Alkanes can also form cyclic structures

1. Alkanes can also form cyclic structures Cyclopropane Cyclobutane Cyclopentane Cyclohexane General formula for cycloalkanes: CnH2n Can be conveniently represented using line segment formulae

2. Note: Cycloalkane nomenclature can be extended to include substitution Methylcyclohexane 1,3-Dimethylcyclohexane

3. Only one cycloalkane has a planar structure: cyclopropane All others have non-planar structure Ideal tetrahedral angle is 109.5o sp3 hybridised carbons with bond angles very different to 109.5o will be less stable (higher in energy) Bond angle approaching 60o Cyclopropane Cyclopropane is said to suffer from angle-strain All C-H bonds in cyclopropane are eclipsed

4. Cyclopentane has almost zero angle-strain To relieve torsional strain due to eclipsed C-H bonds, cyclopentane relaxes into a non-planar structure One CH2 group out of the plane of the ring

5. Cyclohexane A planar structure would have internal bond angles of 120o and eclipsed C-H bonds Actual structure relaxes into a chair conformation This reduces the bond angle to 109o • Geometry about each Carbon very close to tetrahedral ideal • Angle strain ~ zero

6. All C-H bonds staggered, i.e. torsional strain ~ zero Newman projection along any C-C bond The chair conformation contains two different hydrogen environments 6 Equatorial Hydrogens 6 Axial Hydrogens

7. At temperatures below 230 K (-43C): • can observe that two different types of hydrogen environment are present on cyclohexane Above this temperature, observe only one hydrogen environment Reason: cyclohexane molecules are not static above 230 K i.e. exist in different conformations Undergo ring inversion Boat conformation Exists in trace quantities Note: hydrogens axial in one chair conformation equatorial in the other

8. Ball-and-stick model of boat cyclohexane

9. What if one of the cyclohexane hydrogens were replaced by a methyl group? Cyclohexane Methylcyclohexane The two chair conformations are no longer equivalent One has the methyl group in an axial position; one in an equatorial position

10. These interconvert by ring inversion (exist in equilibrium) [Inversion proceeds through boat conformations which exist in trace amounts] Can simplify diagram by omitting the C-H bonds Methyl equatorial Methyl axial

11. Sources of alkanes • Lower Mol. Mt. (~ < 5 Carbons): natural gas • Larger Mol. Wt.: petroluem of crude oil Crude oil: complex mixture of hydrocarbons Separated into fractions based on boiling point ranges Boiling point related to molecular weight, i.e to number of carbons • < 5 Carbons: gases at room temperature • 5 Carbons < ~18 Carbons: liquids at room temperature • > 18 Carbons: solids at room temperature

12. Increasing molecular size results in increasing tendency to form condensed phases • Associated with weak intermolecular interactions between alkane molecules • London dispersion forces: weak electrostatic attractions between induced dipoles, i.e. are… • Van der Waals’ forces between electrons of one molecule and nuclei of another • Extent of attraction increases with increasing molecular size • Weak interactions compared to hydrogen bonding or ionic bonding

13. Solubility of alkanes • ‘Like dissolves like’: alkanes soluble in other alkanes, e.g petroleum • [Soluble: single liquid phase results upon mixing] • Alkanes insoluble in water, i.e are hydrophobic • Mixtures with water separate into two liquid phases: aqueous and hydrocarbon

15. Reactions of alkanes • Relatively inert; contain only stable C-C and C-H bonds • Some important reactions: 1. Combustion, e.g. 2 C4H10 + 13 O2→ 8 CO2 + 10 H2O DH = - 2877 kJ mol-1 i.e. exothermic 2. Steam reforming CH4 + H2O → 3H2 + CO N2 ↓ ↓ NH3 CO2 → Urea +

16. 3. Reaction with halogens

17. 4. Catalytic cracking • Fragmentation of alkanes into smaller molecules, e.g: • The products of these reactions are a new type of hydrocarbon • They are said to be ‘unsaturated’ compared to alkanes • i.e., have fewer Hydrogens per Carbon than alkanes, which are said to be ‘saturated’

18. Unsaturated hydrocarbons contain Carbon-Carbon multiple bonds Classes of unsaturated hydrocarbons are defined by the types of Carbon-Carbon multiple bonds they contain Alkenes: contain Carbon-Carbon double bonds Carbon-Carbon double bond Alkynes: contain Carbon-Carbon triple bonds Carbon-Carbon triple bond Carbon valency of four maintained in alkenes and alkynes

19. Alkenes Older name: Olefins Characterised by presence of Carbon-Carbon double bonds General structural formula Where ‘R’ = Hydrogen or alkyl group Two Carbons and all four ‘R’ groups are lying on the same plane Bond angles about each Carbon ~ 120o

20. Three sp2 hybridised orbitals can be arrayed to give trigonal geometry The remaining 2pz orbital is orthogonal to the three sp2 orbitals View along z axis View along xy plane

21. sbond formation results from overlap of two sp2 hybridised orbitals s [A s-antibonding orbital is also formed, but this is not occupied by electrons] Overlap of the pz orbitals results in formation of a p bond [A p-antibonding orbital is also formed, but this is not occupied by electrons] p

22. p orbital: has a nodal plane on which lies on the bond axis pelectron density lies above and below the plane containing the two Carbons and four ‘R’ groups View along the Carbon-Carbon bond Note: constitutes onep molecular orbital i.e. constitutes onep bond when occupied p • Carbon-Carbon double bond: • One s bond; One p bond s • Both occupied by two electrons

23. Rotation about a Carbon-Carbon double bond requires opening up of the p bond • Requires large input of energy (~ 268 kJ mol-1) • Hence, rotation about C=C bonds does not occur at room temperature • Consequently, a new form of isomerism becomes possible for alkenes • Consider an alkene with one Hydrogen and one alkyl group ‘R’ bonded to each Carbon • Two structures are possible or

24. This form of isomerism is known as Cis-Trans isomerism • [older term: geometrical isomerism] • The cis isomer is that with like groups on the same side of the C=C • The trans isomer is that with like groups on opposite sides of the C=C Cis isomer Trans isomer

25. First two members of the alkene series: Ethene (Ethylene) Propene (Propylene) Note: Nomenclature: • Prefix indicates number of carbons • (‘eth…’ = 2C; ‘prop…’ = 3C; etc.) • Suffix ‘…ene’ indicates presence of C=C

26. Could have C=C between C1 and C2 or between C2 and C3 Butene 1-Butene 2-Butene Note: 1. 1-Butene and 2-butene are structural isomers 2. 3. Number indicates starting point of the C=C, i.e. number through the C=C

27. 4. Cis-Trans isomerism is possible for 2-butene • There are two isomeric 2-butenes Trans-2-butene b.p. 3.7oC m.p. -139oC Cis-2-butene b.p. 0.3oC m.p. -106oC

28. Some other alkenes 4-Methyl-2-pentene 2-Methyl-1-butene 1,3-Pentadiene Cis-3-heptene Trans-2-decene

29. Can have cycloalkenes Cyclohexene Cyclopentene 3-Methylcyclopentene Note: 1,4-Cyclohexadiene

30. Lycopene molecular structure

31. p electrons in alkenes are available to become involved in bond formation processes Essential processes in the synthesis of new molecules: formation of new covalent bonds Covalent bonds: pairs of electrons shared between nuclei (atoms) In the synthesis of organic molecules, a major strategy for forming new covalent bonds is: donation of an electron pair by one molecular species… …to form a covalent bond with another, electron deficient molecular species Electron pair donating species are known as nucleophiles Electron pair accepting species are known as electrophiles Reaction of a nucleophile with an electrophile results in the formation of a new covalent bond

32. Alkene hydrogenation • Addition of hydrogen (H2) across a C=C General reaction • Alkene p bond is lost, and two new C-H s bonds formed • Alkene converted to alkane • No reaction in absence of catalyst • Typical catalysts: Palladium (Pd), Platinum (Pt), Nickel (Ni), Rhodium (Rh) or other metals • Catalysts usually supported on materials such as charcoal • E.g. Pd/C “Palladium on Carbon”

33. Examples 1-Hexene Hexane Hexane 1,3-Hexadiene 2-Methyl-1-butene 2-Methylbutane

34. Reaction occurs at the catalyst surface • H2 molecules adsorbed onto catalyst surface • Both Hydrogens added to same face of C=C 1,2-Dimethylcyclohexene Cis-1,2-dimethylcyclohexane • Both Hydrogens added to the same face of the cyclohexene C=C • [Cis/Trans naming system can be extended to cyclic systems]

35. Addition of HX to alkenes General reaction X = Cl, Br, I • C=Cp bond lost; new C-H and C-Xs bonds formed e.g: 2-Chloropropane (only product) 1-Chloropropane (not formed) Propene • To explain this, need to consider the reaction mechanism

36. Reaction mechanism: • detailed sequence of bond breaking and bond formation in going from reactants to products • Addition of HX to alkenes: reaction involves two steps 1st Step: Addition of proton (H+) 2nd Step: Addition of halide (X-) 1st Step • Alkene p electrons attack proton • New C-Hs bond results • Remaining Carbon short 1 electron • Carbon positively charged

37. Addition of H+ to the alkene p bond forms a new C-Hs bond and a carbocation intermediate • [or carbonium ion] 2nd Step New C-Xs bond results Halide ion attacks electron deficient carbon

38. Reaction of HCl with CH3-CH=CH2 1st Step: addition of H+ to form a carbocation intermediate Two possible modes of addition or I.e. two possible carbocation intermediates

39. Classification of carbocations Primary (1o) Carbocation Secondary (2o) Carbocation Tertiary (3o) Carbocation 2o Carbocation 1o Carbocation

40. The relative order of stability for carbocations is: Most stable 3o > 2o > 1o Least stable • This is because carbocations can draw electron density along sbonds; known as an inductive effect • This effect is significant for alkyl substituents, but weak for Hydrogens Least stabilised Most stabilised

41. Addition of HCl to CH3-CH=CH2 proceeds so as to give the more stable of the two possible carbocation intermediates, i.e: Not formed Addition of chloride then gives 2-chloropropane exclusively Cl- Additions of HX to alkenes which follow this pattern are said to obey Markovnikov’s rule “Reaction proceeds via the more stable possible carbocation intermediate”

42. Other examples not 2-Methylpropene 2-Bromo-2-methyl- propane 1-Bromo-2-methyl- propane not 1-Methylcyclohexene 1-Chloro-1-methyl- cyclohexane 1-Chloro-2-methyl- cyclohexane 2-Butene 2-Chlorobutane (Symmetrical alkene) Same structure

43. Addition of water to alkenes • Follows same pattern as addition of HX • Acid catalysis required Propene 2-Hydroxypropane (2-Propanol) Mechanism: 1. Protonation of C=C so as to give the more stable carbocation intermediate

44. 2. Attack on the carbocation by water acting as a nucleophile 3. Loss of proton to give the product and regenerate the catalyst

45. Acid catalysed addition of water often difficult to control • A Mercury (II) mediated version often used - oxymercuration 1-Methylcyclopentene 1-Hydroxy-1-methyl- cyclopentane • Gives exclusively Markovnikov addition Hydroboration 1-Methylcyclopentene 1-Hydroxy-2-methyl- cyclopentane • Gives exclusively anti-Markovnikov addition Mechanisms of these reactions beyond the scope of this module

46. Alkene hydroxylation • Alkene p bond lost; two new C-OHs bonds formed Alkene epoxidation Epoxides • Alkene p bond lost; two new C-Os bonds are formed to the same Oxygen

47. Examples Propene Propane-1,2-diol 1,2-Epoxypropane Cyclopentene 1,2-Epoxycyclopentane Cis-1,2-cyclopentanediol

48. Ozonolysis of alkenes • Ozone (O3): strong oxidising agent • Adds to C=C with loss of both the p and s bonds • Products formed are known as ozonides Ozonide • Ozonides usually not isolated, but further reacted with reducing agents • Formation of two molecules each containing C=O (Carbonyl) groups

49. Overall process: Examples 1-Butene Aldehydes Ketone 2,3-Dimethyl-2-butene

50. 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)