Alkanes

AS Chemistry

Organic Chemistry: alkanes Following the new OCR syllabus

What Are Alkanes? • Alkanes are saturated hydrocarbons • They contain only Hydrogen(H) and Oxygen(O) atoms joined by single bonds • They are fairly un-reactive except in combustion reactions such as methane gas to power Bunsen burners in the lab

Sources Of Alkanes • Alkanes are obtained mainly from crude oil • Crude oil is a mixture of thousands of compounds • They can be separated by a process called ‘fractal distillation’

The first 5 alkanes Carbon atoms

Name

Formula

1

Methane

CH4

2

Ethane

C2H6

3

Propane

C3H8

4

Butane

C4H10

5

Pentane

C5H12

3d

The first four alkane prefixes have historical names, after which Latin ordinals are used, these will be familiar from maths i.e. PENTagon, HEXagon.

Task 1 The sixth alkane is ‘Hexane’, it has the formula C6H14. Using the structures of the first alkanes and the 3d model below, work out its structure. Use simplified notation [5minutes]

Task 1

H

H

H

H

H

H

H

C

C

C

C

C

C

H

H

H

H

H

H

Hexane

H

Alkanes as fuels The Bunsen Flame- an exothermic reaction CH4 + O2  CO2 + H2O

+



+

Isomers • Isomers are chemical substances with the same molecular formula but different structures • Hydrocarbons like alkanes with 4 or more carbon atoms form isomers

• Lets take butane as an example, it has the molecular formula C4H10. H H

H

H

H

C

C

C

C

H

H H

H

Iso-butane Branched hydrocarbon

H C

H

C C

H

H C

H H

Melting point: -138 Celsius Boiling point: 0 Celsius

H

H H

N-butane Straight chain

H H

Melting point: -159 Celsius Boiling point: 0 Celsius

Task 2 Draw the three possible isomers of pentane [20minutes]

Task 2 H

H

H

H

C

H C

C

H

C

C

H

H H

H

H

H

H H C

H H

H

C H

H

C

C H

C H

H

H H

H H

H C

H H

H

C

C

C H

C H

H

H

H H

How many isomers can be formed? No. No. Name

Formula

Carbon atoms

Isomers possible

Butane

C4H10

Pentane

C5H12

5

3

Hexane

C6H14

6

5

Heptane

C7H16

7

9

Octane

C8H18

8

18

Nonane

C9H20

9

35

Decane

C10H22

10

75

Dodecane

C12H26

12

355

4

2