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halogenoalkanes Flashcards

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 for the FRS of methane and chlorine

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 for the FRS of methane and chlorine

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 for the FRS of methane and chlorine

A

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

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25
what is the overall equation of FRS
main products is the halogenoalkane and hydrogen-halogen
26
what is the overall reaction equation of methane and chlorine
Cl2 + CH4 = CH3Cl + HCl
27
what can happen after the other FRS reactions
the chlorine or halogen atoms can be regenerated -each time amount of hydrogen decreases and halogen increases
28
how can u promote further substitution
use excess chlorine and will keep substitution
29
how can u reduce the chances of further substitution
use excess methane and takes away as many chlorine radicals as possible
30
what are the most common halogenoalkanes in the atmos
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
31
what are the uses of some common CFCs
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
how can CFCs be harmful to the ozone layer
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
what is ozone
03 and is formed in the upper atmosphere by free radical reactions
34
how is ozone formed
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
how is ozone beneficial
absorbs UVA and UVB and protects us from skin cancer
36
what happens when the ozone absorbs UV light- equation
dissociates to form O2 and an oxygen atom O3 = *O* + O2 O is a radical
37
how is ozone layer kept constant
rate of ozone production = rate of ozone decomposition
38
steps of the FRS of chlorine to show the decomposition of ozone
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 Cl*O + O3 = 2O2 + Cl* =propagation radical must go on the oxygen of the chlorine oxide
39
how can more ozone decompose after propagation
the chlorine radical produced after 2nd propagation can go on to react with more ozone
40
how are the chlorine radicals known as catalysts
they are not destroyed and keep on decomposing ozone so much so that there is now a hole in the ozone layer
41
when did the use of CFCs get banned by UN
1987 montreal 24 countries signed the declaration
42
what is used instead of CFCs
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
what is nucleophilic substitution
a reaction in which an electron-rich nucleophile attacks a positive charge or partial positive charge to replace an atom/ group of atoms.
44
how are the carbon atoms susceptible to attack by the nucleophile
halogen is more electroneg than the carbon so the C of halogen bon is delta +ve
45
what is a nucleophile
a lone pair donor- eg it has a lone pair in outer shell that can be used to form a bond
46
what happens to the halogen in a substitution reaction
the halogen is replaced by another atom
47
what are the 3 nucleophiles
OH- CN- NH3
48
what do the nucleophiles do to the carbon
attack the region of +ve charge and substitute into it relpacing the carbon
49
what is the nucleophilic substitution reaction which OH-
R-X + NaOH = R-OH +NaX halogen replaced and gives halide
50
nucleophilic sub - what are the conditions of the OH-
aqueous + warm
51
what do the double headed arrow represent
movement of pair of electrons
52
what do make sure of when drawing the reactions - arrow
the arrow from nucleophile must go from lone pair - touch the arrow from C- halogen must be from middle
53
what type of fission does nucleophilic substitution involve
heterolytic - both move e- move to halogen
54
what is the reaction with KCN
R-X + KCN = R-CN + KX
55
What are the conditions for the reaction KCN
ethanolic and warm
56
what does ethanolic mean
dissolved in ethanol
57
what is the nucleophilic substitution reaction with ammonia
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
how are alcohols formed from haloalkanes
hydrolysis
59
what is the process of the nucleophilic substitution of haloalkanes
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
condtions of nucleophilic sub with OH
warm and aqueous of either NaOH or KOH
61
what is the general equation for the nucleophilic sub with OH of haloalkanes
R-X + OH- = ROH + X-
62
how does the rate of reaction for hydrolysis change down the group
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
how to test for the rate of reaction of hydrolysis of haloalkanes
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
what are the results of the test for the rate of reaction of hydrolysis of haloalkanes
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
why is water not mainly used for hydrolysis reactions
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
what is the definition of hydrolysis
reaction in which water is used to break down molecules into two by breaking the bond
67
what happens in an elimination reaction
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
what does the OH- ion acts as in an elimination reaction
a base
69
what happens in elimination
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
general equation of elimination
R-X + KOH = C=C + KX + H2O
71
what does elimination mechanism look like
-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
what conditions are favoured for elimination
ethanolic and hot
73
what happens to the electrons in elimination to form an alkene
the pair of electrons move to C-C bond as they have been broken from C-H
74
how does being a primary,secondary or tertiary halogenalkane affect the rate of reaction- electrophilic addition
-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
what do 1" and 2" and 3" halogenoalkanes favour eilimination or sub
1"- sub 2" - both 3"-elimination
76
what is the main organic product that is formed when using a large XS of chlorine from the reaction of chlorine and methane in free radical sub
CCl4
77
what are some properties of CFCs that make them useful as refrigerants and propellants in aerosols
- non-toxic - non-flammable - unreactive
78
what is important to remember abt the first propagation equation
-that it produces the start organic molecule as a radical -so don't need to add halogen on to it
79
what is the second reaction of ammonia in nucleophilic sub called
-deprotonation/removal of H+ -prodcues a NH4+ ions -then subsequently an ammonium halogen salt

Chemistry - AS Level (16 decks)

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