1. Alkanes
and
Cycloalkanes
Unit 4

2. Structure Hydrocarbon: a compound composed only of carbon and hydrogen
Saturated hydrocarbon: a hydrocarbon containing only single bonds
Carbon can form four single covalent bonds to other atoms
Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain
General formula: Cn + H2n + 2
Aliphatic hydrocarbon: another name for an alkane

3. Hydrocarbons H - C y d r o c a b n s A l k e Aromatic O i g m u t p zx N h B S U

4. Structure OF ALKANES Shape tetrahedral about carbon
all bond angles are approximately 109.5°

5. Drawing Alkanes Line-angle formulas
an abbreviated way to draw structural formulas
each vertex and line ending represents a carbon

6. Isomers
relationships among isomers

7. Constitutional Isomerism
Constitutional (structural) isomers: compounds with the same molecular formula but a different connectivity of their atoms
Exhibit different properties: example: C4H10

8. Constitutional Isomerism
Do these formulas represent constitutional isomers?
find the longest carbon chain
number each chain from the end nearest the first branch
compare chain lengths as well the identity and location of branches

9. Constitutional Isomerism
The number of isomers increases rapidly as the number of carbon atoms increases

10. Nomenclature - IUPAC Suffix -ane specifies an alkane
Prefix tells the number of carbon atoms

11. Alkyl and Aryl Groups
Chapter 19

12. Nomenclature - IUPAC Parent name: the longest carbon chain
When a hydrogen is removed from an alkane, an alkyl group results
Substituent: a group bonded to the parent chain
alkyl group: a substituent derived by removal of a hydrogen from an alkane; When we remove the hydrogen atom, the name -ane suffix is changed to -yl (given the symbol R-)

13. Nomenclature - IUPAC
1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. The parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number

14. Nomenclature - IUPAC
5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first
indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc.
use commas to separate position numbers

15. Nomenclature - IUPAC
6. If there are two or more different substituents,
list them in alphabetical order
number from the end of the chain that gives the substituent encountered first the lower number

16. Nomenclature - IUPAC
7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization
alphabetize the names of substituents first and then insert these prefixes

17. Nomenclature - IUPAC
Alkyl groups

18. Nomenclature - Common
The number of carbons in the alkane determines the name
all alkanes with four carbons are butanes, those with five carbons are pentanes, etc.
iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

19. Classification of C & H
Primary (1°) C: a carbon bonded to one other carbon
1° H: a hydrogen bonded to a 1° carbon
Secondary (2°) C: a carbon bonded to two other carbons
2° H: a hydrogen bonded to a 2° carbon
Tertiary (3°) C: a carbon bonded to three other carbons
3° H: a hydrogen bonded to a 3° carbon
Quaternary (4°) C: a carbon bonded to four other carbons

20. Cycloalkanes General formula CnH2n
five- and six-membered rings are the most common and most stable
Structure and nomenclature
to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring
if only one substituent, no need to give it a number
if two substituents, number from the substituent of lower alphabetical order
if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

21. Cycloalkanes Monocyclic Compounds Cycloalkanes with only one ring
Attach the prefix cyclo-

22. Substituted Cycloalkanes

23. Cycloalkanes Line-angle drawings each line represents a C-C bond
each vertex and line ending represents a C
Example: isopropylcyclopentane

24. Bicycloalkanes
Bicycloalkane: an alkane that contains two rings that share two carbons
Bicyclo[4.4.0]decane
(Decalin)
Bicyclo[4.3.0]nonane
(Hydrindane)
Bicyclo[2.2.1]heptane
(Norbornane)

25. Bicycloalkanes Nomenclature
parent is the alkane of the same number of carbons as are in the rings
number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc.
show the lengths of bridges in brackets, from longest to shortest

26. IUPAC - General prefix-infix-suffix
prefix tells the number of carbon atoms in the parent
infix tells the nature of the carbon-carbon bonds
suffix tells the class of compound
Nature of Carbon-Carbon
Bonds in the Parent Chain
Suffix
Class
Infix -e
hydrocarbon
-an-
all single bonds
-ol
alcohol
-en-
one or more double bonds
-al
aldehyde
-yn-
one or more triple bonds
-amine
amine
-one
ketone
-oic acid
carboxylic acid

27. Conformations
Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
Newman projection: a way to view a molecule by looking along a carbon-carbon single bond—the best way to judge the stability of the different conformations of a molecule

28. Conformations
Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

29. Conformations
Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

30. Cyclopropane
angle strain: the C-C-C bond angles are compressed from 109.5° to 60°
torsional strain: there are 6 sets of eclipsed hydrogen interactions
strain energy is about 116 kJ (27.7 kcal)/mol

31. Cyclobutane
puckering from planar cyclobutane reduces torsional strain but increases angle strain
the conformation of minimum energy is a puckered “butterfly” conformation
strain energy is about 110 kJ (26.3 kcal)/mol

32. Cyclopentane
puckering from planar cyclopentane reduces torsional strain, but increases angle stain
the conformation of minimum energy is a puckered “envelope” conformation
strain energy is about 42 kJ (6.5 kcal)/mol

33. Cyclohexane
Chair conformation: the most stable puckered conformation of a cyclohexane ring
all bond C-C-C bond angles are 110.9°
all bonds on adjacent carbons are staggered

34. Cyclohexane
In a chair conformation, six H are equatorial and six are axial

35. Cyclohexane
For cyclohexane, there are two equivalent chair conformations
all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

36. Cyclohexane
Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other
there are four sets of eclipsed C-H interactions and one flagpole interaction
a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

37. Bicycloalkanes
Norbornane drawn from three different perspectives

38. Bicycloalkanes
Adamantane

39. Geometric Isomers 1 Same side: cis- 2 2 Opposite side: trans- 1
cis-1,2-dimethylcyclohexane 2 2
Opposite side: trans- 1
trans-1-ethyl-2-methylcyclohexane

40. Physical Properties
Low-molecular-weight alkanes (methane....butane) are gases at room temperature
Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature
High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature
Cycloalkanes are non-polar, relatively inert

41. Physical Properties
Constitutional isomers have different physical properties

42. Effect of Branching on Boiling Point
Branching lowers b.p. because of decreased surface contact between molecules.
The long-chain isomer (n-pentane) has the greatest surface area and the highest boiling point.
As the amount of chain branching increases, the molecule becomes more spherical and its surface area decreases.
The most highly branched isomer (neopentane) has the smallest surface area and the lowest boiling point.

43. Oxidation of Alkanes
Oxidation is the basis for their use as energy sources for heat and power
heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

44. Heat of Combustion
Heat of combustion for constitutional isomers

45. Sources of Alkanes Natural gas 90-95% methane Petroleum
gases (bp below 20°C)
naphthas, including gasoline (bp °C)
kerosene (bp °C)
fuel oil (bp °C)
lubricating oils (bp above 350°C)
asphalt (residue after distillation)
Coal

46. Gasoline
Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties