1. Features of Alkanes Objectives:
To look at the physical properties of the alkanes
To introduce cycloalkanes
To compare and contrast the physical properties of cycloalkanes and straight chain alkanes;

2. Long chain Alkanes

3. Increasing length As hydrocarbon chains increase in length
The become heavier
They have more possible isomers
The have higher melting and boiling points
They contain increasing amounts of energy per mole.

4. What is a cyclo-alkane
They are alkanes containing a ring of three or more carbons
The have the molecular formula CnH2n

5. Where do they come from?
Petroleum or Crude oil contains approximately 150 hydrocarbons
Approximately half are alkanes or cycloalkanes
1 – 4 C =
5 – 12 C =
12 – 15 C =
15 – 25 C =

6. Other organic sources
Pentacosane (25 C) present on the waxy cuticle of most insects
Hentriacontane 31 C is found in beeswax
Heptacosane, nonacosane and hentriacontane have all been found on the surface coating of bees eggs.

7. Physical properties
Boiling point of un-branched alkanes increases with length
Gases = 1 – 4 C
Liquids 5 – 17 C
Solids 18+ C

8. Boiling points
Attractive forces between the molecules that hold them together are called Van der Waals forces
To vapourise the hydrocarbon these must be overcome by putting in energy
The longer the chain – the more energy required so the higher the boiling point

9. Un-branched vs branched alkanes
Explain what this graph shows

10. Branched structures The more branched the more compact
The more compact the less attraction points it has with other molecules
The less attraction the lower the boiling point.
Branched alkanes therefore have lower BPs than their un-branched isomers.

11. Properties Insoluble in water Unreactive Extremely weak acids
Highly combustible in oxygen
Release much energy in exothermic reactions.

12. Heats of Combustion (-ΔH°) of several alkanes and branched alkane
Compound
Number of carbons per chain
-ΔH°
Kj/mol
Hexane 6C
4 163
Heptane 7C
4 817
Octane 8C
5 471
Nonane 9C
6 125
Decane
10C
6 778
Undecane
11C
7 431
Dodecane
12C
8 086
Hexadecane
16C
10 701
2-Methylpentane
5 + 1
4 157
2-methylyhexane
6 + 1
4 812
2-methylheptane
7 + 1
5 466

13. What two energy patterns do you observe in the previous slide?1 2

14. Heat of combustion and stability
The lower the heat of combustion, the more stable the molecule
Branched chains have lower heats of combustion than un-branched ones
Therefore branched chains are more stable than un-branched ones
Un-branched chains tend to crack!
Cycloalkanes also have lower heats of combustion than un-branched chains with the same numbers of carbons

15. Oxidation numbers of Carbon in alkanes
Oxidation number of carbon in methane is -4 as methane is CH4 and each H is worth +1
When alkanes are burnt in air CO2 is produced.
In this Carbon has an oxidation number of ?
This means Carbon has been?
Most organic compounds contain carbon in different oxidation states (i.e. with different oxidation numbers)