12.2 Chemical reactions of the alkanes

Question 1

Do alkanes react with common reagents

Answer 1

They do not react with most

Question 2

What are the reasons for the lack of reactivity of alkanes

Answer 2

Their C-C and C-H σ- bonds are strong
C-C bonds are non-polar
The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar

Question 3

Despite their low reactivity, all alkenes react with a plentiful supply of ______ to produce ______ ______ and ______. This reaction is called combustion

Answer 3

Oxygen
Carbon dioxide
Water

Question 4

Why are alkanes used as fuels

Answer 4

They are readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxic products

Question 5

Methane info

Answer 5

Greenhouse gas with global warming potential of 22
Vast quantity stored in ice-saturated arctic tundra of Siberia and N. Canada
Formed by decomposition of organic material

Question 6

What do alkanes burn completely to produce in a plentiful supply of oxygen

Answer 6

Carbon dioxide and water

Question 7

What happens if oxygen is limited during the combustion of alkanes

Answer 7

The hydrogen atoms in the alkane are always oxidised to water, but combustion of the carbon may be incomplete, forming carbon monoxide, or carbon itself as soot

Question 8

Carbon monoxide info

Answer 8

Colourless, odourless and highly toxic gas.
Combines irreversibly with haemoglobin in red blood cells to form cherry-pink compound called carboxyhaemoglobin which prevents haemoglobin from transporting oxygen around body

Question 9

When do alkanes react with halogens

Answer 9

In the presence of sunlight

High energy UV radiation present in sunlight provides initial energy for a reaction to take place

Question 10

What is an example of radical substitution

Answer 10

The mechanism for the bromination of methane

Question 11

Three stages of the mechanism for the bromination of methane (radical substitution)

Answer 11

Initiation, propagation and termination

Question 12

What happens in the initiation stage (radical substitution)

Answer 12

STEP 1
Reaction is started when covalent bond in bromine molecule is broken by homolytic fission
Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals
Energy for this bond is provided by UV radiation
Br-Br->Br•+•Br

Question 13

What is a radical

Answer 13

A very reactive species with an unpaired electron

Shown with a single dot to represent the electron

Question 14

What happens in the propagation stage (radical substitution)

Answer 14

STEP 2
Reation propagates through two steps, a chain reaction
CH4+Br•->•CH3+HBr
•CH3+Br2->CH3Br+Br•
In first, bromine radical Br• reacts with C-H bond in methane, forming methyl radical, •CH3 and molecyle of HBr
In second, each methyl radical reacts with another bromine forming bromomethane with new bromine radical
New bromine radical reacts with another CH4, continues as cycle
Terminated when radicals collide

Question 15

What happens in the termination stage (radical substitution)

Answer 15

STEP 3
Two radicals collide, forming molecule with all electrons paired, Number of possible termination steps with different radicals in reaction mixture
Br•+•Br->Br2
•CH3+•CH3->C2H6
•CH3+VBr->CH3Br
When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction

Question 16

Limitations of radical substitution in organic synthesis

Answer 16

Another bromine radical may collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane, further substitutions may continue until all hydrogen atoms have been substituted resulting in a mixture of molecules
For methane, only one monobromo compound is possible; if the carbon chain is longer, we will get a mixture of monosubstituted isomers