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Chemistry - AS Level > halogenoalkanes > Flashcards

halogenoalkanes Flashcards

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1
Q

what are halogenoalkanes

A

Halogenoalkanes are a family of compounds in which a halogen atom has replaced one or more hydrogen atoms in an alkane.

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2
Q

what are the 3 types of halogenoalkanes

A

Primary halogenoalkane - the carbon atom attached to the halogenoalkane atom is only attached to 1 alkyl group
Secondary halogenoalkane - the carbon atom attached to the halogenoalkane atom is attached to 2 alkyl groups
Tertiary halogenoalkane - the carbon atom attached to the halogenoalkane atom is attached to 3 alkyl groups.

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3
Q

how are the halogenoalkanes reactive

A

There is a large difference in the electronegativities between the carbon atom and the halogen atom in a halogenoalkane. This results in a polar carbon-hydrogen bond which makes them susceptible to attacks by nucleophiles and halogenoalkanes will undergo nucleophilic substitution reactions. The presence of the polar carbon-halogen bond is the reason why halogenoalkanes are more reactive than alkanes.

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4
Q

How can the rate of the reaction be measured?

A

When halogenoalkanes react with an aqueous silver nitrate solution, a silver halide precipitate is formed. The faster the precipitate is formed, the quicker the rate of reaction.
From the three halogenoalkanes commonly used (chloroalkanes, bromoalkanes and iodoalkanes, the pale-yellow precipitate silver iodide is produced the quickest and the white precipitate silver chloride is produced the slowest, confirming the reactivity of the halogenoalkanes.
So, the rate of reaction increases as the carbon-halogen bond strength decreases.

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5
Q

what isthe explanation for the reactivity of the halogenoalkanes

A

Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities

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6
Q

what is the weakest bond

A

C-I

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7
Q

what is the strongest bond

A

C-F due to difference in electroneg

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8
Q

what can be used to test for the halogens

A

Aqueous silver nitrate

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9
Q

how can aqueous silver nitrate be used to test for the halogens

A

Reacting halogenoalkanes with aqueous silver nitrate solution will result in the formation of a precipitate
The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes

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10
Q

what is the difference in reactivity in the halogens and the resulting tests

A

The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the silver fluoride is the slowest (slowest nucleophilic substitution reaction)
This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes

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11
Q

what are the colours of the precipitates for the halogens

A

chlorides- white, silver chloride
bromides- cream, silver bromide
iodides- pale-yellow, silver iodide

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12
Q

what is the mechanism that is used to produce halogenoalkanes from an alkane and a halogen

A

free radical substitution

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13
Q

why can the alkanes only undergo substitution reactions

A

they are saturated as they have no double bonds so undergo substitution reactions where a hydrogen is replaced by another atom

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14
Q

what are free radicals

A

atoms/a group of atoms with an unpaired electron in outer shell - the only particles that can react with alkanes as they are every reactive- easily lose this pair

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15
Q

what type of fission is used in free radical substitution

A

homolytic
-each atom takes one of the electrons that is in the pair when the bond is broken

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16
Q

what must be present for fission to occur

A

UV light to provide energy

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17
Q

what are the 3 steps of FRS

A

-initiation
-propagation
-termination

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18
Q

what reaction can be used to demonstrate FRS

A

methane and chlorine

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19
Q

describe the initiation step

A

the breaking of the covalent bond producing free radicals usually 2

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20
Q

what is the initiation equation

A

Cl2 = 2Cl*

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21
Q

describe the propagation step

A

reaction of the radical with methane to produce a molecule and free radical

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22
Q

what are the propagation equations

A

Cl* + CH4 = *CH3 + HCl
CH3 + Cl2 = CH3Cl + Cl

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23
Q

describe the termination step

A

free radical combine stopping the chain reaction and so there is no more radicals

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24
Q

what are the termination equations

A

Cl* + CH3 = CH3Cl
Cl + Cl* = Cl2
*CH3 + *CH3 = CH3CH3

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25
Q

what is the overall equation of FRS

A

main products is the halogenoalkane and hydrogen-halogen

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26
Q

what is the overall reaction equation of methane and chlorine

A

Cl2 + CH4 = CH3Cl + HCl

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27
Q

what can happen after the other FRS reactions

A

the chlorine or halogen atoms can be regenerated
-each time amount of hydrogen decreases and halogen increases

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28
Q

how can u promote further substitution

A

use excess chlorine and will keep substitution

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29
Q

how can u reduce the chances of further substitution

A

use excess methane and takes away as many chlorine radicals as possible

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30
Q

what are the most common halogenoalkanes in the atmos

A

The most common halogenoalkanes are chlorofluorocarbons (CFCs)
These compounds contain carbon atoms with chlorine and fluorine atoms attached to them
E.g. CCl3F and CCl2F2

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31
Q

what are the uses of some common CFCs

A

CFCs have many uses due being unreactive and non-flammable and non-toxic
They are used as refrigerators
Propellants for aerosols
As solvents for dry cleaning

32
Q

how can CFCs be harmful to the ozone layer

A

CFCs absorb a lot of UV radiation in the upper atmosphere
The CFCs are then broken down by the UV light causing the formation of chlorine radicals
These radicals react with ozone and break down the ozone layer

33
Q

what is ozone

A

03 and is formed in the upper atmosphere by free radical reactions

34
Q

how is ozone formed

A

UV light from sun provides energy to break 0=0 in 02 molecules = initiation
02 = O + O
The oxygen atoms form free radicals with two unpaired electrons o are very unreactive and react with other molecules to form ozone
O + O2 = O3

35
Q

how is ozone beneficial

A

absorbs UVA and UVB and protects us from skin cancer

36
Q

what happens when the ozone absorbs UV light- equation

A

dissociates to form O2 and an oxygen atom
O3 = O + O2

O is a radical

37
Q

how is ozone layer kept constant

A

rate of ozone production = rate of ozone decomposition

38
Q

how do chlorine atoms cause ozone decomposition

A

UV light breaks Cl-Cl bond in the CFCs
CCl3F = CCl2F + Cl - initiation
the chlorine radicals then react with the ozone and decompose it
Cl* + O3 = ClO* + O2
the chlorine oxide radical then can react with the ozone and decompose it
ClO + O3 = 2O2 + Cl
=propagation

radical must go on the oxygen of the chlorine oxide

39
Q

how can more ozone decompose after propagation

A

the chlorine radical produced after 2nd propagation can go on to react with more ozone

40
Q

how are the chlorine radicals known as catalysts

A

they are not destroyed and keep o decomposing ozone so much so that there is now a hole in the ozone layer

41
Q

when did the use of CFCs get banned by UN

A

1987
montreal
24 countries signed the declaration

42
Q

what is used instead of CFCs

A

Hydrofluorocarbons (HFCs) are compounds that contain a carbon attached to hydrogen and fluorine atoms only
Eg. CH2F-CF3
these are used as they do not have a C-Cl bond so cannot break down into chlorine radicals, so ozone is not decomposed

43
Q

what is nucleophilic substitution

A

a reaction in which an electron-rich nucleophile attacks a positive charge or partial positive charge to replace an atom/ group of atoms.

44
Q

how are the halogen atoms susceptible to attack by the nucleophile

A

halogen is more electroneg than the carbon so the C of halogen bon is delta +ve

45
Q

what is a nucleophile

A

a lone pair donor- eg it has a lone pair in outer shell that can be used to form a bond

46
Q

what happens to the halogen in a substitution reaction

A

the halogen is replaced by another atom

47
Q

what are the 3 nucleophiles

A

OH- CN- NH3

48
Q

what do the nucleophiles do to the carbon

A

attack the region of +ve charge and substitute into it relpacing the carbon

49
Q

what is the nucleophilic substitution reaction which OH-

A

R-X + NaOH = R-OH +NaX
halogen replaced and gives halide

50
Q

what are the conditions of the OH-

A

aqueous + warm

51
Q

what do the double headed arrow represent

A

movement of pair of electrons

52
Q

what do make sure of when drawing the reactions - arrow

A

the arrow from nucleophile must go from lone pair - touch
the arrow from C- halogen must be from middle

53
Q

what type of fission does nucleophilic substitution involve

A

heterolytic - both move e- move to halogen

54
Q

what is the reaction with KCN

A

R-X + KCN = R-CN + KX

55
Q

What are the conditions for the reaction KCN

A

ethanolic and warm

56
Q

what does ethanolic mean

A

dissolved in ethanol

57
Q

what is the nucleophilic substitution reaction with ammonia

A

The nucleophile in this reaction is the ammonia, NH3 molecule
An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with the halogenoalkane
- undergo standard reaction first - however this reaction does not produce the halogen and alkane - too many hydrogens
-so second reaction occurs where the hydrogen donates its electrons to the more positive nitrogen
-this then produces the halogen and alkane

R-X+ 2NH3 = R-NH2 +NH4X

58
Q

how are alcohols formed from haloalkanes

A

hydrolysis

59
Q

what is the process of the nucleophilic substitution of haloalkanes

A

The nucleophile in this reaction is the hydroxide, OH- ion
-the C-X bond is polar and thus the carbon is partially positive so attracts the lone pair of electrons from the OH- ion
-the OH- ion then attacks the slightly +ve carbon, kicking out the halogen and forming a new bond between the C and OH- ion
- the C-X bond breaks and both electrons are taken by the halogen

60
Q

condtions of nucleophilic sub with OH

A

warm and aqueous of either NaOH or KOH

61
Q

what is the general equation for the hydrolysis of haloalkanes

A

R-X + OH- = ROH + X-

62
Q

how does the rate of reaction for hydrolysis change down the group

A

increases
The rate of this reaction depends on the type of halogen in the halogenoalkane
The stronger the C-X bond,
the higher the bond enthalpy- harder to form Cl- ions
the slower the rate of the reaction
In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

63
Q

how to test for the rate of reaction of hydrolysis of haloalkanes

A

acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
Set up three test tubes in a 50 degrees water bath, with a mixture of ethanol and acidified silver nitrate
Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stopwatch
Time how long it takes for the precipitates to form
The precipitate will form as the reaction progresses and the halide ions are formed

64
Q

what are the results of the test for the rate of reaction of hydrolysis of haloalkanes

A

A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
The yellow precipitate will form the fastest
This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest
The white precipitate will form the slowest
This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest

65
Q

why is water not mainly used for hydrolysis reactions

A

This reaction could also be done with water as the nucleophile, but it is very slow
The hydroxide ion is a better nucleophile than water as it carries a full negative charge
In water, the oxygen atom only carries a partial charge

66
Q

what is the definition of hydrolysis

A

reaction in which water is used to break down molecules into two by breaking the bond

67
Q

what happens in an elimination reaction

A

In an elimination reaction, an organic molecule loses a small molecule
In the case of halogenoalkanes this small molecule is a potassium halide
the reaction competes with nucleophilic substitution

68
Q

what does the OH- ion acts as in an elimination reaction

A

a base

69
Q

what happens in elimination

A

The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product

70
Q

general equation of elimination

A

C-H and C-X + KOH = C=C + KX + H2O

71
Q

what does elimination mechanism look like

A

-arrow from OH- ion to a hydrogen
-arrow from this hydrogen to carbon
-arrow from middle of C-X bond adjacent to C-H bond

72
Q

what conditions are favoured for elimination

A

ethanolic and hot

73
Q

what happens to the electrons in elimination to form an alkene

A

the pair of electrons move to C-C bond as they have been broken from C-H

74
Q

how does being a primary,secondary or tertiary halogenalkane affect the rate of reaction

A

-tertiary is the fastest - as once the halogenoalkane is ionised the carbocation will be rapidly attacked by the nucleophile and the RDS will only involve the haloalkane
-subsequently followed by secondary then primary

75
Q

what do 1” and 2” and 3” halogenoalkanes favour eilimination or sub

A

1”- sub
2” - both
3”-elimination

Chemistry - AS Level (16 decks)

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