1. Alkanes and Cycloalkanes
Conformational and Geometric Isomerism

2. Hydrocarbons Petroleum Saturated Unsaturated Aromatic
Single-bonded C’s
Alkanes or cycloalkanes
Unsaturated
At least 1 double- or triple-bonded pair of C’s
Alkenes—double
Alkynes—triple
Can be cyclic
Aromatic
Structures like benzene

3. Alkane Structure Simplest Bond angle CnH2n + 2 Normal v. branched
n-alkanes
Methylene group
—CH2
Homologous series
Consecutive building with predictable chemical and physical properties

4. Nomenclature of Organic Compounds
Common
Sources, structure, or uses
Limonene
Cubane
IUPAC
Systematic method
Unique name
Write structures from names
1-methyl-4-(1-methylethenyl)-cyclohexene
pentacyclo[ ,5.03,8.04,7]octane

5. First 10 Unbranched Alkanes
Name
# of C
Molcular Formula
Structural Formula
# Structural Isomers
methane 1
CH4
ethane 2
C2H6
CH3CH3
propane 3
C3H8
CH3CH2CH3
butane 4
C4H10
CH3CH2CH2CH3
pentane 5
C5H12
CH3(CH2)3CH3
hexane 6
C6H14
CH3(CH2)4CH3
heptane 7
C7H16
CH3(CH2)5CH3 9
octane 8
C8H18
CH3(CH2)6CH3 18
nonane
C9H20
CH3(CH2)7CH3 35
decane 10
C10H22
CH3(CH2)8CH3 75

6. IUPAC Rules for Alkanes
1. The –ane ending will be used for all saturated
hydrocarbons.
2. Alkanes without branches are named according to
the number of C’s
3. Branched alkanes use a root or parent name for the
longest continuous chain of C’s

7. IUPAC Rules for Alkanes
4. Anything not on the root chain is known as a substituent. Saturated substituents are known as alkyl groups, and names come from # of C’s.
methyl
ethyl
propyl

8. IUPAC Rules for Alkanes
n-propyl
isopropyl
n-butyl
isobutyl
sec-butyl

9. IUPAC Rules for Alkanes
5. The root chain is numbered such that the first substituent along the chain receives the lowest possible number.
Any other substituents are also located by name and location. Every substituent must be named and numbered.
If two or more of the same substituent are present, prefixes such as di, tri, tetra, penta, etc. are used
If two or more different types of substituents are used, they are listed alphabetically, ignoring prefixes such as di, tri, n, iso, sec, tert unless necessary to alphabetize

10. IUPAC Rules for Alkanes
5. Separate numbers from words with a hyphen; separate numbers from numbers with a comma. There are no spaces within an IUPAC name. Let’s use some C skeletons as our first examples:

11. IUPAC Rules for Alkanes

12. IUPAC Rules for Alkanes

13. Halogen Substituents
The letter R represents alkyl group. R-H means any alkane.
The halogens that may take the place of the H: F, Cl, Br, and I
The name of halogen loses ine suffix and becomes o: fluoromethane, chloromethane, bromomethane, iodomethane
Common names treat R as substituent and halogen as root chain

14. Alkane Sources Petroleum Mixture of alkanes and cycloalkanes
Gasoline v. diesel
LPG—1° propane
butane
Natural Gas
~80% methane and 5-10% ethane
Liquefied for transportation (-160°C, 1 m3 l = 600 m3 g)

15. Alkane Properties and Intermolecular Forces
Insoluble in water
H-bonding v. LDF
Lower bp for MW than other organics
bp rises as chain length increases
bp falls as chain becomes more branched
Name
Boiling Point (°C)
n-pentane 36
2-methylbutane 28
2,2-dimethylpropane 10

16. Alkane Conformations Stereoisomers
Atoms connected in same order but arranged differently in space
Sigma bonds
Single bonds allow rotation
Staggered
Eclipsed
Rotamers or Conformers
Dash-wedge
Sawhorse
Newman projections

17. Staggered Conformer of Ethane

18. Eclipsed Conformer of Ethane

19. Cylcoalkane Nomenclature
Ring of C’s
cyclo is prefix on alkane name
With 1 substituent, no number needed
More than 1 substituent, numbers needed
Alphabetic priority gets lowest number
cylcopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctane
bp = -32.7°C °C °C °C °C °C

20. Cycloalkane Nomenclature
CH3
CH3
CH3
CH3
CH2CH3
methylcyclooctane
NOT…
1,2-dimethylcyclooctane
NOT…
1-ethyl-2-methylcyclooctane
NOT…

21. Cylcoalkane Conformations
Cyclopropane
Planar with 60° C-C-C (only cycloalkane that is planar)
Cyclobutane
88° C-C-C (predicted 90°)
Cyclopentane
105° C-C-C (predicted 108°)
Cyclohexane
109.5° C-C-C
Chair conformation

22. Cyclohexane Chair Conformations
Axial…3 up, 3 down
Equatorial, 6 in mean plane of C’s
Flipping occurs easily at RT; slows as T decreases
Important in biomolecules like glucose

23. Cyclohexane Boat Conformation
Why is this conformation less stable than the chair conformation?

24. Cis-Trans Isomerism in Cycloalkanes
Type of stereoisomers or geometric stereoisomers in which substituents are on the same side (cis) or the opposite side (trans) of a ring
Different physical and chemical properties
Not readily converted like conformers are

25. Conformers (Rotamers)
Isomer Review
Interconvertible?
Bond Pattern?
Isomers
Stereoiosomers
Conformers (Rotamers)
Configurational
Structural Isomers

26. Reactions of Alkanes Relatively inert Oxidation (Combustion)
Excess O2 produces CO2, most oxidized form of C (ox # is +4)
Insufficient O2 means lower ox # of C CO C
CH2O
CH3CO2H
Halogenation (substitution rxn)
Excess halogen results in more substitution…even mixtures
Cycloalkanes make pure organic products

27. Oxidation of Alkanes
CH4 + 2O2  CO2 + 2H2O 2CH4 + 3O2  2CO + 4H2O CH4 + O2  C + 2H2O CH4 + O2  CH2O + H2O 2C2H6 + 3O2  2 CH3CO2H + 2H2O

28. Halogenation of Alkanes
energy
CH4 + Cl2  CH3Cl + HCl
Process is known as chlorination (others are fluorination, bromination, and iodination)
Excess of halogen, more H’s are substituted
CCl4 can be produced with enough chlorine.
+ F2  HF
light F

29. Halogenation: Free Radical Chain Mechanism
Rxn Mechanism
Most rxns have multiple steps
Chain-initiating step
Breaking of halogen molecule
Chain-propagating step
Radical is formed (odd # of unshared e-)
Each step: radical is consumed, but another radical is produced
Chain-terminating step
No new radicals are formed and all radicals are combined with some other radical

30. Halogenation: Free Radical Chain Mechanism
light
R-H + Cl-Cl  R-Cl + H-Cl
Chain-initiating step
Chain-propagating steps
Possible chain-terminating steps

31. Reaction Summary
Combustion
Halogenation

32. Reaction Mechanism Summary
Halogenation