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 CH 4 + Cl 2 CH 3 Cl + 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. Cl Hydrogen Chloride Methane + = H Cl H Chlorine Chloromethane +

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

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

12. Stage 2 Keeping it going Propagation

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

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

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

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

17. And on and on and on and on

18. Stage 3 Grinding to a halt Termination

19. Three different ways this can happen

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

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

22. Methyl radical Methyl radical 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. Ethane

23. Remember the evidence that proves this mechanism 1.Small amounts of ethane are found in the products of the reaction 2.The amount of chloromethane produced is proportional to the amount of UV light absorbed 3.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 ReactionsTetrahedral PlanarPlaner

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 Br 2 molecule approaches the double bond and becomes polarised

42. Step 2 The polarisation becomes so strong the Br 2 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 C 2 H 4 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

51. Addition of Cl 2 and HCl to Ethene The same mechanism applies In the case of Cl 2 the Cl + species attacks the C=C double bond then the Cl - attacks to form 1,2 di-chloroethane In the case of HCl the H + ion attacks the C=C double bond then the Cl - attacks to form chloroethane

53. NB, a cyclic intermediate is not formed in these cases as the Cl and H atoms are too small

54. Addition Reactions Planar Tetrahedral

55. Tetrahedral Carbons C 3 H 8

57. Learning check What is an addition reaction? Describe the mechanism of an addition reaction for Br 2 and Ethene What is the evidence for this mechanism? What is formed in the reaction between Br 2 and Ethene but not in the reaction between Cl 2 and HCl and Ethene?

58. Why is this not formed? What is a catalyst for addition reactions? Name some addition reactions

59. Polymerisation Reactions It is possible for molecules containing double bonds to undergo addition reactions among themselves Molecules of ethene for example may link with each other to form a long chain of carbon atoms

61. The compound Polythene which is formed is an example of a polymer Polymers are long chains of molecules made by joining together many small molecules Polymers have a repeating structure and can have a molecular mass of many thousand

62. The small unit which make up polymers are called monomers Ethene is a monomer used to make polythene Polythene is a very useful material and is used to make plastic bags, bowls etc.

63. Polypropene is another important polymer used in toys, beakers, chairs etc. These polymers are called Addition Polymers because they are made from addition reactions Crude oil is the raw material for these polymers!

64. Learning Check What is the starting material for polymers? What type of reaction is polymerisation? How do polymers form? What are the repeating units in a polymer called? Name some common polymers Give a use for these polymers

65. Elimination Reactions When we made ethene in the lab we used ethanol as our starting material and removed a water molecule from it An elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

66. The removal of water from an alcohol is the only type of elimination reaction you need to know Since water is removed it is also known as a Dehydration Reaction

68. Elimination Reaction Tetrahedral Planar

70. Learning Check What is an elimination reaction? Describe an elimination reaction you performed in the school laboratory What is another name for this type of reaction? What catalyst was used?

71. Redox Reactions When a primary alcohol reacts with an oxidising agent it is converted to an aldehyde Suitable oxidising agents are Acidified Sodium Dichromate or Acidified Potassium Permanganate

72. If the oxidising agent is in excess the aldehyde is further oxidised to a carboxylic acid

73. If a secondary alcohol reacts with an oxidising agent a ketone will be formed

74. A bottle of wine left open in air will soon become oxidised and the ethanol will change into a dilute solution of ethanoic acid (vinegar)

75. Your liver oxidises ethanol to ethanal in your body It can oxidise about 8g of ethanol an hour Excessive quantities of ethanol can cause cirrhosis of the liver The ethanal is eventually oxidised to carbon dioxide + water

76. Sodium Dichromate was used in older breathalyser tests as it changes from orange to green when it oxidises alcohol

77. It is important to note that in the preaparation of aldehydes the acidified sodium dichromate is added slowly and the aldehyde is distilled off immediately to prevent further oxidation to carboxylic acids In the production of carboxylic acids the reaction is refluxed as we want to achieve complete oxidation of the alcohol

78. Carboxylic acids can be reduced to aldehydes and then to alcohols in the presence of a nickel catalyst using hydrogen to add to the molecules

80. Learning Check What is does redox stand for? What will a primary alcohol become oxidised to? What will a secondary alcohol become oxidised to? What will an aldehyde become oxidised to? Why does wine turn acidic if left open? Name an oxidising agent used in the laboratory

81. Reactions as Acids If a piece of sodium is added to ethanol hydrogen gas is released A hydrogen atom in the ethanol is replaced with a sodium atom The ethanol is thus behaving as an acid as it is losing a H + ion Alcohols are much weaker acids than water

82. Acid Nature of The Carboxylic Acid Group What makes it possible for the hydrogen atom of the carboxyl group to break off as a proton? 2 reasons.................

83. 1. Inductive Effect In the carboxyl functional group the H atom is attached to an oxygen atom in a C-H- O chain The carbon atom is slightly positive and tends to attract electrons from the Oxygen atom in the OH group This electron pulling effect is called the inductive effect

84. This drawing away of electrons facilitates the ionisation of the H atom in the O-H bond This means the COOH group can lose a proton quite easily and hence show acidic properties

85. 2. Stability of Carboxylate Ion When the carboxyl group loses a proton it forms a negative ion called the carboxylate ion

86. Chemists have found that the 2 Carbon/Oxygen bonds are the same length suggesting the actual structure is an intermediate of the image below

87. The negative charge is not localised on one oxygen but is spread over 3 atoms giving it extra stability This stability means it will lose the proton easily

89. Learning Check What is an acid? There are 2 reasons carboxylic acids readily donate a proton what are they? When an alcohol reacts with a sodium ion what is formed? When a carboxylic acid reacts with a sodium ion what is formed? When a carboxylic acid reacts with sodium hydroxide what is formed? When a carboxylic acid reacts with sodium carbonate what is formed?

90. Organic Synthesis Organic Synthesis is the process of making organic compounds from simpler starting materials

91. Chemists specialising in organic synthesis have to plan steps and design experiments for making new and useful substances The new molecule being made is called the Target Molecule Many medicines and other useful substances are made in organic synthesis

92. Learning Check What is meant by organic synthesis? When chemists are trying to make something ne what is the molecule they are trying to make called?

93. In the pharmaceutical industry new organic compounds are continuously being designed Organic synthesis involves bond breaking and bond making The following is a summary of some of the steps to make new substances out of others and back again!