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C12- ALKANES > 12.5- THE FORMATION OF HALOGENOALKANES > Flashcards

12.5- THE FORMATION OF HALOGENOALKANES Flashcards

1
Q

When will an alkane react with a halogen? (2)

A

put mixture of alkane and halogen into bright sunlight
or
shine photoflood lamp onto mixture

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2
Q

What will the alkane and halogen react to form?

A

halogenoalkane

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3
Q

What component of light starts the reaction to form halogenoalkenes?

A

ultraviolet component

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4
Q

At what condition do alkanes not react with halogens?

A

in the dark at room temperature

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5
Q

What happens when you shine UV light onto a mixture of hexane and bromine? (hint- change)

A

changes from red-brown colour to colourless and misty fumes of hydrogen bromide appear

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6
Q

What type of reaction takes place when UV light is shone on a mixture of hexane and bromine?

A

substitution reaction

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7
Q

Why is it called a substitution reaction when UV light is shone on a mixture of hexane and bromine?

A

one or more of the hydrogen atoms in the alkane has been replaced by a bromine atom

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8
Q

What is the equation of the main reaction when UV light is shone onto a mixture of hexane and bromine?

A

C6H14 (g) + Br2 (l) = C6H13Br (l) + HBr (g)

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9
Q

What is the halogenoalkane formed when UV light is shone on a mixture of hexane and bromine?

A

bromohexane

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10
Q

What type of substitution is the reaction between the alkane and halogen with UV?

A

free-radical substitution

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11
Q

What does free-radical substitution start off?

A

chain reaction

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12
Q

How many stages are there in the chain reaction from free-radical substitution?

A

3 stages

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13
Q

What are the 3 stages in the chain reaction from free-radical substitution?

A

initiation
propagation
termination

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14
Q

What is the initiation steps? CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)

A

breaking the Cl-Cl bond to form two chlorine atoms

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15
Q

Why does the chlorine molecule break in initiation?

A

chlorine molecule absorbs energy of single quantum of UV light
energy of one quantum of UV light greater than Cl-Cl bond energy, so bond will break

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16
Q

How does the Cl-Cl bond break?

A

homolytically

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17
Q

Why does the Cl-Cl bond break homolytically?

A

both the atoms are the same

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18
Q

What happens when the Cl-Cl bond is broken homolytically?

A

one electron goes to each chlorine atom

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19
Q

What does the Cl-Cl breaking homolytically result in?

A

two separate chlorine atoms

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20
Q

What does the . on Cl. represent?

A

unpaired electron

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21
Q

What is the Cl. called?

A

free radical

22
Q

How reactive are free radicals?

A

highly reactive

23
Q

Why is the C-H bond in the alkane not broken in initiation?

A

needs more energy to break than is available in quantum of UV radiation

24
Q

In how many steps does propagation take place?

A

2

25
Q

What is the first stop of propagation?

CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)

A

chlorine free radical takes hydrogen atom from methane to form hydrogen chloride- stable compound
leaves methyl free radical .CH3

Cl. + CH4 = HCl + .CH3

26
Q

What is the second step if propagation?

Cl. + CH4 = HCl + .CH3

A

methyl free radical also very reactive + reacts with a chlorine molecule
produces another chlorine free radical + a molecule of chloromethane- stable compound

.CH3 + Cl2 = CH3Cl + Cl.

27
Q

What is the effect of the two steps in propagation?

CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)

A

produce hydrogen chloride, chloromethane, and new Cl. free radical

28
Q

What is the Cl. free radical ready to react with?

A

more methane and repeat the two steps

29
Q

What is the chain part of the chain reaction?

A

Cl. free radical reacting with more methane and repeating step 2

30
Q

How many times can the steps within propagation happen?

A

thousands of times

31
Q

What happens in termination?

A

free radicals are removed

32
Q

How can the free radicals be removed?

.CH3 + Cl2 = CH3Cl + Cl.

A

Cl. + Cl. = Cl2
two chlorine free radicals react together to give chlorine

.CH3 + .CH3 = C2H6
two methyl free radicals react together to give ethane

Cl. +.CH3 = CH3Cl
chlorine free radical + methyl free radical react together to give chloromethane

33
Q

What do two free radicals react to form?

A

stable compound with no unpaired electrons

34
Q

What other products are formed as well as the main ones, chloromethane and hydrogen chloride? (4)
CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)

A

some ethane produced at termination stage

dichloromethane may be made at propagation stage, if chlorine radical reacts with some chloromethane that has already formed
CH3Cl + Cl. = .CH2Cl + HCl
followed by .CH2Cl + Cl2 = CH2Cl2 + Cl.

35
Q

What can be produced in free-radical substitution with longer-chain alkanes?

A

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

36
Q

Why are chain reactions not useful?

A

they produce such a mixture of products

37
Q

What can chain reactions occur without?

A

without light at high temperatures

38
Q

What from the stratosphere is believed to be destroying the ozone layer?

A

chlorofluorocarbons (CFCs)

39
Q

What is ozone?

A

molecule made from three oxygen atoms, 03

40
Q

What does ozone decompose to?

A

oxygen

41
Q

What does too much ozone at ground level cause?

A

lung irritation + degradation of paints + plastics, but high in atmosphere has vital role

42
Q

Why is the ozone layer important?

A

it protects the Earth from the harmful exposure to too many UV rays

43
Q

Examples of what will happen without the ozone layer? (2)

A

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

too much UV radiation causes skin cancer in people damaging DNA

44
Q

What can chlorine free radicals be formed from?

A

CFCs

45
Q

How are chlorine free radicals formed from CFCs?

A

C-Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl.

46
Q

What are ozone molecules attacked by?

A

chlorine free radicals

47
Q

Equation for chlorine free radical attacking ozone molecule

A

Cl. + O3 = ClO. + O2

48
Q

What does the resulting free radical from Cl. + O3 = ClO. + O2 do?

A

ClO. attacks ozone and regenerate Cl.

49
Q

Equation for ClO. attacking ozone and regenerating Cl.

A

ClO. + O3 = 2O2 + Cl.

50
Q

What does adding the two equations: Cl. + O3 = ClO. + O2 and ClO. + O3 = 2O2 + Cl show?

A

chlorine free radical not destroyed in this process

51
Q

What does the chlorine free radical act as in the breakdown of the ozone to oxygen?

A

catalyst

52
Q

Overall equation for ozone being broken down into oxygen

A

203 = 302

C12- ALKANES (5 decks)

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