1. Chemistry 20
Chapter 2
Alkanes

2. Carbon Carbon has four valence electrons; hydrogen has one. •
Review
Carbon
Carbon has four valence electrons; hydrogen has one. •
• C • H •
To obtain an octet, carbon forms four bonds.
H H
• •
H  C  H H C H CH4 , methane
H H H

3. Hydrocarbons Large family of organic compounds
Composed of only carbon and hydrogen
Saturated hydrocarbons
Alkanes
Unsaturated hydrocarbons
Alkenes, Alkynes & Aromatics
C - C
C = C
C C

4. Alkanes

5. Expanded structural formula (Lewis):
Methane
Tetrahedral
Expanded structural formula (Lewis):
showing each bond line.
CH4
Molecular formula

6. Ethane
C2H6
Molecular formula
Expanded structural formula
CH3 – CH3
Condensed structural formula: with each carbon atom and
its attached hydrogen atoms.

7. Alkanes
CnH2n+2
n: number of carbon atoms

8. Naming of Alkanes
CnH2n+2
Prefix + ane

9. Line-angle Formula Propane CH3-CH2-CH3 Butane CH3-CH2-CH2-CH3 Pentane
CH3-CH2-CH2-CH2-CH3

10. Naming Substituents In the IUPAC system:
Removing a H from an alkane is called alkyl group.
-ane
Halogen atoms are named as halo.
-ine
-yl -O
-OH Hydroxyl
-NO2 Nitro

11. Number the carbon atoms starting from the end nearest a substituent.
STEP 2
Number the carbon atoms starting
from the end nearest a substituent.
STEP 3
Give the location and name of each
substituent (alphabetical order) as a
prefix to the name of the main chain.

12. Give the name of: CH3 
CH3─CH─CH2─CH3
STEP Longest chain is butane.
STEP Number chain. CH3
STEP 3 Locate substituents and name.
2-Methylbutane

13. Give the name of: CH3 CH3
 
CH3─CH─CH─CH3
STEP Longest chain is butane.
STEP Number chain. CH3 CH3
 
STEP 3 Locate substituents and name.
2,3-dimethylbutane

14. Cl CH3
 
CH3─CH2─CH─CH─CH3
STEP Longest chain is pentane.
STEP Number chain from end nearest substituent.
Cl CH3
 
CH3─CH2─CH─CH─CH3
STEP 3 Locate substituents and name alphabetically.
3-Chloro-2-methylpentane

15. CH CH3
| |
CH3─CH─CH2 ─CH─CH ,4-dimethylpentane
Cl CH3
CH3─CH2─CH─CH2─C─CH2─CH3 | Cl
3,5-dichloro-3-methylheptane

16. STEP 1 Longest chain has 5 carbon atoms (Pentane).
CH2 CH3 
CH3─CH─CH2─CH3
STEP Longest chain has 5 carbon atoms (Pentane).
STEP Number chain from end nearest substituent.
STEP Locate substituent and name.
3-Methylpentane 2 1 3 4 5

17. Constitutional Isomers
Have the same molecular formula.
Have different atom arrangements (different structural formula).
CH3CH2CH2CH3
Butane
C4H10
CH3
2-Methylpropane
CH3CHCH3
C4H10

18. Cyclic Hydrocarbon - Cycloalkane
Carbon atoms are joined to form a ring.
Cyclobutane = =
Cyclopentane =
Cyclohexane

19. Naming of Cycloalkanes
Prefix “cyclo-” + the name of the open-chain alkane
Substituents:
One substituent: no location number
Two or more substituents: number the ring beginning
with the substituent of lower alphabetical order. Cl
CH3 Br
NO2 OH 1 2 3
Chlorocyclohexane
1-Bromo-2-methylcyclohexane
1-Chloro-3-hydroxyl-2-nitrocyclopentane

20. Conformation - 3D shapes
Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.
The following are three conformations for a butane molecule. 1 2 3 4

21. Conformation - 3D shapes
The most stable conformation of a cyclopentane is the envelope conformation.
The most stable conformation of a cyclohexane is the chair conformation.
All bond angles are approximately 109.5°.

22. Conformation - 3D shapes
In a chair conformation,
six C-H bonds are equatorial bonds.
six C-H bonds are axial bonds.
Equatorial
Axial

23. Conformation - 3D shapes
The more stable conformation of a substituted cyclohexane ring has the substituent group(s) as equatorial rather than axial. H H H H
More Stable

24. Cis & Trans Stereoisomers
The same molecular formula and the same connectivity of their atoms,
but a different arrangement of their atoms in space.
mp & bp of cis < mp & bp of trans

25. Cis & Trans Stereoisomers
Stereoisomers: Isomers that have the same molecular formulas and the same
connectivity of their atoms but a different orientation of their atoms in space.

26. Physical Properties of Alkanes
Nonpolar
Insoluble in water.
Lower density than water.
Low boiling and melting points.
Gases with 1-4 carbon atoms.
(methane, propane, butane)
Liquids with 5-17 carbon atoms.
(kerosene, diesel, and jet fuels)
Solids with 18 or more carbon atoms.
(wax, paraffin, Vaseline)

27. Boiling & melting points of Alkanes
Number of carbon atoms ↑
bp & mp ↑
Number of branches ↑
bp & mp ↓
CH3
CH3CH2CH2CH3
CH3CHCH3

28. Chemical reactions of Alkanes
Low reactivity
1- Combustion:
Alkanes react with oxygen.
CO2, H2O, and energy are produced.
Alkane + O CO2 + H2O + heat
CH4 + 2O2 CO2 + 2H2O + energy

29. Chemical reactions of Alkanes
Low reactivity
2- Halogenation:
Alkanes react with Halogens.
CH4 + Cl2 CH3Cl + HCl
Chloromethane
Heat or light
Dichloromethane
CH3Cl+ Cl2 CH2Cl2 + HCl
Heat or light
CH2Cl2 + Cl2 CHCl3 + HCl
Trichloromethane
Heat or light
CHCl3 + Cl2 CCl4 + HCl
Tetrachloromethane
Heat or light

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