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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
Q

what are the conditions for a nucleophilic substitution reaction with CN- ions?

A

water and ethanol solvent

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26
Q

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

A

nitrile

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27
Q

what is the reagent in a nucleophilic substitution reaction with NH3?

A

excess NH3

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28
Q

why is excess NH3 needed in a nucleophilic substitution reactions?

A

it is a 2 step process

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29
Q

what are the conditions for a nucleophilic substitution reaction with NH3?

A

high heat and pressure, ethanol solvent

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30
Q

what is the product of a nucleophilic substitution reaction with NH3?

A

amine

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31
Q

what is the reagent required for an elimination reaction of a halogenoalkane?

A

NaOH or KOH

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32
Q

what are the conditions required for an elimination reaction of a halogenoalkane?

A

ethanolic solvent (or any alcoholic solvent e.g. butanolic)

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33
Q

what is the product of an elimination reaction of a halogenoalkane?

A

alkene

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34
Q

what is the function of the OH- ion in an elimination reaction of a halogenoalkane?

A

The OH- ion acts as a base (proton acceptor)

35
Q

draw the mechanism for the elimination reaction of 2-bromopropane

36
Q

draw the mechanism for the each of the 3 possible products of the elimination reaction of 2-bromobutane

37
Q

draw the mechanism for the reaction between bromoethane and ammonia

38
Q

draw the mechanism for the reaction between methane and potassium cyanide

39
Q

draw the mechanism for the substitution reaction between bromoethane and KOH

40
Q

what is a base?

A

proton acceptor

41
Q

what is a carbocation?

A

a positive molecular ion (cation) where the charged atom is carbon

42
Q

what is the positive inductive effect?

A

the ability to lose electron density through a covalent bond

43
Q

what is the reaction and conditions needed to turn an alkane into a haloalkane?

A

free radical substitution
halogen
UV light

44
Q

what is the reaction and conditions needed to turn a haloalkane into an alkene?

A

elimination
ethanolic NaOH

45
Q

what is the reaction and conditions needed to turn a haloalkane into an alcohol?

A

nucleophilic subsitution
aqueous NaOH

46
Q

what is the reaction and conditions needed to turn a haloalkane into an amine?

A

nucleophilic substitution
excess NH3

47
Q

what is the reaction and conditions needed to turn a haloalkane into a nitrile?

A

nucleophilic substitution
KCN
water and ethanol solvent

48
Q

what is the reaction and conditions needed to turn an alcohol into an alkene?

A

elimination
hot, concentrated H2SO4

49
Q

what does an alkane produce when it undergoes free radical substitution?

A

haloalkane

50
Q

what does a haloalkane produce when it undergoes nucleophilic substitution?

A

nitrile, amine, or alcohol

51
Q

what does a haloalkance produce when it undergoes elimination?

52
Q

what does an alcohol produce when it undergoes elimination?

53
Q

what is the reagent required for the electrophilic addition of hydrogen halides?

A

HCl or HBr

54
Q

what are the conditions required for the electrophilic addition of hydrogen halides?

A

there are none

55
Q

what is the product of the electrophilic addition of hydrogen halides?

A

haloalkane

56
Q

draw the mechanism for the electrophilic addition reaction between ethene and bromine

57
Q

what is the reactive intermediate in an electrophilic addition reaction of hydrogen halides?

A

carbocation

58
Q

what are the qualities of reactive intermediates?

A

Reactive intermediates are unstable and quickly either react further or decompose into their reactants

59
Q

what does the stability of a carbocation depend on?

A

Carbocations have different stabilities depending on the number of alkyl groups bonded to the positive C atom

60
Q

why does the number of alkyl groups bonded to the C+ atom affect a carbocation’s stability/

A

Alkyl groups push electron density onto whatever they are bonded to; positive inductive effect

61
Q

why are tertiary carbocations more stable than primary?

A

because there is a greater positive inductive effect from 3 alkyl groups in tertiary carbocation, than 1 alkyl group in primary carbocation

62
Q

will the major or minor product of electrophilic addition form in greater quantity?

63
Q

in the electrophilic addition of hydrogen bromide and prop-1-ene, what will be the major and minor product and why?

A
  • 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
Q

what is the reagent for the electrophilic addition of a halogen and an alkene?

65
Q

why can’t F2 or I2 be used for the electrophilic addition of alkenes?

A

Fluorine adds uncontrollably with alkenes,and the addition of iodine is unfavorable, so these are not useful preparative methods.

66
Q

what are the conditions required for the electrophilic addition of halogens and alkenes?

67
Q

what is the product of the electrophilic addition of halogens and alkenes?

A

haloalkane

68
Q

why does bromine react with alkenes, even though bromine is a non-polar molecule?

A

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
Q

what is hydrolysis?

A

splitting molecules using water.

70
Q

does hydrolysis occur at the same rate for all haloalkanes?

71
Q

outline a Method to determine speed of hydrolysis of haloalkanes

A
  • 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
Q

describe the trend in rates of hydrolysis up the halogen group

A
  • 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
Q

what is ozone?

A

a pale blue poisonous gas

74
Q

where does ozone form?

A

the stratosphere

75
Q

why is ozone essential to the survival of life on earth?

A

Acts as a barrier to harmful UV-B radiation; can cause skin cancer, cataracts, and plant leaf damage

76
Q

how is ozone formed?

A
  • 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
Q

what can ozone be broken down by?

A

a reaction with chlorine radicals

78
Q

what are Chlorofluorocarbons ?

A

Haloalkanes that contain both chlorine and fluorine

79
Q

what were CFC’s used in?

A

Historically used in coolants in refrigerators

80
Q

why are CFC’s damaging to the atmosphere?

A

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
Q

why are ‘ozone-friendly’ alternatives to CFC’s biodegradable?

A

Ozone-friendly’ alternatives break down rapidly in the lower atmospher as they are more reactive in comparison to CFC’s

82
Q

Explain why 1,1,1-trifluoroethane does not lead to the depletion of the ozone in the upper atmosphere

A
  • 1,1,1-trifluoroethane does not contain Cl
  • C-F bonds are strong and do not break
83
Q

what is the mechanism for the breakdown of ozone?

A

free radical substitution

84
Q

outline the mechanism for the breakdown of ozone

A
  • Initiation: production of a chlorine radical
    • CCl3F ⇒ Cl* + *CCl2F
  • Propagation:
    • Cl* + O3 ⇒ ClO* + O2
    • ClO* + O3 ⇒ 2O2 + Cl*

1.Chem A-level (23 decks)

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