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1.Chem A-level > 3.3.3 Halogenoalkanes Knowledge > Flashcards

3.3.3 Halogenoalkanes Knowledge Flashcards

1
Q

how are haloalkanes formed?

A

Formed by reacting an alkane with a halogen; occurs via the mechanism of free-radical substitution OR electrophilic addition

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2
Q

what is a disadvantage of using free-radical substitution to form haloalkanes?

A

a wide range of products can form as a result of further substitution; to reduce the amount of further substitution use excess alkane

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3
Q

what is an essential condition for free-radical substitution to form haloalkanes?

A

UV light

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4
Q

what is a free radical?

A

a species with an unpaired electron

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5
Q

what are the 3 stages of free-radical substitution?

A
  • Stage 1: Initiation
  • Stage 2: Propagation
  • Stage 3: Termination
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6
Q

what is the purpose of Initiation in free-radical substitution?

A

Produces first radicals - homolytic fission - bond broken by UV light

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7
Q

what is the purpose of Propagation 1 in free-radical substitution?

A

Produces more radicals which continue the chain; halogen radical attacks other molecule and takes a hydrogen. Forms an alkyl radical and hydrogen halide

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8
Q

what is the purpose of Propagation 2 in free-radical substitution?

A

Alkyl radical produced in step 1 of propagation takes a halogen atom from a halogen molecule (not all of the halogen molecules were split in Initiation). This forms a haloalkane and reproduces the halogen free-radical that started propagation step 1 (acts as a catalyst)

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9
Q

what is the purpose of Termination in free-radical substitution?

A

two free radicals combine; there are always three possibilities

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10
Q

write the initiation equation for the free radical substitution of this overall equation: CH4 + Cl2 ⇒ CH3Cl + HCl

A

Cl2 ⇒ 2Cl*

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11
Q

write the propagation 1 equation for the free radical substitution of this overall equation: CH4 + Cl2 ⇒ CH3Cl + HCl

A

CH4 + Cl* ⇒ HCl + *CH3

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12
Q

write the propagation 2 equation for the free radical substitution of this overall equation: CH4 + Cl2 ⇒ CH3Cl + HCl

A

CH3 + Cl2 ⇒ CH3Cl + Cl

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13
Q

write the 3 termination equations for the free radical substitution of this overall equation: CH4 + Cl2 ⇒ CH3Cl + HCl

A
  • e.g. CH3 + Cl ⇒ CH3Cl
  • e.g. Cl* + Cl* ⇒ Cl2
  • e.g. *CH3 + *CH3 ⇒ C2H6
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14
Q

Outline the mechanism for the reaction between F2 and butane to produce 2-fluorobutane

A
  • Initiation: F2 ⇒ 2F*
  • Propagation:
    • F* + CH3CH2CH2CH3 ⇒ CH3*CHCH2CH3 + HF
    • CH3CHCH2CH3 + F2 ⇒ F + CH3CHFCH2CH3
  • Termination
    • 2F* ⇒ F2
    • CH3CHCH2CH3 + F ⇒ CH3CHFCH2CH3
    • 2CH3*CHCH2CH3 ⇒ C8H18
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15
Q

Outline a mechanism for the reaction of chloromethane with chlorine to form dichloromethane

A
  • Initiation: Cl2 ⇒ 2Cl*
  • Propagation:
    • Cl* + CH3Cl ⇒ *CH2Cl + HCl
    • CH2Cl + Cl2 ⇒ CH2Cl2 + Cl
  • Termination:
    • 2Cl* ⇒ Cl2
    • 2*CH2Cl ⇒ C2H4Cl2
    • CH2Cl + Cl ⇒ CH2Cl2
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16
Q

what is a nucleophile?

A

electron pair donor

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17
Q

what are curly arrows used to show?

A

the movement of electrons in mechanisms

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18
Q

what are the rules for drawing curly arrows?

A
  • Must start from: a covalent bond or a lone pair of electrons
  • Must go to: a positive/partially positive atom or somewhere a bond is being formed
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19
Q

why do nucleophilic substitution reactions occur?

A
  • Haloalkanes are much more reactive than alkanes because they contain polar bonds
  • The C-halogen bond is polar because the halogen has a greater electronegativity than the carbon
  • This means that nucleophiles will be attracted to the C atom bonded to the halogen
  • As the C bonded to the halogen already has 4 bonds, the halogen atom leaves as the nucleophile forms a bond to the C atom
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20
Q

what is the reagent in a nucleophilic substitution reaction with OH- ions?

A

NaOH or KOH

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21
Q

what is the condition required for a nucleophilic substitution reaction with OH- ions?

A

aqueous solvent and RTP

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22
Q

what is the product of a nucleophilic substitution reaction with OH- ions?

A

Alcohol

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23
Q

what is the process in a nucleophilic substitution reaction with OH- ions?

A

heterolytic fission

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24
Q

what is the reagent in a nucleophilic substitution reaction with CN- ions?

A

KCN-

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25
what are the conditions for a nucleophilic substitution reaction with CN- ions?
water and ethanol solvent
26
what is the product of a nucleophilic substitution reaction with CN- ions?
nitrile
27
what is the reagent in a nucleophilic substitution reaction with NH3?
excess NH3
28
why is excess NH3 needed in a nucleophilic substitution reactions?
it is a 2 step process
29
what are the conditions for a nucleophilic substitution reaction with NH3?
high heat and pressure, ethanol solvent
30
what is the product of a nucleophilic substitution reaction with NH3?
amine
31
what is the reagent required for an elimination reaction of a halogenoalkane?
NaOH or KOH
32
what are the conditions required for an elimination reaction of a halogenoalkane?
ethanolic solvent (or any alcoholic solvent e.g. butanolic)
33
what is the product of an elimination reaction of a halogenoalkane?
alkene
34
what is the function of the OH- ion in an elimination reaction of a halogenoalkane?
The OH- ion acts as a base (proton acceptor)
35
draw the mechanism for the elimination reaction of 2-bromopropane
36
draw the mechanism for the each of the 3 possible products of the elimination reaction of 2-bromobutane
37
draw the mechanism for the reaction between bromoethane and ammonia
38
draw the mechanism for the reaction between methane and potassium cyanide
39
draw the mechanism for the substitution reaction between bromoethane and KOH
40
what is a base?
proton acceptor
41
what is a carbocation?
a positive molecular ion (cation) where the charged atom is carbon
42
what is the positive inductive effect?
the ability to lose electron density through a covalent bond
43
what is the reaction and conditions needed to turn an alkane into a haloalkane?
free radical substitution halogen UV light
44
what is the reaction and conditions needed to turn a haloalkane into an alkene?
elimination ethanolic NaOH
45
what is the reaction and conditions needed to turn a haloalkane into an alcohol?
nucleophilic subsitution aqueous NaOH
46
what is the reaction and conditions needed to turn a haloalkane into an amine?
nucleophilic substitution excess NH3
47
what is the reaction and conditions needed to turn a haloalkane into a nitrile?
nucleophilic substitution KCN water and ethanol solvent
48
what is the reaction and conditions needed to turn an alcohol into an alkene?
elimination hot, concentrated H2SO4
49
what does an alkane produce when it undergoes free radical substitution?
haloalkane
50
what does a haloalkane produce when it undergoes nucleophilic substitution?
nitrile, amine, or alcohol
51
what does a haloalkance produce when it undergoes elimination?
alkene
52
what does an alcohol produce when it undergoes elimination?
alkene
53
what is the reagent required for the electrophilic addition of hydrogen halides?
HCl or HBr
54
what are the conditions required for the electrophilic addition of hydrogen halides?
there are none
55
what is the product of the electrophilic addition of hydrogen halides?
haloalkane
56
draw the mechanism for the electrophilic addition reaction between ethene and bromine
57
what is the reactive intermediate in an electrophilic addition reaction of hydrogen halides?
carbocation
58
what are the qualities of reactive intermediates?
Reactive intermediates are unstable and quickly either react further or decompose into their reactants
59
what does the stability of a carbocation depend on?
Carbocations have different stabilities depending on the number of alkyl groups bonded to the positive C atom
60
why does the number of alkyl groups bonded to the C+ atom affect a carbocation's stability/
Alkyl groups push electron density onto whatever they are bonded to; positive inductive effect
61
why are tertiary carbocations more stable than primary?
because there is a greater positive inductive effect from 3 alkyl groups in tertiary carbocation, than 1 alkyl group in primary carbocation
62
will the major or minor product of electrophilic addition form in greater quantity?
major
63
in the electrophilic addition of hydrogen bromide and prop-1-ene, what will be the major and minor product and why?
- 2-bromopropane is formed from a secondary carbocation, where 1-bromopropane is formed from a primary carbocation. - The secondary carbocation is more stable than the primary due to the positive inductive effect. Therefore, 2-bromopropane is the major product.
64
what is the reagent for the electrophilic addition of a halogen and an alkene?
Cl2, Br2
65
why can't F2 or I2 be used for the electrophilic addition of alkenes?
Fluorine adds uncontrollably with alkenes,and the addition of iodine is unfavorable, so these are not useful preparative methods.
66
what are the conditions required for the electrophilic addition of halogens and alkenes?
none
67
what is the product of the electrophilic addition of halogens and alkenes?
haloalkane
68
why does bromine react with alkenes, even though bromine is a non-polar molecule?
The C=C bond is electron rich and induces a dipole in Br2; the delta positive Br is attracted to the C=C double bond.
69
what is hydrolysis?
splitting molecules using water.
70
does hydrolysis occur at the same rate for all haloalkanes?
no
71
outline a Method to determine speed of hydrolysis of haloalkanes
- Place a measured sample of each haloalkane (equal molar amounts) into separate test tubes and place in a water bath at 50 degrees - Into another test tube place a solution of ethanol, water, and aqueous silver nitrate and place in the same water bath - Once at equal temperature add equal volumes of the ethanol mixture and haloalkane solutions together - Time how long it takes for each ppt to form - The rate of the reaction can be determined by calculating 1/time taken for the ppt to occur
72
describe the trend in rates of hydrolysis up the halogen group
- As the bond enthalpy decreases it is easier to break the C-Halogen bond. The stronger the bond the harder it is to break and the slower the rate of reaction will be as more energy is required to break it. - The C-I bond is weakest, so has fastest reaction - C-Cl bond is strongest, so has slowest reaction
73
what is ozone?
a pale blue poisonous gas
74
where does ozone form?
the stratosphere
75
why is ozone essential to the survival of life on earth?
Acts as a barrier to harmful UV-B radiation; can cause skin cancer, cataracts, and plant leaf damage
76
how is ozone formed?
- ordinary, stable O2 is split into two by UV radiation into two oxygen atoms - one of the oxygen atoms combines with an oxygen molecule to form ozone - O2 ➡ 2O* - the quantity of ozone is maintained in a balance with oxygen, in equilibrium - O2 + O ⇌ O3
77
what can ozone be broken down by?
a reaction with chlorine radicals
78
what are Chlorofluorocarbons ?
Haloalkanes that contain both chlorine and fluorine
79
what were CFC's used in?
Historically used in coolants in refrigerators
80
why are CFC's damaging to the atmosphere?
They are damaging to the ozone layer as the breakdown of ozone can be caused be a single chlorine free-radical. This is becuase Cl* is regenerated in the final propagation step and causes a chain reaction in the decomposition of ozone
81
why are 'ozone-friendly' alternatives to CFC's biodegradable?
Ozone-friendly' alternatives break down rapidly in the lower atmospher as they are more reactive in comparison to CFC's
82
Explain why 1,1,1-trifluoroethane does not lead to the depletion of the ozone in the upper atmosphere
- 1,1,1-trifluoroethane does not contain Cl - C-F bonds are strong and do not break
83
what is the mechanism for the breakdown of ozone?
free radical substitution
84
outline the mechanism for the breakdown of ozone
- Initiation: production of a chlorine radical - CCl3F ⇒ Cl* + *CCl2F - Propagation: - Cl* + O3 ⇒ ClO* + O2 - ClO* + O3 ⇒ 2O2 + Cl*
85
why is the subsitution reaction between water and haloalkanes slow?
water is a poor nucleophile
86
how can you measure the rate of hydrolysis reactions of haloalkanes?
- add aqueous silver nitrate - the halide leaving group will combing with a silver ion to form a silver halide precipitate - the ppt only forms when the halide ion has left the haloalkane so the rate of formation of the ppt can be used to compare the reacticity of different haloalkanes
87
why do tertiary haloalkanes undergo a different mechanism of subsitutuiopn with hydroxide ions?
the tertiary acrbocation is stabilised by the alkyl groups around it which prevent the hydroxide ion from attacking the haloalkane in the same way as secondary and primary
88
draw the mechanism for the nucleophilic substitution of hydroxide ions with a tertiary haloalkane
89
what can chloroalkanes and chlorofluoralkanes be used as?
solvents
90
what can haloalkanes be used as?
refrigerants, pesticides, aerosol propellants

1.Chem A-level (24 decks)

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