Chapter 2 Introduction to Hydrocabons

1. Chapter 2 Introduction to Hydrocabons Carbon Backbone, Nomenclature, Physical & Chemical Properties

2. HYDROCARBONS • Compounds composed of only carbon and hydrogen atoms (C, H). Eachcarbon has 4 bonds. • They represent a “backbone” when other “heteroatoms” (O, N, S, .....) are substituted for H. (The heteroatoms give function to the molecule.) • Acyclic (without rings); Cyclic (with rings); Saturated: only carbon-carbon single bonds; Unsaturated: contains one or more carbon-carbon double and/or triple bond

3. HYDROCARBONS • Alkanes contain only single ( ) bonds and have the generic molecular formula: [CnH2n+2] • Alkenes also contain double ( + )bonds and have the generic molecular formula: [CnH2n] • Alkynes contain triple ( + 2)bonds and have the generic molecular formula: [CnH2n-2] • Aromatics are planar, ring structures with alternating single and double bonds: eg. C6H6

4. Types of Hydrocarbons Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds. Each C atom is trigonal planar with sp2 hybridized orbitals. There is no rotation about the C=C bond in alkenes.

5. Types of Hydrocarbons Each C atom is linear with sp hybridized orbitals. Each C--C bond is the same length; shorter than a C-C bond: longer than a C=C bond. The concept of resonance is used to explain this phenomena.

6. Propane It is easy to rotate about the C-C bond in alkanes.

7. Naming Alkanes C1 - C10 : the number of C atoms present in the chain. Each member C3 - C10differs by one CH2 unit. This is called a homologous series. Methane to butane are gases at normal pressures. Pentane to decane are liquids at normal pressures.

9. Nomenclature of Alkyl Substituents

10. Examples of Alkyl Substituents

11. Constitutional or structural isomers have the same molecular formula, but their atoms are linked differently. Naming has to account for them.

13. A compound can have more than one name, but a name must unambiguously specify only one compound C7H16 can be any one of the following:

14. Alkanes (Different types of sp3 carbon atoms) • Primary, 1o, a carbon atom with 3 hydrogen atoms: [R-CH3] • Secondary, 2o, a carbon atom with 2 hydrogen atoms: [R-CH2-R] • Tertiary, 3o, a carbon atom with 1 hydrogen atom: [R-CH-R] R • Quaternary, 4o, a carbon atom with 0 hydrogen atoms: CR4

15. Different Kinds of sp3 Carbons and Hydrogens

16. Nomenclature of Alkanes 1. Determine the number of carbons in the parent hydrocarbon 2. Number the chain so that the substituent gets the lowest possible number

17. 3. Number the substituents to yield the lowest possible number in the number of the compound (substituents are listed in alphabetical order) 4. Assign the lowest possible numbers to all of the substituents

18. 5. When both directions lead to the same lowest number for one of the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents 6. If the same number is obtained in both directions, the first group receives the lowest number

19. 7. In the case of two hydrocarbon chains with the same number of carbons, choose the one with the most substituents 8. Certain common nomenclatures are used in the IUPAC system

21. Cycloalkane Nomenclature CnH2n

22. Cycloalkanes • Cycloalkanes are alkanes that contain a ring of three or more carbons. • Count the number of carbons in the ring, and add the prefix cyclo to the IUPAC name of the unbranched alkane that has that number of carbons. Cyclopentane Cyclohexane

23. CH2CH3 Cycloalkanes • Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent. Ethylcyclopentane

24. H3C CH3 CH2CH3 Cycloalkanes • Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent. • List substituents in alphabetical order and count in the direction that gives the lowest numerical locant at the first point of difference. 3-Ethyl-1,1-dimethylcyclohexane

25. For more than two substituents,

26. 2.17Physical Properties of Alkanesand Cycloalkanes

27. Crude oil Naphtha (bp 95-150 °C) Kerosene (bp: 150-230 °C) C5-C12 C12-C15 Light gasoline (bp: 25-95 °C) C15-C25 Gas oil (bp: 230-340 °C) Refinery gas C1-C4 Residue

28. Fig. 2.15

29. Example of Intramolecular Forces: Protein Folding Ion-dipole (Dissolving) 40-600kJ/mol 10-40kJ/mol 150-1000kJ/mol 0.05-40kJ/mol 700-4,000kJ/mol

30. Intermolecular Forces • Ion-Dipole Forces(40-600 kJ/mol) • Interaction between an ion and a dipole (e.g. NaOH and water = 44 kJ/mol) • Strongest of all intermolecular forces.

31. Ion-Dipole & Dipole-Dipole Interactions: like dissolves like • Polar compounds dissolve in polar solvents • & non-polar in non-polar

32. Intermolecular Forces Dipole-Dipole Forces (permanent dipoles) 5-25 kJ/mol

33. Intermolecular Forces Dipole-Dipole Forces

34. Boiling Points & Hydrogen Bonding

35. Hydrogen Bonding • Hydrogen bonds, a unique dipole-dipole (10-40 kJ/mol).

37. Intermolecular Forces Hydrogen Bonding

38. DNA: Size, Shape & Self Assembly http://www.umass.edu/microbio/chime/beta/pe_alpha/atlas/atlas.htm Views & Algorithms 10.85 Å 10.85 Å

39. Intermolecular Forces • London or Dispersion Forces • An instantaneous dipole can induce another dipole in an adjacent molecule (or atom). • The forces between instantaneous dipoles are called London or Dispersion forces ( 0.05-40 kJ/mol).

40. van der Waals Forces The boiling point of a compound increases with the increase in van der Waals force..and the Gecko!

41. Gecko: toe, setae, spatulae6000x Magnification Full et. al., Nature (2000) 5,000 setae / mm2 600x frictional force; 10-7 Newtons per seta http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html Geim, Nature Materials (2003) Glue-free Adhesive 100 x 10 6 hairs/cm2

42. Boiling Points of Alkanes • governed by strength of intermolecular attractive forces • alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent • only forces of intermolecular attraction are induced dipole-induced dipole forces

43. Induced dipole-Induced dipole Attractive Forces • two nonpolar molecules • center of positive charge and center of negative charge coincide in each + – + –

44. Induced dipole-Induced dipole Attractive Forces • movement of electrons creates an instantaneous dipole in one molecule (left) + – + –

45. Induced dipole-Induced dipole Attractive Forces • temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right) – + – +

46. Induced dipole-Induced dipole Attractive Forces • temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right) – – + +

47. Induced dipole-Induced dipole Attractive Forces • the result is a small attractive force between the two molecules – – + +

48. Induced dipole-Induced dipole Attractive Forces • the result is a small attractive force between the two molecules – – + +

49. Boiling Points • Increase with increasing number of carbons • more atoms, more electrons, more opportunities for induced dipole-induced dipole forces • Decrease with chain branching • branched molecules are more compact with smaller surface area—fewer points of contact with other molecules

50. Intermolecular Forces London Dispersion Forces Which has the higher attractive force?