Substitution Reactions of Alkanes

Question 1

why are alkanes considered to be fairly unreactive

Answer 1

• because they contain only carbon and hydrogen atoms

- and only have single bonds

Question 2

why do alkane’s single bonds make them unreactive

Answer 2

• they are not very polar
• so they dont undergo reactions with substances that are reactive
• such as acids, alkalis and reactive metals

Question 3

what is a simple example of a substitution reaction between methane and chlorine gas

Answer 3

CH4 + Cl2 = CH3Cl + HCl

Question 4

what is happening during the substitution reaction and what is it called

Answer 4

• one of the hydrogen atoms from methane have been replaced by a chlorine atom
• in this case, it would be called chlorination

Question 5

what is the broader name given to substitution reactions using halogens

Answer 5

halogenation

Question 6

what does a mechanism do in chemistry

Answer 6

• it tries to explain the actual changes that happen during a reaction
• especially with the bonding between the atoms

Question 7

what is a mechanism in chemistry (definition)

Answer 7

• a sequence of two or more steps
• each one represented by an equation
• that shows how a reaction takes place

Question 8

what happens when you mix chlorine with methane at room temperature

Answer 8

no reaction occurs

Question 9

what are the two conditions that need to be met in order for a reaction to occur

Answer 9

• there needs to be a sufficient rise in temperature

- or it needs to be exposed to ultraviolet radiation (sunlight)

Question 10

what does the ultraviolet radiation affect

Answer 10

• the chlorine

- not the alkane (methane)

Question 11

what does the ultraviolet light do to the chlroine

Answer 11

• it breaks the chlorine molecule (Cl2) into chlorine atoms

Question 12

how are the outer electrons of this broken chlorine molecule distributed

Answer 12

• the bond in a chlorine molecule consists of a shared pair of electrons
• which are equally shared between the two atoms
• so when broken, each chlorine atoms takes one electron from the shared pair

Question 13

what is this kind of bond breaking between chlorine molecules, for example, called

Answer 13

homolytic fission

Question 14

what equation shows the chlorine molecule being broken down into two chlorine atoms and what is this step called

Answer 14

• Cl2 = Cl. + Cl. (notice the dots)

- this step is called initiation

Question 15

what does the dot represent

Answer 15

an unpaired electron

Question 16

if the dot represents and unpaired electron, what does that make and not make Cl.

Answer 16

• it doesnt make it an ion or a molecule

- but instead it is called a free radical

Question 17

what is a free radical

Answer 17

• a species with an unpaired electron
• ‘species’ refers to any substance that can be represented by a formula
• which includes atoms, molecules, ions and radicals

Question 18

how would chlorine radicals and methane molecules interact and why

Answer 18

• when they collide the chlorine radical will remove a hydrogen atom from the methane molecule
• this is because radicals are very reactive

Question 19

what is the equation for this process and what is this step called

Answer 19

• Cl. + CH4 = CH3. + HCl

- this called propagation (1)

Question 20

what would the newly created molecular radical now react with, along with this formula

Answer 20

• it would react with chlorine molecules to produce chlorine radicals
• CH3. + Cl2 = CH3Cl + Cl.

Question 21

what is that step called

Answer 21

propagation (2)

Question 22

why are the two steps called propagation

Answer 22

• because both equations involve one radical reacting with molecule
• which produces one radical and one molecule

Question 23

what is the overall product when the two propagation steps are combined

Answer 23

CH4 = CH3Cl

Question 24

what is the last reaction that can take place

Answer 24

• two radicals colliding and reacting with each other

- to form a stable molecule

Question 25

why is a molecule produced

Answer 25

the two unpaired electrons (one from each radical) are shared to form a covelant bond

Question 26

what are the three different possibilities of two radicals reacting together in this chlorine and methane reaction

Answer 26

• Cl. + Cl. = Cl2
• Cl. + CH3. = CH3Cl
• CH3. + CH3. = C2H6

Question 27

what is the name of this final step to the reaction and what does it mean

Answer 27

• termination
• meaning that the sequence of reactions come to an end
• because two reactive species are converted into one unreactive species

Question 28

when chlorine reacts with a methane molecule in a substitution reaction to form chloromethane, is that the end of the reaction or can that new molecule continue to react with chlorine

Answer 28

• it can continue to react with more chlorine molecules
• by displacing the remaining hydrogen atoms
• and making the molecule more ‘saturated’ with chlorine atoms

Question 29

what are the 3 following products that could be produced when reacting chloromethane with chlorine

Answer 29

• dichloromethane
• trichloromethane
• tetrachloromethane

Question 30

what are the equations for the production of those three products

Answer 30

• CH3Cl + Cl2 = CH2Cl2 + HCl
• CH2Cl2 + Cl2 = CHCl3 + HCl
• CHCl3 + Cl2 = CCl4 + HCl

Question 31

why isnt this reaction between chlorine and methane not a good method for the preparation of chloromethane

Answer 31

• the yield will be low because of these further reactions

- and the products would have to be separated