1. Alkane

2. Introduction
Alkanes are saturated hydrocarbon, that is they contain only carbon-hydrogen (C-H) and carbon-carbon (C-C) single bonds
The general formula of alkane = CnH2n+2
Where n is the number of carbon

3. Nomenclature
The IUPAC names of alkanes with unbranched chain of carbon atoms consists of two parts:
A prefix: Indicates the number of carbon atoms in the chain
The suffix –ane to show the compound is a saturated hydrocarbon; alkane

4. The prefix that correspond to the number of carbon atoms in a chain1
Meth- 2
Eth- 3
Prop- 4
But- 5
Pent- 6
Hex- 7
Hept- 8
Oct- 9
Non- 10
Dec- 11
Undec- 12
Dodec-

5. Molecular formula and structural formula
Name
Molecular formula
Condensed structural formula
Methane
CH4
Ethane
C2H6
CH3CH3
Propane
Butane
Pentane
Hexane
Heptane

6. Alkyl group
An alkyl group is a radical that has general formula CnH2n+1, formed by removing a hydrogen atom from alkane
Alkyl group are named by dropping the suffix –ane from the parent alkane and adding the suffix –yl.
For example alkyl group CH3-CH2- is named ethyl

7. Alkyl groups Alkane Name of alkyl group Condensed structure Methane
Methyl
CH3-
Ethane
Ethyl
CH3-CH2-
Propane
Propyl
CH3-CH2-CH2-
Isopropane
Isopropyl
CH3-CH- |
CH3
Butane
Butyl
CH3-CH2-CH2-CH2-
Isobutane
Isobutyl
CH3-CH-CH2-
Pentane
Pentyl
CH3-CH2-CH2-CH2-CH2-

8. Rules for naming branched
Locate the longest continuous chain of carbon atoms which determines the parent name for the alkane
For example
substituent
Parent chain

9. 4. Thus the alkane named is 3-methylhexane and not 4-methylhexane
Number the carbon atoms in the parent chain beginning from the end that is nearest to the substituent so that the substituent is attached at the lowest number carbon atom.
3. Use the number obtained in 2, to state the location of the substituent group.
4. Thus the alkane named is 3-methylhexane and not 4-methylhexane
Methyl group

10. 5. When two or more alkyl groups are present are of the same kind, use the prefix di-, tri-, tetra-, and so on. The numbers indicating the position of the alkyl groups are separated by commas.
2,4-dimethylhexane

11. 6. When two or more alkyl groups are present, each group is given a number corresponding to its location on the longest chain. The groups should be listed alphabetically in the name of the alkane. i.e. ethyl before methyl.
4-ethyl-2-methylhexane

12. Write the IUPAC names for the following alkanes

13. Naming cycloalkanes
Alkanes having one or more rings of carbon atoms are called cycloalkanes.
Cycloalkanes are named by placing the prefix cyclo- to the names of the corresponding alkane. For example
Cyclopropane
(Skeletal formula)

14. 1-ethyl-3-methylcyclohexane
1,3-dimethylcyclohaxane
1-ethyl-4-methylcyclohexane

15. Name the following cycloalkanes

16. Classification of carbon and hydrogen atoms
A carbon is classified as primary, secondary, tertiary or quaternary depending on the number of carbon atoms bonded to it.
Primary
Primary
Secondary
Tertiary
Quaternary

17. Exercise
How many primary, secondary, tertiary and quaternary carbon atoms are there in 2,2,4-trimethylpentane?
5-primary carbon atoms
1-secondary carbon atoms
1-tertiary carbon atoms
1-quaternary carbon atom

19. General preparation of alkanes
Alkanes can be prepared by decarboxylation of the sodium carboxylate salt with soda lime. Decarboxylation involves the removal of the carboxyl group, -COO- and replacing it with a hydrogen atom
RCOONa + NaOH  RH + Na2CO3

20. 2. Wurtz reaction: The action of sodium on alkyl halide in ether 2 R-X + 2 Na  R-R + 2NaX For example

21. Physical properties
The lower alkanes (methane to butane), are colorless gases at room conditions
The higher member (C5 to C18) are liquids and solids with pleasant smell

22. Physical properties Boiling point
Alkane molecules have low polarity. Only weak van der walls forces are present between molecules. Hence the boiling point of alkane is lower than those of other organic compounds
As the number of electrons in the molecule increases (with increasing number of carbon atoms), the strength of these attractive forces also increases. More energy is needed to break this forces between the molecules when boiling. Hence the boiling point rises with increasing number of carbon atoms in the molecule.
The boiling points of unbranched alkanes show a regular increases with increasing number of carbon atoms in the molecules

23. Physical properties Melting point
Like boiling points, the melting points of alkanes also increase with an increase in the number of carbon atoms
Density
Alkanes are the least dense of all groups of organic compounds and have densities less than water
Solubility
Alkanes are almost insoluble in water because of their low polarity and their inability to form hydrogen bonds. However liquid alkanes are micscible with each other

24. Exercise
Arrange the compounds below in order of increasing boiling point
As the number of carbon atoms increases, intermolecular forces between molecules become greater, resulting in increasing boiling point. Therefore propane<pentane<nonane

25. Chemical properties Combustion of Alkane
Alkanes do not react with air at room temperature but if heated, they burn readily to give carbon dioxide and water in an exorthermic reaction.
C3H8 + O2  3CO2 + 4H2O
Try:
C2H6
C4H10

26. Halogenation of alkanes
Substitution is a reaction where an atom or a group of atoms in a molecule is substituted by a different atom or group of atoms.
The reaction takes place in light (uv light) due to chemical inertness
For example:
CH4 + Cl2  CH3Cl + HCl
CH3Cl + Cl2  CH2Cl2 + HCl
CH2Cl2 + Cl2  CHCl3 + HCl
CHCl3 + Cl2  CCl4 + HCl

27. Exercise
Write halogenation substitution for the reaction of one mole of propane with one mole of chlorine

28. The mechanism of chlorination of methane:- Free radical substitution
The mechanism consists of three steps:- Initiation, Propagation and termination
Initiation:
Involves the photochemical homolytic fission of the Cl-Cl bond in the chlorine molecule
The two chlorine atoms in the bond. The bond will break homolytically producing two chlorine atoms, each with one unpaired electron. Known as a free radical.
It is easier to break the Cl-Cl bond (+242 kJ/mol) than the C-H bond (+435 kJ/mol). Thus in the initiation step only chlorine radicals are produced although both the chlorine and methane molecules are exposed to sunlight.

29. The dot indicates the unpaired electron
Step 1
The dot indicates the unpaired electron
The half arrows shows the movement of the electrons

30. Chain propagation
The chlorine radical formed is very reactive. It is reactive enough to break the C-H bond when it attacks the methane molecules, producing hydrogen chloride, HCl and a methyl radical

31. The methyl free radical can react with a chlorine molecule to produce chloromethane, CH3Cl and a new chlorine free radical.

32. This chain reaction continues as follows

33. Chain termination
Free radicals can combine with each other and bring the chain to an end –termination steps

34. Summary
Alkanes are saturated hydrocarbons and may exist as unbranched chains, branched chain or rings
A mixture of alkanes is found in crude oil and natural gas

35. Summary Radical substitution reactions have three stages:
Initiation – Where radicals are formed
Propagation – Where products are formed and the radicals are regenerated
Termination – Where radicals are removed