Chemical reactions of the alkanes

Question 1

Are alkanes reactive?

Answer 1

No - don’t react with most common reagents

Question 2

What are the three reasons for why alkanes lack reactivity?

Answer 2

1) C-C and C-H sigma bonds are strong
2) C-C bonds are non polar
3) The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar

Question 3

What type of reaction do alkanes undergo?

Answer 3

Combustion

Question 4

How do alkanes combust?

Answer 4

They react with a plentiful supply of oxygen to produce carbon dioxide and water

Question 5

What do combustion reactions give out?

Answer 5

Heat

Question 6

Why are alkanes used as fuels? (3 reasons)

Answer 6

1) Readily available
2) Easy to transport
3) Burn in a plentiful supply of oxygen without releasing toxic products

Question 7

What do alkanes do in a plentiful supply of oxygen?

Answer 7

They burn completely to produce carbon dioxide and water - complete combustion

Question 8

What do alkanes do in a limited supply of oxygen?

Answer 8

Hydrogen atoms are oxidised to water, but combustion of carbon may be incomplete, forming carbon monoxide and sometimes carbon itself as soot - incomplete combustion

Question 8

Why do alkanes not react fully in incomplete combustion?

Answer 8

There isn’t enough oxygen for complete combustion

Question 9

How many additional oxygen molecules are required for complete combustion as you descend down the homologous series?

Answer 9

1.5 oxygen molecules

Question 10

What is the equation for complete combustion of heptane?

Answer 10

C7H16 (l) + 11O2 (g) –> 7CO2 (g) + 8H2O (l)

Question 11

What is the equation for incomplete combustion of heptane when CO is formed?

Answer 11

C7H16 (l) + 7.5O2 (g) –> 7CO (g) + 8H2O (l)

Question 12

What is the equation for incomplete combustion of heptane when C is formed?

Answer 12

C7H16 (l) + 4O2 (g) –> 7C (s) + 8H2O (l)

Question 13

What do alkanes react with in the presence of sunlight?

Answer 13

The halogens

Question 14

Why do alkanes react with halogens only in the presence of sunlight?

Answer 14

The high-energy UV radiation present in sunlight provides the initial energy for a reaction to take place

Question 15

What is a reaction mechanism?

Answer 15

A series of steps showing how electrons are thought to move during the reaction

Question 16

What is the mechanism for bromination of methane an example of?

Answer 16

Radical substitution

Question 17

What are the three steps of bromination of methane?

Answer 17

1) Initiation
2) Propagation
3) Termination

Question 18

What happens in the initiation stage?

Answer 18

The reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission - each bromine takes one electron from the pair, forming two highly reactive bromine radicals

Question 19

Where is the energy for initiation provided from?

Answer 19

UV radiation

Question 20

What is a radical?

Answer 20

A very reactive species with an unpaired electron

Question 21

How is a radical shown?

Answer 21

With a single dot to represent the electron

Question 22

What happens in the propagation stage?

Answer 22

The reaction propagates through two propagation steps, a chain reaction

Question 23

What happens in the first propagation step?

Answer 23

A bromine radical reacts with a C-H bond in methane, forming a methyl radical, and a molecule of hydrogen bromide

Question 24

What happens in the second propagation step?

Answer 24

Each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, together with a new bromine radical

Question 25

What does the new bromine radical do?

Answer 25

Reacts with another CH4 molecule as in the first propagation step

Question 26

Why is propagation a chain reaction?

Answer 26

The two steps cycle through in a chain reaction

Question 27

When is propagation terminated?

Answer 27

When two radicals collide

Question 28

What happens in the termination stage?

Answer 28

Two radicals collide, forming a molecule with all electrons paired

Question 29

Why does termination stop the reaction?

Answer 29

When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction

Question 30

What are the two limitations of radical substitution in organic synthesis?

Answer 30

1) Further substitution
2) Substitution at different positions in a carbon chain

Question 31

What is further substitution?

Answer 31

Another bromine radical can collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane - further substitution can continue until all hydrogen atoms have been substituted, resulting in a mixture

Question 32

When does substitution at different positions in the carbon chain occur?

Answer 32

If the carbon chain is longer - there will be a mixture of monosubstituted isomers by substitution at different positions in the carbon chain