WARNING! This document contains visual aids for lectures It does not contain lecture notes It does not contain act

1. WARNING! • This document contains visual aids for lectures • It does not contain lecture notes • It does not contain actual lectures • Failure to attend lectures can harm your performance in module assessment • Printing out handouts of PowerPoint documents • From ‘File’ menu, select ‘Print’ • Set ‘Print range’ to ‘All’; set ‘Print what:’ to ‘Handouts’ • Set ‘Slides per page’ to ‘3’ (recommended to facilitate taking of notes), ‘4’ or ‘6’ • Click on ‘OK’

2. Addition of bromine (Br2) to alkenes General reaction • Alkene p bond lost; two new C-Br s bonds formed • Stereospecific reaction observed with cycloalkenes Cyclopentene Trans-1,2-dibromo- cyclopentane (no cis-isomer)

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

4. 2nd 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. Chlorine also adds to alkene C=C bonds 1,2-Dichlorobutane 1-Butene

6. Molecular formula C6H6 Benzene • 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 sp2 hybridised Carbons • 6 pz orbitals overlap to form a continuous cyclic p system p electron density located above and below the plane of the ring • 6 p electrons • All 6 C-C bonds equivalent • [Not a representation of benzene p molecular orbitals]

9. An orbital representation of the bonding in benzene.

10. Arrangement of 6 p electrons in a closed cyclic p systems is especially stable • Said to possess aromaticity • Aromatic systems very common (e.g. benzene and its derivatives) Representing the p system in benzene • Represents p 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 p 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 p 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 p electrons in benzene are said to be resonance delocalised over the entire ring system • Resonance delocalisation is generally energetically favourable • Resonance delocalisation of 6 p 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 sp3 nor sp2 hybridised Carbon consistent with this geometry

17. Hybridisation 2e- 1e- 1e- 1e- 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, 2px lies along the x axis and 2py along the y axis]

19. Overlap of sp orbitals on two Carbons results in s bond formation s = • [s* also formed; not occupied by electrons] • px orbitals overlap to form a p bond in the xz plane p [p* also formed; not occupied] • py orbitals overlap to form a p bond in the yz plane p [p* also formed; not occupied]

20. C≡C consists of one s bond and two p bonds • The s bond lies along the C-C bond axis • The bond axis lies along the intersection of orthogonal planes • One p 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-Butyne 2-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 p bonds • Both p bonds lost; four new C-Hs bonds formed Heptane 3-Heptyne • [Conversion of alkyne to alkane]

26. Possible to modify the catalyst so as to reduce its activity (poisoning) Lindlar’s catalyst Pd/PbO/CaCO3 • Pd: catalytic metal • PbO: poison • CaCO3: supporting material • Hydrogenation of alkynes using Lindlar’s catalyst removes only one p 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. NH3; or Li, liq. NH3] • This gives specifically Trans-alkenes 3-Heptyne Trans-3-heptene

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

30. Addition of bromine (Br2) to alkynes • Can have addition to one or both alkyne p bonds Alkyne Trans-1,2-dibromo- alkene 1,1,2,2-tetra- bromoalkane 1,1,2,2-Tetrabromoethane Ethyne (Acetylene) Trans-1,2-dibromo- 1-butene 1-Butyne

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

32. Review: quantifying acid strength: pKa Conjugate base Acid Proton • 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 Ka’ • Assume concentration of water stays constant; remove [H2O] term to give the dissociation constant Ka

34. The stronger the acid HA, the greater the dissociation • The stronger the acid,the greater the value of Ka • Range of Ka values is vast; inconvenient numbers • For convenience, take logs; define: pKa = - log10Ka • Stronger acid; greater Ka; smaller pKa • Weaker acid; smaller Ka; greater pKa • ‘Strong acid’: HCl pKa = -7.0 • ‘Weak acid’: CH3CO2H pKa = 4.76

35. pKa 50.0 44.0 25.0 Conjugate bases • Ethane and ethene are effectively devoid of acidity • Ethyne dissociates to a miniscule extent • Reflects the relative stability of the conjugate bases Least stable Most 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 pKa25 • 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 of1-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 Chloride anion displaced Acetylide anion attacks methyl Carbon Chloromethane New C-C bond formed

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 • Covalent bond formation the occurs as a result of movement of pairs of electrons • Such mechanisms are known as polar mechanisms

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 (CH4) 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 Propagation Initiation Propagation

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

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

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. [1] [2] [3] [4] 34% 16% 28% 22% Observed ratio of products • 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 3o > 2o > 1o 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