Unsaturated hydrocarbons

1. Unsaturated hydrocarbons

2. Unsaturated hydrocarbons

3. ALKENES

4. Alkene’s shape Ethene (ethylene) Propene (propylene)

5. Industrial Preparation and Use of Alkenes

6. Reactions of alkenes Alkane Alcohol H2O H2 X2, H2O Halohydrin oxidation 1,2-Diol X2 ozonolysis HX 1,2-Dihalide :CH2 Carbonyl compound Cyclopropane Halide

7. Addition – elimination reactions Addition Elimination

8. Preparation of alkenes 1. Dehydrohalogenation 2. Dehydration + H-OH

9. Stability of alkenes • Cis alkenes are less stable than trans alkenes • Less stable isomer is higher in energy

10. Comparing Stabilities of Alkenes • Evaluate heat given off when C=C is converted to C-C • More stable alkene gives off less heat • trans-Butene generates 5 kJ less heat than cis-butene

11. Stability of alkenes tetrasubstituted > trisubstituted > disubstituted > monosusbtituted S T A B I L I T Y FREE ENERGY

12. Electrophilic Addition to Alkenes

13. Electrophilic Addition Energy Path • Two step process • First transition state is high energy point

14. Orientation of electrophilic addition (Markovnikov’s rule) Sole product Not formed

15. Energy diagram ENERGY Primary carbocation G#prim Tertiary carbocation G#tert (CH3)2C=CH2 + HCl (CH3)2CHCH2Cl (CH3)3CCl REACTION PROGRESS

16. Carbocation stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals The fourth orbital on carbon is a vacant p-orbital The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents

17. Carbocation stability Measured by dissociation enthalpy of C-Cl bond

18. Inductive stabilization of cation species

19. Carbocation rearrangements

20. Addition of Halogens to Alkenes • Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process • F2 is too reactive and I2 does not add • Cl2 reacts as Cl+ Cl- • Br2 is similar

21. Addition of Br2 to Cyclopentene • Addition is exclusively trans – stereospecific reaction

22. Mechanism of Bromine Addition • Br+ adds to an alkene producing a cyclic ion • Bromonium ion, bromine shares charge with carbon • Gives trans addition

23. Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromonium ion is a reactive electrophile and bromide ion is a good nucleophile Stereospecific anti addition

24. Addition of Hypohalous Acids to Alkenes: Halohydrin Formation This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)

25. Mechanism of Formation of a Bromohydrin • Br2 forms bromonium ion, then water adds • Orientation toward stable carbocation species

26. An Alternative to Bromine Bromine is a difficult reagent to use for this reaction N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

27. Addition of Water to Alkenes Hydration of an alkene is the addition of H-OH to to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

28. Mechanism of AlkeneHydration

29. Alkene hydrogenation Addition of H-H across C=C Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)

30. Mechanism of Alkene Hydrogenation

31. Electrophilic (polar) addition of HBr to 2-methylpropene

32. Radical addition of HBr to 2-methylpropene (formation of anti-Markovnikov product) Sole product Not formed

33. Energy profile for radical addition of HBr to 2-methylpropene ENERGY Primary radical Tertiary radical (CH3)2C=CH2 + HBr (CH3)3CBr (CH3)2CHCH2Br REACTION PROGRESS

34. Radical stability order > > > Tertiary > Secondary > Primary > Methyl S T A B I L I T Y

35. Alkene oxidation - epoxidation Epoxidation results in a cyclic ether with an oxygen atom Stereochemistry of addition is syn

36. Osmium Tetroxide Catalyzed Formation of Diols

37. Oxidation of Alkenes:Cleavage to Carbonyl Compounds Ozone, O3, adds to alkenes to form molozonide Reduce ozonide to obtain ketones and/or aldehydes

38. Oxidation of Alkenes:Cleavage to Carbonyl Compounds

39. Permanganate Oxidation of Alkenes

40. ALKYNES

41. Alkyne’s shape Ethyne (acetylene) 1.20 Angstroem 1.06 Angstroem

42. Electronic Structure of Alkynes Carbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized px and py orbitals forming π bonds. The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond. The bond is shorter and stronger than single or double Breaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)

43. 1-Butyne

44. Preparation of alkynes 1. Dehydrohalogenation of 1,2-dihalides 2. Dehydrohalogenation of vinyl halides

45. Bond energy in alkynes H = 48 kcal/mol H = 64 kcal/mol H = 88 kcal/mol

46. Reactivity of alkynes 1. Addition of halides 2. Addition of hydrogen halides

47. Reactivity of alkynes 3. Hydration of alkynes Keto-enol tautomerism

48. Reactivity of alkynes 4. Reduction of alkynes

49. Reactivity of alkynes 5. Acetylide formation 6. Acetylide alkylation

50. Alkyne Acidity: Formation of Acetylide Anions • Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic • Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion • The sp-hybridization at carbon holds negative charge relatively close to the positive nucleus