1. Carbon Backbone, Nomenclature, Physical & Chemical Properties
Chapter 4
Introduction to
Hydrocabons
Carbon Backbone, Nomenclature, Physical & Chemical Properties

2. HYDROCARBONS
Compounds composed of only carbon and hydrogen atoms (C, H). Each carbon 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.
Organic molecules can be divided into four categories. Alkanes have only single bonds. Alkenes contain a C-C double bond, and alkynes contain a C-C triple bond. Benzene is an example of an aromatic hydrocarbon.
Each C atom is trigonal planar with sp2 hybridized orbitals.
There is no rotation about the C=C bond in alkenes.

5. Question 4.1
What is the hybridization of the starred carbon in humulene (shown)?
A) sp
B) sp2
C) sp3
D) 1s2 2s2 2p2

6. Question 4.2
What is the hybridization of the starred carbon of geraniol?
A) sp
B) sp2
C) sp3
D) 1s2 2s2 2p2

7. Types of Hydrocarbons
Each C atom is linear with sp hybridized orbitals.
Organic molecules can be divided into four categories. Alkanes have only single bonds. Alkenes contain a C-C double bond, and alkynes contain a C-C triple bond. Benzene is an example of an aromatic hydrocarbon.
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.

8. It is easy to rotate about the C-C bond in alkanes.
Propane
It is easy to rotate about the C-C bond in alkanes.
The structures of propane show that adjacent carbon atoms are able to rotate freely around C-C single bonds.

9. Naming Alkanes C1 - C10 : the number of C atoms present in the chain.
Each member C3 - C10 differs 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.

11. Nomenclature of Alkyl Substituents

12. Examples of Alkyl Substituents

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

14. Question 4.3 How many hydrogens are in a molecule of isobutane? A) 6

16. C7H16 can be any one of the following:
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:

17. Question 4.4
How many isomeric hexanes exist?
A) 2
B) 3
C) 5
D) 6

18. Question 4.5
The carbon skeleton shown at the bottom right accounts for 9 carbon atoms. How many other isomers of C10H22 that have 7 carbons in their longest continuous chain can be generated by adding a single carbon to various positions on this skeleton?
A) 2
B) 3
C) 4
D) 5

19. 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

20. Different Kinds of sp3 Carbons and Hydrogens

21. Question 4.6
In 3-ethyl-2-methylpentane, carbon #3 (marked by a star) is classified as:
A) primary (1°)
B) secondary (2°)
C) tertiary (3°)
D) quaternary (4°)

22. Question 4.7
How many primary carbons are in the molecule shown at the bottom right?
A) 2
B) 3
C) 4
D) 5

23. 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

24. 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

25. 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

26. 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

28. Question 4.7 The correct structure of 3-ethyl-2-methylpentane is:
A) B)
C) D)

29. Cycloalkane Nomenclature
CnH2n
Cycloalkane Nomenclature 26

30. 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 6

31. Cycloalkanes
Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.
CH2CH3
Ethylcyclopentane 6

32. 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.
CH2CH3
H3C
CH3
3-Ethyl-1,1-dimethylcyclohexane 6

33. For more than two substituents,

34. Question 4.8
Which one contains the greatest number of tertiary carbons?
A) 2,2-dimethylpropane
B) 3-ethylpentane
C) sec-butylcyclohexane
D) 2,2,5-trimethylhexane

35. Physical Properties of Alkanes and Cycloalkanes4

36. Crude oil Naphtha Kerosene (bp 95-150 °C) (bp: 150-230 °C) C5-C12
Light gasoline
(bp: °C)
C15-C25
Gas oil
(bp: °C)
Refinery gas
C1-C4
Residue 2

38. Question 4.9
Arrange octane, 2,2,3,3-tetramethylbutane and 2-methylheptane in order of increasing boiling point.
A) 2,2,3,3-tetramethylbutane < octane < 2-methylheptane
B) octane < 2-methylheptane < 2,2,3,3- tetramethylbutane
C) 2,2,3,3-tetramethylbutane < 2-methylheptane < octane
D) 2-methylheptane < 2,2,3,3- tetramethylbutane < octane

39. Crude Oil and Uses of Alkanes
The gasoline fraction of crude oil only makes up about 19%, which is not enough to meet demand.

40. Weak Intermolecular Attractive Forces
van der Waals Forces
Weak Intermolecular Attractive Forces
The boiling point of a compound increases with the increase in van der Waals force…and a Gecko uses them to walk!

41. Gecko: toe, setae, spatulae 6000x Magnification
Full et. al., Nature (2000)
5,000 setae / mm2
600x frictional force; 10-7 Newtons per seta
Geim, Nature Materials (2003)
Glue-free Adhesive
100 x 10 6 hairs/cm2
GECKOS tiny tropical lizards are able to run up walls and along ceilings extremely fast, yet they can stick to a sheet of polished glass with only one foot. The secret of their success lies in the rows of tiny hairs on the bottom of their feet. Thousands of these hairs, called setae, are arrayed like the bristles of a toothbrush across a gecko’s toes.
Microscopy reveals that the tip of each seta is divided into hundreds of tiny“spatulae”, each pointing in a different direction and tipped with a cone-shaped structure. This shape suggests a suction mechanism, but suction relies on air pressure—and gecko feet are known to stick to walls even in a vacuum. Robert Full of the University of California, Berkeley, Nature 2000.
Using a tiny micro-electro-mechanical force sensor, they conducted various experiments to measure the stickiness of a single seta. The maximum adhesive force that could be exerted by a single seta had already been estimated, by measuring the total force exerted by a foot and dividing by the number of setae (around 5,000 per square millimetre). But to their surprise, researchers found that a single seta can actually exert ten times as much force as this. Setae are, in other words, even stickier than expected—giving thegecko a surprisingly large safety margin.adhesive force is about 600 times greater than the simple frictional force between lizard skin and the surface. And a seta will stick to a surface most firmly if it is first pushed into the surface and then pulled along it by a few millionths of a metre. These findings suggest that setae operate at a molecular level, and exploit intra-molecular forces, called van der Waals forces, for their stickiness.
They detach by curling up the tips of their toes before moving, forming a sort of reverse fist. This allows them to peel their feet off the surface gently at a critical angle without damage, much like peeling a sticky label off a jar withot tearing it. The researchers found that setae reliably detach from the surface at an angle of about 30°.

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

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

44. Intermolecular Forces
Dipole-Dipole Forces
(permanent dipoles)
Dipole-Dipole Forces
Dipole-dipole forces exist between neutral polar molecules.
Polar molecules need to be close together.
Weaker than ion-dipole forces.
There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble.
If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity.
5-25 kJ/mol

45. Intermolecular Forces
Dipole-Dipole Forces

46. Boiling Points & Hydrogen Bonding Hydrogen Bonding
Special case of dipole-dipole forces.
By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.
H bonded to an electronegative element : F, O, and N.
H-bonds are strong.
Eg. Ice Floating
Solids are usually more closely packed than liquids;
Therefore, solids are usually more dense than liquids.
However, Ice is an exception due to H-bonding.
Ice (solid) is less dense than water (liquid).
The H-O bond length is 1.0 Å.
The O…H hydrogen bond length is 1.8 Å.
Ice has waters arranged in open, regular hexagons.

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

49. 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 ( kJ/mol).
London Dispersion Forces
Weakest of all intermolecular forces.
It is possible for two adjacent neutral molecules to affect each other.
The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom); a dipole forms.
London dispersion forces increase as molecular weight increases.
London dispersion forces exist between all molecules.
London dispersion forces depend on the shape of the molecule.

50. 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 6

51. 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 8

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

53. Question 4.10 Which alkane has the highest boiling point? A) hexane
B) 2,2-dimethylbutane
C) 2-methylpentane
D) 2,3-dimethylbutane

54. Boiling Points Increase with increasing number of carbons
more atoms, more electrons, more opportunities for induced dipole-induced dipole forces
Heptane bp 98°C
Octane bp 125°C
Nonane bp 150°C 8

55. 2,2,3,3-Tetramethylbutane: bp 107°C
Boiling Points
Decrease with chain branching
branched molecules are more compact with smaller surface area—fewer points of contact with other molecules
Octane: bp 125°C
2-Methylheptane: bp 118°C
2,2,3,3-Tetramethylbutane: bp 107°C 8

56. Sources and Uses of Alkanes
Gasoline is a mixture of straight, branched, and aromatic hydrocarbons (5–12 carbons in size).
Large alkanes can be broken down into smaller molecules by CRACKING.
Straight chain alkanes can be converted into branched alkanes and aromatic compounds through REFORMING.
After using these processes, the yield of gasoline is about 47% rather than 19%.

57. Chemical Properties: Combustion of Alkanes
All alkanes burn in air to give carbon dioxide and water. 10

58. Heats of Combustion Heptane 4817 kJ/mol 654 kJ/mol Octane 5471 kJ/mol
Nonane
6125 kJ/mol
What pattern is noticed in this case? 11

59. Heats of Combustion Increase with increasing number of carbons
more moles of O2 consumed, more moles of CO2 and H2O formed 8

60. Heats of Combustion 5471 kJ/mol 5 kJ/mol 5466 kJ/mol 8 kJ/mol
What pattern is noticed in this case? 13

61. Reaction Coordinate Diagrams
ENERGY Diagrams /
Reaction Coordinate Diagrams
5471 kJ/mol
5466 kJ/mol O2 + 25 2
5458 kJ/mol
5452 kJ/mol O2 + 25 2 O2 + 25 2 O2 + 25 2
8CO2 + 9H2O 14

62. Heat of Combustion Patterns
Increase with increasing number of carbons
more moles of O2 consumed, more moles of CO2 and H2O formed
Decrease with chain branching
branched molecules are more stable (have less potential energy) than their unbranched isomers 8

63. Important Point
Isomers can differ in respect to their stability.
Equivalent statement:
Isomers differ in respect to their potential energy.
Differences in potential energy can be measured by comparing heats of combustion. (Worksheet problems) 12