the formation of halogenoalkanes

Question 1

how is a halogenoalkane formed

Answer 1

when you put a mixute of an alkane and a halogen into bright sunlight, or shine a photoflood lamp onto the mixture, the alkane and the halogen will react to form a halogenoalkane

the ultraviolet component of the light starts the reaction

Question 2

what are the three stages of free-radical substitution

Answer 2

initiation

propagation

termination

Question 3

define the initiation stage

Answer 3

the initiation step of the reaction is breaking the covalent bond to form two free radicals

Question 4

explain how the covalent bond is broken

Answer 4

the molecule absorbs the energy of a single quantum of ultraviolet light

the energy of one quantum of UV light is greater than the covalent bond energy, so the bond will break

since both atoms are the same, the covalent bond breaks homolytically. so, one electron goes to each atom

this results in two separate atoms they are called free radicals

Question 5

why does the C-H bond not break during the intiation step

Answer 5

the C-H bond in the alkane needs more energy to break than is available in a quantum of ultraviolet radiation, so this bond does not break

Question 6

give an example equation of the initiation stage

Answer 6

Br-Br ——-) Br. .Br

Question 7

define the propagation reaction

Answer 7

free radicals are used up and created in a chain reaction

Question 8

describe the propagation reaction

Answer 8

the chlorine free radical takes a hydrogen atom from methane to form a stable compound. this leaves a methyl free radical

the methyl free radical is also very reactive and reacts with a molecule.

this produces another free radical and a stable compound

Question 9

explain the effect of the two steps of propagation

Answer 9

the effect of these two steps produces a stable compound and a new free radical.

this is ready to react with more methane and repeat the two steps

Question 10

define the termination step

Answer 10

the free radicals are removed.

if two free radicals join together they make a stable molecule. the two unpaired electrons from a covalent bond

Question 11

what are the three possible termination reactions

using chlorine as an example

Answer 11

two chlorine free radicals react together to give chlorine

Cl. + Cl. = Cl2

two methyl free radicals react together to give ethane

CH3. + CH3 = C2H6

A chlorine free radical and a methyl free radical react together to give chloromethane

Question 12

how is dichloromethane made at the propagation stage

Answer 12

Dichloromethane may be made at the propagation stage, if a chlorine radical reacts with some chloromethane that has already formed

Question 13

what occurs when longer-chain alkanes are used in the chain reaction

Answer 13

there will be many isomers formed because of the Cl. can replace any of the hydrogen atoms

Question 14

why are chain reactions unuseful

Answer 14

they are not very useful because they produce such a mixture of products.

they will also occur without light at high temperatures

Question 15

what destroys the Ozone layer

Answer 15

CFCs

chloroflurorocarbons

Question 16

define Ozone

Answer 16

ozone is a molecule made from three oxygen atoms, O3
it decomposes to oxygen

Question 17

what can too much ozone at ground level cause

Answer 17

causes lung irritation and degradation of paints and plastics

Question 18

why is the Ozone layer important

Answer 18

it protects the earth from harmful exposure to too many ultraviolet rays

Question 19

give an example of what would happen without the Ozone layer

Answer 19

plankton in the sea, which are at the very bottom of the food chain of the oceans, need protection from too much Uv radiation

also, too much UV radiation causes skin cancer in people by damaging DNA

Question 20

explain why chain reactions are important using chlorine as an example

Answer 20

chlorine free radicales are formed from CFCs because the C-Cl bond breaks homolytically in the pressure of UV radiation to produce chlorine free radicals, Cl. . ozone molecules are then attacked by these

the resulting free radicals also attack ozone and regenerate cl.

chlorine free radical is not destroyed in this process. it acts as a catalyst in the breakdown of Ozone to oxygen