main types of Organic Reaction to learn • Substitution Reactions • Esterification Reactions • Addition Reactions • Polymerisation Reactions • Elimination Reactions • Redox Reactions • Reactions as Acids • Organic Synthesis

3. Substitution Reactions

4. A substitution reaction is a chemical reaction where an atom or group of atoms in a molecule is replaced by an atom or another group of atoms • A very common example of substitution reactions is the reaction between alkanes and halogens (chlorine/Bromine) in the presence of UV light

5. Mechanism for reaction • The mechanism of a reaction tells you the detailed step by step processes that occur in a reaction. • For example in the following reaction CH4+ Cl2 CH3Cl + HCl What exactly happened between the molecules when they were reacting?

6. We know roughly what has happened and we can come up with a simple mechanism

7. H H Cl Cl Cl Cl Methane Chloromethane = + + Hydrogen Chloride Chlorine

8. Simple mechanism H Cl Cl Chloromethane Methane Hydrogen and Chlorine have swapped places Substitution Hydrogen Chloride Chlorine

9. We need to find out the following • How does this happen? • What causes it? • How do we know that what we suggest is correct?

10. Stage 1 Initiation Getting Started

11. Both species are the same Called Homolytic Fission Ultra violet light breaks the bond Chlorine molecule Cl2 2 Chlorine radicals each with an unpaired electron i.e. 7 electrons in outside shell

12. Stage 2 Propagation Keeping it going

13. H Cl Let’s put in the 2 electrons in this bond Chlorine radical Methane The chlorine radical pulls the hydrogen and one electron across to it. Hydrogen chloride Methyl radical The methyl radical is now free to react with a chlorine molecule

14. Cl Cl Chlorine radical Methyl radical Chlorine Chloromethane Chlorine radical can now go and react with a methane molecule

15. H Cl Let’s put in the 2 electrons in this bond Chlorine radical Methane The chlorine radical pulls the hydrogen and one electron across to it. Hydrogen chloride Methyl radical The methyl radical is now free to react with a chlorine molecule

16. Cl Cl Chlorine radical Methyl radical Chlorine Chloromethane Chlorine radical can now go and react with a methane molecule

17. And on and on and on and on

18. Stage 3 Termination Grinding to a halt

19. Three different ways this can happen

20. Cl Cl Chlorine molecule Reaction stops No free radicals to keep it going Chlorine radical Chlorine radical

21. Cl Chlorine radical Methyl radical Chloromethane forms Reaction stops Because there are no free radicals to keep it going

22. Methyl radical Methyl radical Ethane Reaction stops because no free radicals to keep it going The formation of ethane proves that this is the mechanism Reaction speeded up by sources of free radicals such as tetraethyl lead.

23. Remember the evidence that proves this mechanism • Small amounts of ethane are found in the products of the reaction • The amount of chloromethane produced is proportional to the amount of UV light absorbed • If tetra methyl lead is added to the mixture there is a marked increase in the rate of reaction, we know that tetra methyl lead decomposes to form methyl radicals which feed into the system to speed it up

24. 4. If ethane is used instead of methane then butane will be formed rather than ethane (as 2 ethyl radicals combine)

25. The chlorination of methane is commonly called a Free Radical Substitution Reaction

26. Esterification • Esters are formed when an alcohol and carboxylic acid react together • This is called a condensation reaction as water is formed • It could also be called a substitution reaction since the H of the OH group of the carboxylic acid molecule is replaced by an alkyl group

28. Formation of esters is Esterification • It is important to remember the reaction can go backwards also ie. The ester formed can react with the water to form the carboxylic acid and alcohol • This reverse reaction is called Hydrolysis • Hydrolysis will happen very easily in the presence of a base such as NaOH

29. This base hydrolysis of esters results in the formation of the sodium salt of the carboxylic acid • NB see fig 23.9 p 364 • This reaction is very important in the manufacture of soap and is commonly referred to as Saponification

30. Substitution Reactions Tetrahedral Tetrahedral Planar Planer

32. Learning Check • What is a substitution reaction? • Can you describe in 4 steps and using diagrams the mechanism of a substitution reaction? • What is this mechanism called? • What is a radical? • What causes the formation of radicals?

33. Learning Check • Can you describe how 3 pieces of evidence support the theory of free radical substitution? • Can you name 3 types of substitution reaction? • What 2 molecules react in an esterificateion raection

34. Learning Check • What type of reaction is an esterification raection • What is the reverse of an esterification raection called? • What is the product of this type of raectiopn?

35. Addition Reactions • An addition reaction is one in which two substances react together to form a single substance • Addition reactions are very important in industry as a whole range of new products can be formed

36. Consider the following reactions

37. You may notice that in these addition reactions double bonds were broken and replaced with single bonds • An important feature of addition reactions is that saturated compounds are formed from unsaturated compounds

38. Margarine Hydrogenation of C=C double bonds is used in the manufacture of margarine It is believed that unsaturated fats are better for health. Butter contains many saturated fats whereas sunflower oils etc don’t However oils are not good for spreading on bread By controlling the degree of saturation in margarines we can manipulate their melting points to make them spreadable and control the amount of saturation to try to make them healthier

39. Mechanism for Addition Reactions • The reaction between ethene and bromine can occur in the dark indicating it is different to free radical substitution • There are 4 overall steps in this mechanism

40. Step 1 • The C=C double bond is a region of high negative charge • The Br2 molecule approaches the double bond and becomes polarised

42. Step 2 • The polarisation becomes so strong the Br2 molecule splits in Br+ and Br – • As two different species are firmed this is called Heterolytic fission

43. Step 3 • The Br+ species in order to gain the two electrons it needs to give it a stable octet of electrons attacks the C2H4 molecule • It forms a covalent bond with one of the carbon atoms • The other carbon atom is left with a positive charge and is now called a carbonium ion

44. There is evidence to suggest the carbonium ion has a cyclic structure known as a cyclic bromonium ion • Because bromine has a relatively large size it allows this to form

46. Step 4 • The carbonium ion is attacked by the Br- ion • This forms 1,2-dibromoethane

47. Ionic Addition • This mechanism is often called ionic addition since ions add across the C=C double bond

48. Evidence for this mechanism • When ethene reacts with bromine water in in the presence of sodium chloride solution 2 other compounds (1-bromo-2-chloro ethane) and ( 2- bromo ethanol) are formed as well as 1,2 di-bromoethane

49. 1-bromo-2-chloro ethane is formed when the carbonium ion is attacked by the Cl- ion • This is evidence for the existence of the carbonium ion

50. 2- bromo ethanol is formed when the carbonium ion is attacked by the water molecule • Again this is evidence for the presence of the carbonium ion