1. Unit 2 – Alkanes and Chemical Reactions
Structure and Stereochemistry of Alkanes
Nomenclature of alkanes and cycloalkanes
Physical Properties
Conformational Analysis
The Study of Chemical Reactions
Kinetics and Thermodynamic Quantities
Free Radical Halogenation
Reactive Intermediates and Transition States

2. Hydrocarbons The simplest organic compounds are the hydrocarbons:
organic compounds that contain only carbon and hydrogen
four general types:
alkanes
alkenes
alkynes
aromatic hydrocarbons

3. Hydrocarbons Alkanes are often called saturated hydrocarbons
they contain the maximum number of hydrogen atoms per carbon atom.
Alkenes, alkynes, and aromatic hydrocarbons are called unsaturated hydrocarbons
they contain fewer H atoms than an alkane with the same number of carbon atoms

4. Alkanes
You must know the names and formulas for the 10 simplest alkanes:
CH4 methane
CH3CH3 ethane
CH3CH2CH3 propane
CH3CH2CH2CH3 butane
CH3CH2CH2CH2CH3 pentane
CH3CH2CH2CH2CH2CH3 hexane

5. Alkanes
You must know the names and formulas for the 10 simplest alkanes:
CH3CH2CH2CH2CH2CH2CH3 heptane
CH3CH2CH2CH2CH2CH2CH2CH3 octane
CH3CH2CH2CH2CH2CH2CH2CH2CH3 nonane
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane

6. Alkanes
The alkanes form a homologous series with a general molecular formula of CnH(2n+2)
Homologous series:
a series of compounds in which each member differs from the next member by a constant unit
Alkanes differ from each other by -CH2-

7. Alkanes Example: Which of the following are alkanes:
C2H6, C3H6, C5H12, C4H8
Example: What is the formula for an alkane with 12 carbons?

8. Alkanes The previous alkanes are straight-chain alkanes:
all of the carbon atoms are joined in a continuous chain
also called “normal” alkanes (n-alkanes)
Alkanes containing 4 or more carbons can also form branched alkanes
one or more of the carbon atoms form a “branch” or side-chain off of the main chain

9. Alkanes 2-methylbutane 2,2-dimethylpropane
An example of a straight chain alkane:
C5H12 pentane
Examples of branched alkanes:
C5H12
2-methylbutane
2,2-dimethylpropane

10. Alkanes
The three structures shown previously for C5H12 are structural (constitutional) isomers:
compounds with the same molecular formula but different bonding arrangements
Structural isomers have different properties:
different melting points
different boiling points
often different chemical reactivity

11. Alkane Nomenclature pentane 2-methylbutane or isopentane
Organic compounds can be named either using common names or IUPAC names.
pentane
2-methylbutane or
isopentane
2,2-dimethylpropane or
neopentane

12. Alkane Nomenclature “iso”
The common name for any alkane containing a CH3 group on the second carbon in the chain is “isoalkane.”
“iso”
Isobutane
(4 C total)
Isohexane (6 C total)

13. Alkane Nomenclature
Most of the time, organic chemists use the IUPAC names for organic compounds.
LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE DISCUSS.

14. Alkane Nomenclature Base name: heptane To name an alkane:
Find the longest continuous chain of carbon atoms and use the name of that chain as the base name of the compound:
the longest chain is often NOT written in a straight line
Base name:
heptane

15. Alkane Nomenclature A substituent
Number the carbon atoms in the longest chain starting at the end of the chain closest to a substituent
a group attached to the main chain that has taken the place of a hydrogen atom on the main chain
A substituent

16. Alkane Nomenclature Name and give the location of each substituent.
Common substituents:
Halo group
a halogen atom
“Halo” groups are named using “halo”:
Cl chloro
Br bromo
I iodo
F fluoro
Nitro group
-NO2

17. Alkane Nomenclature Common substituents: alkyl group
A group that is formed by removing an H atom from an alkane
the alkyl group attaches to the main chain at the carbon that has lost its H

18. Alkane Nomenclature
Alkyl groups are named by replacing the “ane” ending of the parent alkane with the “yl” ending.

19. Alkane Nomenclature

20. Alkane Nomenclature

21. Alkane Nomenclature Complex alkyl substituents
Use the longest alkyl chain of the substituent as the base name of the substituent
Number the substituent chain with the “head carbon” as carbon 1
List substituents on the alkyl chain with the appropriate numbers
Use parentheses around the name of the group

22. Alkane Nomenclature Methyl group
3-methylheptane
Note: Separate numbers from letters using a hyphen. Separate groups of numbers using commas.

23. Alkanes Know these. Note: Ignore these prefixes when alphabetizing.
Alkane Nomenclature:
When two or more substituents are present, list them in alphabetical order:
isopropyl before methyl
t-butyl or sec-butyl before chloro
When more than one of the same substituent is present (i.e. two methyl groups), use prefixes to indicate how many. Give the location of each as well.
Di = two
Tri = three
Tetra = four
Penta = five
Hexa = six
Know these.
Note: Ignore these prefixes when alphabetizing.

24. Alkane Nomenclature Additional rules:
When there are two “longest” chains of equal length, use the chain with the greater number of substituents.
correct
incorrect

25. Alkane Nomenclature Additional rules:
If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second substituent.
correct
incorrect
3-chloro-2,5-dimethylhexane

26. Alkanes
Example: Name the following compounds:

27. Alkanes
Example: Name the following compound:

28. Alkanes
You must also be able to write the structure of an alkane when given the IUPAC name.
Identify the main chain and draw the carbons in it.
Identify the substituents (type and #) and attach them to the appropriate carbon atoms on the main chain.
Add hydrogen atoms to the carbons to make a total of 4 bonds to each carbon

29. Alkanes Example: Draw the structure for the following compounds:
3, 3-dimethylpentane
4-sec-butyl-2-methyloctane
1,2-dichloro-3-methylheptane
2-nitropropane

30. Alkane Nomenclature
Example: Draw the structural isomers of hexane (C6H14). Name each isomer.
Use a systematic approach to draw structural isomers:
Draw the unbranched isomer for the first structure.
For other structures, remove 1 or more carbons (and/or functional groups) from the unbranched isomer and reposition to make unique compounds

31. Types of Carbon Atoms Primary carbon (1o) a carbon bonded to
one other carbon
Secondary carbon (2o)
two other carbons
Tertiary carbon (3o)
three other carbons

32. Physical Properties Solubility Alkanes are nonpolar hydrophobic
do not dissolve in water
soluble in nonpolar or weakly polar organic solvents
Density:
varies from ~0.5 - ~0.8 g/mL
less dense than water (1.0 g/mL)
Alkanes float on water

33. Physical Properties Boiling Point
In general, boiling point increases as the molecular weight of the alkane increases
larger molecules have greater surface area and higher London dispersion forces of attraction
must be overcome for vaporization and boiling to occur

34. BP (branched) < BP (n-alkane)
Physical Properties
Boiling Point (cont)
Given the same total number of carbon atoms:
BP (branched) < BP (n-alkane)
Branched alkanes are more compact.
less surface area
smaller London dispersion forces
lower BP

35. Physical Properties Melting Points:
In general, melting point increases as MW increases
irregular, sawtooth pattern

36. Physical Properties Melting Point:
Alkanes with odd number of carbons have lower than expected melting points (compared to the previous alkane with an even number of carbons)
Even # carbons
better packing in solid structure
higher MP
Odd # carbons
do not pack as well
lower MP

37. MP (branched) > MP (n-alkane)
Physical Properties
Melting Points:
Given the same total number of carbon atoms:
MP (branched) > MP (n-alkane)
branched alkanes have more compact structure
better packing
higher MP

38. Sources & Uses of Alkanes
Alkanes are derived primarily from petroleum and petroleum by-products:
Refining via fractional distillation gives useful mixtures of alkanes:
C2 - C4 liquified petroleum gas
C4 - C9 gasoline
C8 - C16 kerosene
C10 - C18 diesel
C16+ heavy/mineral oil

39. alkane smaller alkanes + alkenes
Reactions of Alkanes
Catalytic Cracking:
converts alkanes into more valuable mixtures of smaller alkanes and alkenes
alkane smaller alkanes + alkenes
C12H26 D
SiO2 or Al2O3 catalyst D
SiO2 +

40. Alkane shorter alkanes
Reactions of Alkanes
Hydrocracking:
converts higher alkanes into shorter alkanes and eliminates N and S impurities
Alkane shorter alkanes
C12H26 D
H2, SiO2 or Al2O3 catalyst D +
H2, SiO2

41. Reactions of Alkanes Combustion:
a rapid, exothermic redox reaction that converts hydrocarbons into carbon dioxide and water
alkane + O CO2 + H2O
2 C6H O CO H2O
(unbalanced)

42. alkane + X2 mixture of alkyl halides
Reactions of Alkanes
Halogenation:
a reaction in which a halogen atom is substituted for a hydrogen atom on an alkane
alkane + X mixture of alkyl halides
CH4 + Cl CH3Cl + CH2Cl2 + CHCl3 + CCl4
D or hu hu
unbalanced

43. Conformations of Alkanes
The simplest alkane, CH4, is perfectly tetrahedral:
bond angle = 109.5
C-H bond length = 1.09 A
free rotation of the C-H bond
Figure: UN
Caption:
The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an sp3 hybrid carbon. The four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09 Å.

44. Conformations of Alkanes
Ethane:
Two carbons
overlapping sp3 hybrid orbitals form a sigma bond
Figure: UN
Caption:
Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp3 hybrid orbitals forming a sigma bond between them.

45. Conformations of Alkanes
The two methyl groups are relatively free to rotate about the sigma bond between the two carbon atoms
sigma bond maintains its overlap at all times
The different arrangements formed by rotation around a single bond are called conformations.
Conformer:
a specific conformation
a “conformational isomer”

46. Conformations of Alkanes
Conformers are often drawn using Newman projections:
a way of drawing a molecule looking straight down the bond connecting two carbon atoms
front carbon atom is represented by three lines joined together in a Y shape
back carbon is represented by a circle with three bonds pointing out from it

47. Conformations of Alkanes
View from this end =
3-D structure of one conformer of ethane
Newman projection

48. Conformations of Alkanes
An infinite number of conformations are possible for ethane and higher alkanes.
The dihedral angle (q) can have an infinite number of values
angle between the C-H bonds on the front and back carbons q

49. Conformations of Alkanes
Important conformations for ethane:
Eclipsed conformation
Staggered conformation
Figure: 03-06
Caption:
Ethane conformations. The eclipsed conformation has a dihedral angle  = 0, and the staggered conformation has q = 60. Any other conformation is called a skew conformation.
Skew conformation
Molecules are constantly rotating through all possible conformations.

50. Conformations of Alkanes
The conformation of ethane changes constantly at room temperature.
Conformations may have different energies.
Lowest energy conformer is most favored.
Highest energy conformer is least favored.
Conformational analysis:
the study of the energies of different conformations
helps predict which conformation are favored and which reaction may occur

51. Conformations of Alkanes
Staggered conformation of ethane:
lowest energy most favored
electron clouds in the C - H bonds separated as much as possible
Eclipsed conformation of ethane:
highest energy least favored
electron clouds of C - H bonds are closest together

52. Conformations of Alkanes
As ethane rotates from the staggered conformation towards the eclipsed conformation:
potential energy increases due to torsional strain
resistance to rotation or twisting about a bond

53. Conformations of Alkanes
Figure: 03-07
Caption:
The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about
3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly.

54. Conformation of Alkanes
Butane:
4 carbon chain held together by end-to-end overlap of sp3 hybrid orbitals on the carbon atoms
tetrahedral geometry around each carbon
free rotation about any C - C bond
many conformers of differing energies are possible
Newman projections of butane are drawn by looking down the central C2 - C3 bond.

55. Conformations of Alkanes
Figure: 03-10
Caption:
Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations are given specific names.
Totally
eclipsed
(0o)
Gauche
(60o)
Eclipsed
(120o)
Anti
(180o)

56. Conformation of Alkanes
Totally eclipsed conformer of butane:
highest energy due to steric hinderance between the methyl groups
Steric hinderance:
interference between two bulky groups that are close enough together so that their electron clouds repel each other

57. Conformations of Alkanes
Figure: UN
Caption:
The totally eclipsed conformation is about 1.4 kcal (5.9 kJ) higher in energy than the other eclipsed conformations, because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance.

58. Conformations of Alkanes
Eclipsed conformer of butane:
second highest in energy due to repulsion of the methyl group on one carbon and the hydrogen atom on the other
All staggered conformers (gauche and anti) of butane are lower in energy than any of the eclipsed conformers.
Anti conformer of butane:
lowest energy because methyl groups are furthest apart

59. Conformations of Alkanes
Figure: 03-11
Caption:
Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy.