1. Bettelheim, Brown, Campbell and Farrell Chapter 11
Alkanes
Bettelheim, Brown, Campbell and Farrell
Chapter 11

2. Hybrid Orbitals Type Bond Angle Bonds to ? Atoms sp 180o 2 sp2 120o 3
Each hybrid orbital is identical

3. Condensed Structural Formula
Molecular Formula:
Gives number of each type of atom in compound
Structural Formula
Gives number of each type of atom AND shows how the atoms are connected
Condensed Structural Formula
Gives number of each type of atom AND shows how the atoms are connected using “shorthand” structures

4. Line-Angle Structural Formulas
Line represents C-C single bond
Each vertex (point) of angle represents a C atom
Hydrogen atoms are not shown

5. Hydrocarbons Contain only carbon and hydrogen
Saturated--only single bonds
Unsaturated—not just single bonds

6. Alkanes Saturated hydrocarbons Contain only single bonds
May be straight-chains, branched or cyclic
First two alkanes are methane and ethane

7. Alkanes First 10 “straight-chain” alkanes No branches
Generic Formula CnH2n (n = number of C atoms)

8. ALKANES
Straight chain or “normal” alkanes have all of their carbons connected in sequence
C1 connected to C2 connected to C3 connected to C4, etc.
A straight chain has angles of 109.5o between adjacent carbons. Physically the straight chain appears to be “zig-zag” rather than a straight line

9. Constitutional Isomerism (Structural Isomers)
Compounds that have the same molecular formula but different structural formulas
Up to 3 C—only one structure possible
For 4 C (C4H10), two constitutional isomers are possible

10. Constitutional Isomerism
Do the structural formulas in each set represent the same compound or constitutional isomers?

11. Free Rotation around C-C Single Bonds Conformations
Different shapes which molecule can have when it rotates around single bonds

12. Constitutional Isomerism
(a) The same compound—just different conformations
(b) Constitutional isomers—connected differently

13. Constitutional Isomerism
Draw structural formulas for the five constitutional isomers of molecular formula C6H14

14. Constitutional Isomerism
Draw structural formulas for the five constitutional isomers of molecular formula C6H14

15. IUPAC Names for Unbranched Alkanes
The IUPAC name has two parts:
(1) a root name (prefix that shows the number of carbon atoms in the chain)
(2) the suffix -ane: shows that the compound is a saturated hydrocarbon
Root names for 1 to 4 carbons new
Root names for 5 or more carbons same as molecular prefixes

16. Alkyl Group Names
Substituent group derived from alkane by removing a hydrogen atom
commonly represented by the symbol R-
named by dropping the -ane from the name of the parent alkane and adding the suffix -yl

17. IUPAC Names
The name of an alkane with a branched chain of carbon atom consists of:
1. Parent name: the longest chain of carbon atoms
2. Substituent names and locations: the groups bonded to the parent chain

18. IUPAC Alkane Names
1.An unbranched alkane: Prefix showing the number of carbon atoms plus -ane
2.Branched-chain alkanes: Parent name is name of longest carbon chain
3. Name and number each substituent on the parent chain: Use a hyphen between number and name

19. IUPAC Names
4.Number the parent chain from the end that gives the substituents the lower numbers

20. IUPAC Names 5. If the same substituent occurs more than once,
Number parent chain from the end that gives the lower number to the substituent encountered first
Use prefix (di-, tri-, tetra-,penta-, hexa- etc.) to show how many times the substituent
Use a comma to separate numbers

21. IUPAC Names 6. For two or more different substituents
List them in alphabetical order
Number chain to give the lower number to the substituent encountered first
For different substituents in same positions on opposite ends of the parent chain, the substituent that is first in the alphabet gets the lower number

22. IUPAC Names
7. Do NOT alphabetize the prefixes di-, tri-, tetra, etc., or the prefixes sec- and tert- in alphabetizing;
Alphabetize the names of substituents first, and then insert these prefixes

23. Common Names Common names still in use
Number of carbon atoms determines the name
First three alkanes are methane, ethane, and propane
All alkanes of formula C4H10 are butanes, all alkanes of formula C5H12 are called pentanes, etc.
The prefix iso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH-
For more complex alkanes, use the IUPAC system

24. Name these compounds

25. Cycloalkanes
Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring
Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated
Cycloalkanes of ring sizes ranging from 3 to over 30 carbon atoms are found in nature
Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature

26. Generic Formula Alkane: CnH2n+2 n = # of C Cycloalkane: CnH2n
Note that making a ring results in 2 fewer hydrogens than in straight chain

27. Cycloalkanes Nomenclature
Add cyclo- to the name of the corresponding open-chain alkane and name each substituent on the ring
If only one substituent, it does not need to be numbered
If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

28. Conformations - Alkanes
Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a C-C single bond
Three conformations for a butane molecule

29. Conformations - Alkanes
Ring formation limits free rotation
Can only rotate partly without breaking bonds

30. Cyclopentane
The most stable conformation of a cyclopentane ring is an envelope
conformation

31. Cyclohexane
The most stable conformation of a cyclohexane ring is the chair conformation
all bond angles are approximately 109.5°

32. Cyclohexane

33. Cyclohexane In a chair conformation,
six C-H bonds are equatorial (red)
six C-H bonds are axial (blue)

34. Cyclohexane
the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

35. Cis-Trans Isomers Cis: on the same side of ring
Trans: on opposite sides of ring
Look at molecule edge-on…
Same side
Opposite side

36. Cis-Trans Isomers
….or view it from above (use wedge notation)

37. Cis-Trans Isomers
Often helpful to picture cyclohexane ring as hexagon to determine which cis-trans isomer is present
Stereoisomers differ in the orientation of atoms in space (connected to same atoms)
Cis-trans isomers are one type of stereoisomers

38. Physical Properties of Alkanes
Most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity
Electronegativity difference between carbon and hydrogen is = 0.4
Thus C-H bond is nonpolar covalent
Alkanes are nonpolar compounds
Display only weak London dispersion forces between molecules

39. Physical Properties of Alkanes
Melting and boiling points
Low melting and boiling points compared to other types of compounds with same size
Both boiling and melting points of alkanes increase with increasing molecular weight
Greater number of London dispersion forces

40. Physical Properties of Alkanes

41. Physical Properties of Alkanes
Constitutional isomers are different compounds with different physical and chemical properties
C6H14 compounds:

42. Physical Properties of Alkanes
Solubility: “like dissolves like”
Nonpolar alkanes are NOT soluble in water
Do not form hydrogen bonds with water
Soluble in each other and in other nonpolar organic compounds
Density
Densities of g/mL
Less dense than water (1.0 g/mL) so they float on water

43. Chemical Properties of Alkanes
Oxidation (Combustion)
Reaction with oxygen
Products are CO2, H2O and heat
Used as energy sources for heat and power

44. Alkyl Halides Alkanes with halide substituents F Cl Br I
CH2Cl2 CH3Cl CHCl3
CCl3F (Freon-11)

45. Reactions Reaction with halogens (halogenation) Substitution reaction
Requires heat or light as catalyst

46. The Chlorofluorocarbons
Chlorofluorocarbons (CFCs)
Freons
CCl3F (Freon-11) and CCl2F2 (Freon-12)
nontoxic, nonflammable, odorless, noncorrosive
CFCs were used as
refrigerants
industrial cleaning solvents
propellants for aerosol sprays

47. CFC Replacements
Chlorofluorocarbons (CFCs) cause destruction of the Earth’s stratospheric ozone layer
Replacements are hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)
More reactive than CFCs
Destroyed before they reach the stratosphere

48. Sources of Alkanes Natural gas Petroleum 90 to 95 percent methane,
5 to 10 percent ethane, and
Mixture of other relatively low-boiling alkanes, chiefly propane, butane, and 2-methylpropane
Petroleum
Thick, viscous liquid mixture of thousands of compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals

49. Refining of Crude Oil

50. Petroleum Distillation
Fraction Size Distills at oC
Gasoline C4 to C
Kerosene C10 to C
Fuel Oil/Diesel C14 to C
Lubricating Oil C16 to C > 350
Residue >C20
(Asphalt/Greases)