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Alkenes

Chapter 30. Alkenes. 30.1 Introduction 30.2 Nomenclature of Alkenes 30.3 Physical Properties of Alkenes 30.4 Preparation of Alkenes 30.5 Reactions of Alkenes. 30.1 Introduction (SB p.122). The C = C double bond in ethene. Functional group of alkenes :. 30.1 Introduction (SB p.122).

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Alkenes

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  1. Chapter 30 Alkenes 30.1Introduction 30.2Nomenclature of Alkenes 30.3Physical Properties of Alkenes 30.4Preparation of Alkenes 30.5Reactions of Alkenes

  2. 30.1 Introduction (SB p.122) The C = C double bond in ethene Functional group of alkenes:

  3. 30.1 Introduction (SB p.122) Alkenes show geometrical isomerism

  4. 30.2 Nomenclature of Alkenes (SB p.123) Nomenclature of Alkenes 1. Determine the stem name by selecting the longest possible straight chain containing the C = C double bond and use the ending ‘-ene’ 2. Number the parent chain so as to include both carbon atoms of the double bond, and begin numbering with the end of the chain nearer the C = C double bond 3. Designate the position of the C = C double bond by using the number of the first atom of the double bond 4. Designate the positions of the substituents by using the numbers obtained by application of rule 2

  5. 30.2 Nomenclature of Alkenes (SB p.123) Examples:

  6. 30.2 Nomenclature of Alkenes (SB p.123) 5. If two identical groups are present on the same side of the C = C double bond, the compound is designated as cis; if they are on opposite sides, the compound is designated as trans. e.g.

  7. 30.2 Nomenclature of Alkenes (SB p.124) Example 30-1 Give the IUPAC names for the following alkenes: (a) (b) Answer Solution: (a)trans-3,4-dichlorohept-3-ene (b)cis-3,4-dimethyloct-3-ene

  8. 30.2 Nomenclature of Alkenes (SB p.124) (a) (b) (c) Check Point 30-1 Draw the structural formula for each of the following alkenes: (a) cis-hex-3-ene (b) trans-2,3-dihydroxybut-2-ene (c) cis-1,2-dichloroethene Answer

  9. 30.3 Physical Properties of Alkenes (SB p.124)

  10. 30.4 Preparation of Alkenes (SB p.125) Cracking • alkenes can be prepared industrially by cracking of high molecular mass alkanes

  11. 30.4 Preparation of Alkenes (SB p.125) Elimination Reactions Dehydrohalogenation • Dehydrohalogenation is the elimination of a hydrogen halide molecule from a haloalkane

  12. 30.4 Preparation of Alkenes (SB p.125) Examples:

  13. 30.4 Preparation of Alkenes (SB p.126) tertiary > secondary > primary haloalkane haloalkane haloalkane The ease of dehydrohalogenation of haloalkanes decreases in the order:

  14. 30.4 Preparation of Alkenes (SB p.126) Dehydrohalogenation of 2° and 3° haloalkanes can take place in more than one way and a mixture of alkenes is formed alc. KOHCH3CH2CHClCH3 CH3CH = CHCH3 + CH3CH2CH = CH2heat 2-chlorobutane But-2-ene But-1-ene (80%) (20%) Note: the more highly substituted alkene is formed as major product

  15. 30.4 Preparation of Alkenes (SB p.127) The relative stabilities of alkenes decrease in the order:

  16. 30.4 Preparation of Alkenes (SB p.125) Dehydration of Alcohols Dehydration is the removal of a water molecule from a reactant molecule

  17. 30.4 Preparation of Alkenes (SB p.125) The experimental conditions of dehydration depend on the structures of alcohols e.g.

  18. 30.4 Preparation of Alkenes (SB p.125) tertiary > secondary > primary alcohol alcohol alcohol The relative ease of dehydration of alcohols generally decreases in the order: Like dehydrohalogenation, the more highly substituted alkene is formed as the major product

  19. 30.4 Preparation of Alkenes (SB p.128) Example 30-2 Classify the following alcohols as primary, secondary or tertiary alcohols. (a) CH3CHOHCH2CH3 (b) CH3CH2CH2OH (c) (CH3)2COHCH2CH2CH3 Answer Solution: (a) Secondary alcohol (b) Primary alcohol (c) Tertiary alcohol

  20. 30.4 Preparation of Alkenes (SB p.128) (a) Secondary haloalkane (b) Primary haloalkane (c) Tertiary haloalkane Check Point 30-2 Classify the following haloalkanes as primary, secondary or tertiary haloalkanes. (a) (b) (c) Answer

  21. 30.4 Preparation of Alkenes (SB p.129) Addition Reactions Hydrogenation • hydrogenation of alkynes using Lindlar’s catalyst produces alkenes • prevent further hydrogenation of the alkenes formed to alkanes

  22. 30.5 Reactions of Alkenes (SB p.129) Alkenes are more reactive than alkanes Reason: presence of the C = C double bond Energetically favourable!!  alkenes undergo addition reactions and the reactions are exothermic

  23. 30.5 Reactions of Alkenes (SB p.129) Electrons of  bond are more diffuse and less firmly held susceptible to attack by electrophiles Electrophiles such as H+, neutral reagents such as bromine (can be polarized) react with C = C double bond

  24. 30.5 Reactions of Alkenes (SB p.130) Electrophilic Addition Reactions Addition of Hydrogen Bromide Addition of hydrogen bromide to C = C double bond yields a bromoalkane

  25. 30.5 Reactions of Alkenes (SB p.130) Examples:

  26. 30.5 Reactions of Alkenes (SB p.131) Propene reacts with HBr to give 2-bromopropane(major product) and 1-bromopropane (minor product) The formation of two possible products can be explained by the reaction mechanism.

  27. 30.5 Reactions of Alkenes (SB p.131) Reaction Mechanism: Electrophilic Addition Reaction of Hydrogen Bromide to Alkenes The mechanism for the addition of HBr to an alkene involves 2 steps Step 1: Step 2:

  28. 30.5 Reactions of Alkenes (SB p.131) Theoretical Explanation of Markownikoff’s Rule If the alkene is unsymmetrical, two different carbocations can be formed

  29. 30.5 Reactions of Alkenes (SB p.132) 2-bromopropane is the major product because the more stable secondary carbocation is formed in the first step

  30. 30.5 Reactions of Alkenes (SB p.132) Markownikoff’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atomadds to the carbon atom of the carbon-carbon double bond that already hasthe greater number of hydrogen atoms. Example:

  31. 30.5 Reactions of Alkenes (SB p.132) Markownikoff’s rule is related to the stability of the carbocation intermediate formed in the electrophilic addition reaction. The relative stabilities of carbocations:

  32. 30.5 Reactions of Alkenes (SB p.133) Addition of Bromine Alkenes react rapidly with Br2 in 1,1,1-trichloroethane at room temperature and in the absence of light e.g.

  33. 30.5 Reactions of Alkenes (SB p.133) The reddish brown colour of Br2 is decolourized Add Br2 in CH3CCl3 to excess alkene The behaviour of alkenes towards Br2 in CH3CCl3 is a useful test for the presence of carbon-carbon multiple bonds

  34. 30.5 Reactions of Alkenes (SB p.133) Br2 + H2O HBr + HOBr Bromic(I) acid Addition of Bromine Water In an aqueous solution of Br2, the following equilibrium exists The bromine atom bears a partial positive charge while the oxygen atom bears a partial negative charge∵ oxygen is more electronegative than bromine

  35. 30.5 Reactions of Alkenes (SB p.134) When bromic(I) acid reacts with alkenes, bromohydrin is formed e.g.

  36. 30.5 Reactions of Alkenes (SB p.134) Addition of Sulphuric(VI) Acid Alkenes react with cold and concentrated H2SO4 to form alkyl hydrogensulphates e.g.

  37. 30.5 Reactions of Alkenes (SB p.134) The large bulky –OSO3H groupmakes the alkyl hydrogensulphate very unstable. Two possible further reactions take place: 1. Regeneration of alkenes 2.Production of alcohols

  38. 30.5 Reactions of Alkenes (SB p.135) Catalytic Hydrogenation In the presence of metal catalysts (e.g. Pt, Pd or Ni), H2 is added to each atom of C = C double bond to form an alkane e.g.

  39. 30.5 Reactions of Alkenes (SB p.135) • Hydrogenation is useful in analyzing unsaturated hydrocarbons • The number of double or triple bonds present in the unsaturated hydrocarbon molecule can be deduced by the number of moles of hydrogen reacted • Catalytic hydrogenation is used to convert liquid vegetable oil to semi-solid fats in making margarine and solid cooking fats (known as hardening of oils).

  40. 30.5 Reaction of Alkenes (SB p.136) (a) Either one of the following tests: Hex-1-ene can decolourize bromine water or chlorine water in the dark while hexane cannot. Hex-1-ene can decolourize acidified potassium manganate(VII) solution while hexane cannot. Check Point 30-3 (a) What chemical tests would you use to distinguish between two unlabelled bottles containing hexane and hex-1-ene respectively? Answer

  41. 30.5 Reaction of Alkenes (SB p.136) (b) (i) (ii) Check Point 30-3 (b) What is the major product of each of the following reactions? (i) (ii) Answer

  42. 30.5 Reaction of Alkenes (SB p.136) (c) (i) CH3CH2CH3 (ii) (iii) Check Point 30-3 (c) Give the reaction products for the following reactions: Ni (i) CH3CH = CH2 + H2  conc.H2SO4 (ii) CH3CH = CHCH3  (iii) CH3CH = CHCH3 + Br2  Answer

  43. 30.5 Reaction of Alkenes (SB p.136) (d) The increasing order of stability of carbocations is : Tertiary carbocations are the most stable because the three alkyl groups release electrons to the positive carbon atom and thereby disperse its charge. Primary carbocations are the least stable as there is only one alkyl group releasing electrons to the positive carbon atom. Check Point 30-3 (d) Arrange the following carbocations in increasing order of stability. Explain your answer briefly. Answer

  44. 30.5 Reaction of Alkenes (SB p.136) (e) Upon the reaction with hydrogen chloride, it involves the formation of carbocations. Therefore, the order of reaction rates follows the order of the ease of the formation of carbocations, i.e. the stability of carbocations: Therefore, the rates of reactions of the three compounds with hydrogen chloride increase in the order: Check Point 30-3 (e) Based on your answer in (d), arrange the following molecules in the order of increasing rate of reaction with hydrogen chloride. Answer

  45. 30.5 Reactions of Alkenes (SB p.136) (unstable) Ozonolysis Ozonolysis is a widely used method for locating the double bond of an alkene The unstable ozonide is reduced directly by treatment with Zn and H2O

  46. 30.5 Reactions of Alkenes (SB p.137) Overall process of ozonolysis: e.g.

  47. 30.5 Reaction of Alkenes (SB p.137) Example 30-3 Predict the structures of the following hydrocarbons A, B and C using the information given below: Answer

  48. 30.5 Reaction of Alkenes (SB p.138) Solution: A: As C3H6 can be expressed as CnH2n, the hydrocarbon is a molecule with one C = C double bond. When A undergoes ozonolysis, and are formed. ∴The possible structure of A is CH3CH = CH2.

  49. 30.5 Reaction of Alkenes (SB p.138) Solution: B: As C6H10 can be expressed as CnH2n-2 and only one dicarbonyl compound is formed on ozonolysis, the hydrocarbon is an alicyclic molecule with one C = C double bond. ∴ The possible structure of B is .

  50. 30.5 Reaction of Alkenes (SB p.138) Solution: C: As C10H16 can be expressed as CnH2n-4. Two products with totally five carbon atoms are formed. So the original compound is an acyclic molecule with three C = C double bonds. ∴ The possible structure of C is CH3CH = CHCH2CH = CHCH2CH = CHCH3.

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