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2. Addition of bromine (Br 2 ) to alkenes General reaction Alkene  bond lost; two new C-Br  bonds formed Stereospecific reaction observed with cycloalkenes Cyclopentene Trans-1,2-dibromo- cyclopentane (no cis-isomer)

3. Reaction mechanism involves two steps 1 st Step: alkene  electrons attack Bromine Bromide ion and a cyclic epibromonium ion results The large size of Bromine w.r.t Carbon (4 th row vs. 2 nd row) means that it can span two Carbons rather than

4. 2 nd Step: addition of bromide anion Anion approaches epibromonium ion from the face opposite that blocked by bromine With cyclopentene Epibromonium ion and bromide Trans-1,2-dibromo- cyclopentane

5. 1,2-Dichlorobutane 1-Butene Chlorine also adds to alkene C=C bonds

6. Benzene Molecular formula C 6 H 6 All Carbons and Hydrogens equivalent Kekulé structure (1865) = However, does not behave like a typical alkene Less reactive than typical alkenes Only reacts with bromine in presence of a catalyst A substitution rather than an addition reaction occurs not

7. Styrene

8. Also, all benzene C-C bond lengths equal: 139 pm Comparison: C-C 154 pm; C=C 134 pm Planar ring of sp 2 hybridised Carbons 6 p z orbitals overlap to form a continuous cyclic  system  electron density located above and below the plane of the ring 6  electrons All 6 C-C bonds equivalent [Not a representation of benzene  molecular orbitals]

9. An orbital representation of the bonding in benzene.

10. Arrangement of 6  electrons in a closed cyclic  systems is especially stable Said to possess aromaticity Aromatic systems very common (e.g. benzene and its derivatives) Representing the  system in benzene Represents  system well Of limited use in describing reactivity Better to use a combination of Kekulé structures

11. Some points about this representation Neither Kekulé structure alone is an adequate representation of the  bonding in benzene. An adequate representation requires both structures simultaneously The structures are known as resonance forms or resonance contributors Each resonance structure contributes [equally] to the overall  bonding system ‘↔’ is used to show that structures are resonance forms of each other; resonance structures are enclosed in square brackets

12. These are NOT independent species existing in equilibrium The  electrons in benzene are said to be resonance delocalised over the entire ring system Resonance delocalisation is generally energetically favourable Resonance delocalisation of 6  electrons in a closed ring system is especially favourable: aromaticity

13. Graphite

14. Carbon nanotube

15. Aromatic systems in pharmaceuticals atorvastatin (Lipitor®) sildenafil (Viagra®) miconazole

16. Alkynes Older name: Acetylenes Characterised by the presence of Carbon-Carbon triple bonds General structure of alkynes Groups R, C, C and R are co-linear Neither sp 3 nor sp 2 hybridised Carbon consistent with this geometry

17. Hybridisation 2e - 1e -

18. Two sp hybridised orbitals can be arrayed to give linear geometry Two remaining 2p orbitals are mutually orthogonal and orthogonal to the two sp hybridised orbitals [If the two sp orbitals lies along the z axis, 2p x lies along the x axis and 2p y along the y axis]

19. Overlap of sp orbitals on two Carbons results in  bond formation  [  * also formed; not occupied by electrons] = p x orbitals overlap to form a  bond in the xz plane    * also formed; not occupied] p y orbitals overlap to form a  bond in the yz plane [  * also formed; not occupied] 

20. C≡C consists of one  bond and two  bonds The  bond lies along the C-C bond axis The bond axis lies along the intersection of orthogonal planes One  bond lies in each plane, with a node along the bond axis View along the bond axis

21. A triple bond consists of the end-on overlap of two sp-hybrid orbitals to form a σ bond and the lateral overlap of the two sets of parallel oriented p orbitals to form two mutually perpendicular π bonds

22. First two members of the series of alkynes Ethyne (Acetylene) Propyne Nomenclature Prefix indicates number of carbons (‘eth…’, ‘prop…’, etc.) Suffix ‘…yne’ indicates presence of C≡C Butyne Can have C≡C between C1 and C2 or between C2 and C3 1-Butyne2-Butyne These are structural isomers

23. 6-Methyl-3-octyne 1-Heptene-6-yne 4-Methyl-7-nonen-1-yne

24. Linear geometry of alkynes difficult to accommodate in a cyclic structure Hence relatively few cycloalkynes Smallest stable cycloalkyne is cyclononyne Cyclononyne

25. Hydrogenation of alkynes Standard hydrogenation conditions completely remove the  bonds Both  bonds lost; four new C-H  bonds formed 3-Heptyne Heptane [Conversion of alkyne to alkane]

26. Possible to modify the catalyst so as to reduce its activity (poisoning) Lindlar’s catalyst Pd/PbO/CaCO 3 Pd: catalytic metal PbO: poison CaCO 3 : supporting material Hydrogenation of alkynes using Lindlar’s catalyst removes only one  bond [Only two Hydrogens added to C≡C ; products are alkenes] Reaction occurs on catalyst surface; both Hydrogens added to same face of alkyne Specifically Cis-alkenes produced

27. Alkyne Cis-alkene 3-Heptyne Cis-3-heptene

28. Alkynes can also be converted into alkenes by reaction with sodium or lithium metal in liquid ammonia [Na, liq. NH 3 ; or Li, liq. NH 3 ] This gives specifically Trans-alkenes 3-Heptyne Trans-3-heptene

29. Cis-2-hexene Trans-2-hexene

30. Addition of bromine (Br 2 ) to alkynes Can have addition to one or both alkyne  bonds Alkyne Trans-1,2-dibromo- alkene 1,1,2,2-tetra- bromoalkane Ethyne (Acetylene) 1,1,2,2-Tetrabromoethane 1-Butyne Trans-1,2-dibromo- 1-butene

31. Hydration of 1-alkynes [Addition of water] Requires catalysis by mercury (II) salts 1-AlkyneKetones 4-Methyl-1-hexyne Ketone

32. Review: quantifying acid strength: pK a AcidProton Conjugate base Extent of dissociation is medium dependent; hence medium should be defined If not otherwise stated, assume medium is water Acid Base Conjugate acid Conjugate base

33. Can define an equilibrium constant K a ’ Assume concentration of water stays constant; remove [H 2 O] term to give the dissociation constant K a

34. The stronger the acid HA, the greater the dissociation The stronger the acid, the greater the value of K a Range of K a values is vast; inconvenient numbers For convenience, take logs; define: pK a = - log 10 K a Stronger acid; greater K a ; smaller pK a Weaker acid; smaller K a ; greater pK a ‘Strong acid’: HCl pK a = -7.0 ‘Weak acid’: CH 3 CO 2 H pK a = 4.76

35. pKapKa 50.0 44.025.0 Conjugate bases Ethane and ethene are effectively devoid of acidity Ethyne dissociates to a miniscule extent Reflects the relative stability of the conjugate bases Most stable Least stable

36. Order of stability is related to the hybridisation of the Carbons bearing the negative charge Increasing s character assists in stabilising negative charge on Carbon s orbitals locate the excess electron density closer to the positively charged nucleus By comparison, p orbitals have nodal points at the nucleus s p

37. HC≡CH pK a 25 Extent of dissociation almost negligible However, dissociation can be driven to completion by reaction with very strong base Sodium amide (Sodamide) Sodium acetylide This reaction goes entirely to completion

38. The process is general for 1-alkynes Sodium acetylides Reaction of 1-alkynes with sodium amide gives complete conversion into sodium acetylides 1-Pentyne 3-Methyl-1-butyne Acetylide anions

39. Acetylide anions are strong Carbon nucleophiles React with Carbon electrophiles to form new Carbon- Carbon bonds Chloromethane New C-C bond formed Acetylide anion attacks methyl Carbon Chloride anion displaced

40. 2-Pentyne 2-Methyl-3-pentyne Propyne 2-Butyne

41. Recall: Etc. Reaction mechanisms so far have involved nucleophiles reacting with electrophiles… …and ionic intermediates Such mechanisms are known as polar mechanisms Covalent bond formation the occurs as a result of movement of pairs of electrons

42. New covalent bonds can also be formed by processes in which… …each molecular species involved donates one electron Chlorination of alkanes proceeds by such mechanisms Homolytic cleavage [Heterolytic cleavage: cleavage into ions]

43. Methane (CH 4 ) Methyl radical Methyl radical is a neutral species bearing an unpaired electron Is said to be a ‘free radical’ Methyl radical can react with further chlorine molecules This step generates product and further chlorine atom

44. Overall process is a chain reaction Initiation Propagation

45. Chlorination of alkanes other than methane e.g. 2-Methylbutane Substrate contains primary (1 o ), secondary (2 o ) and tertiary (3 o ) Hydrogens 1 o C-H 2 o C-H 3 o C-H

46. Monochlorination of 2-methylbutane: four products obtained [1] [2] [3][4]

47. Four products obtained in unequal amounts If all Hydrogens on the substrate were equally reactive towards chlorine atom, would expect: [1] [2] [3] [4] 50% 25% 17% 8% Based on Expected ratio [1]:[2]:[3]:[4] = 6:3:2:1

48. Observed ratio of products [1] [2] [3] [4] 34% 16% 28% 22% Less of products [1] and [2] than expected More of product [3] than expected Substantially more of product [4] than expected Conclusion: Hydrogens not all equally reactive towards chlorine Relative reactivity most reactive 3 o > 2 o > 1 o least reactive

49. This trend reflects the relative stabilities of the intermediate free radicals more stable than More stable than

50. Primary, secondary, tertiary system used to distinguish between substitutents of the same number of Carbons Propyl group Two possibilities 1-Propyl (‘Propyl’) 2-Propyl or Isopropyl

51. Butyl group Four possibilities 1-Butyl (‘Butyl’) 2-Butyl or sec-Butyl (“secondary-Butyl”) tert-Butyl (“tertiary- Butyl”) [ or 2-Methyl-2-propyl] Isobutyl

52. Free-Radical Polymerization (of Alkenes) Examples

53. SC slides now available on ChemWeb Free radical polymerization mechanism Require a free radical initiator (In) Termination