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AQA A Level Organic Chemistry > 3. Halogenoalkanes > Flashcards

3. Halogenoalkanes Flashcards

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1
Q

What is a nucleophile?

A

A species that has a lone pair of electrons that it can donate

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2
Q

What are nucleophiles attracted to?

A

Electron deficient atoms

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3
Q

What are the common nucleophiles in organic chemistry?

A

:⁻OH, :NH₃ and :⁻CN

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4
Q

What is nucleophilic substitution?

A

A chemical reaction where a nucleophile reacts with a polar molecule, removing the functional group and taking its place

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5
Q

In halogenoalkanes, what is the atom that is replaced by the nucleophile during nucleophilic substitution?

A

The halogen

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6
Q

Why is the halogen atom replaced during nucleophilic substitution?

A

Halogens are more electronegative than carbon, resulting in a polar bond

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7
Q

What causes halogens to be susceptible to nucleophilic attack?

A

Halogens are more electronegative than carbon, and so can form polar bonds

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8
Q

What is :Nu⁻ used to represent?

A

Any nucleophile

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9
Q

What is a primary halogenoalkane?

A

One in which the halogen atom is bonded to an end carbon, and so has two hydrogens also bonded

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10
Q

What is a secondary halogenoalkane?

A

One in which the halogen atom is bonded to a carbon which is bonded to one hydrogen atom and two alky groups

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11
Q

What is a tertiary halogenoalkane?

A

One in which the halogen atom is bonded to a carbon which is bonded to no hydrogen atoms and three alky groups

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12
Q

What does a reaction mechanism illustrate?

A

The route through a reaction, which may involve intermediates

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13
Q

What are curly arrows used to show?

A

The direction of movement of an electron pair in organic reactions

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14
Q

What do the curly arrows specifically show during nucleophilic substitution?

A
  1. The lone pair of electrons of the nucleophile is attracted towards the slightly positive carbon in the molecule
  2. The electrons in the carbon-halogen bond moving towards the halogen atom, leading to the formation of a halide atom
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15
Q

What do halogenoalkanes react with OH⁻ to form?

A
  • alcohol (nucleophilic substitution)

* alkene (elimination)

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16
Q

Where do the OH⁻ free radicals in nucleophilic substitution come from?

A

They will be in solution (e.g. NaOH) - you don’t just have loads of OH⁻ ions by themselves

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17
Q

What do halogenoalkanes react with CN⁻ to form?

A

A nitrile

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18
Q

When a nitrile is formed during nucleophilic substitution, what happens to the length of the carbon skeleton?

A

The carbon skeleton increases by one (e.g. eth- to prop- skeleton)

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19
Q

What bond is there in a CN⁻ ion?

A

Triple (⁻C≡N)

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20
Q

Is NH₃ still a nucleophile, even if it’s not an ion?

A

Yes; a nucleophile just needs to have a lone pair

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21
Q

What do halogenoalkanes react with NH₃ to form?

A

An amine

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22
Q

What happens, during nucleophilic substitution with ammonia, after the halogen has been replaced by an NH₃ molecule?

A

One of the hydrogen is ‘kicked off’ and bonds to another :NH₃ molecule, to form NH₄⁺

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23
Q

What can the amine produced in nucleophilic substitution with ammonia act as?

A

A nucleophile - meaning that this reaction could continue to produce a mixture of products

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24
Q

What are the products that could be produced if the amine produced in nucleophilic substitution with ammonia acts as a nucleophile?

A

2° and 3° amines, and 4° ammonium salts

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25
Q

Overall equation for nucleophilic substitution reaction with chloroethane and sodium hydroxide?

A

CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl

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26
Q

Overall equation for nucleophilic substitution reaction with chloroethane and potassium cyanide?

A

CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl

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27
Q

Overall equation for nucleophilic substitution reaction with bromoethane and ammonia?

A

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

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28
Q

Does bond polarity determine the rate of substitution?

A

No

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29
Q

What is the rate of substitution dependent on?

A

Bond enthalpy - the strength of the carbon-halogen bond

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30
Q

What is bond enthalpy?

A

The average enthalpy change that takes place when breaking 1 mole of a given bond in the molecule of a gaseous species under standard conditions

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31
Q

What happens to the carbon-halogen bond as the halogen atom becomes larger?

A

It becomes longer, weaker and easier to break - and the corresponding halogenoalkanes react more quickly

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32
Q

What type of halogenoalkanes react the most quickly?

A

Iodoalkanes

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33
Q

Why are iodoalkanes the halogenoalkanes that react the most quickly?

A

The C-I is the longest, weakest and easiest to break - as iodine is at the bottom of the group and the largest

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34
Q

What are the reagents of nucleophilic substitution for when an alcohol is produced?

A

NaOH₍ₐᵩ₎ or KOH₍ₐᵩ₎

35
Q

What are the conditions of nucleophilic substitution for when an alcohol is produced?

A

Warm under reflux

36
Q

What does reflux do?

A

Helps the reaction to go to completion

37
Q

General equation for nucleophilic substitution with hydroxide?

A

R-X + OH⁻ → R-OH + X⁻

38
Q

What are the reagents of nucleophilic substitution for when a nitrile is produced?

A

KCN in aqueous ethanol

39
Q

What are the conditions of nucleophilic substitution for when a nitrile is produced?

A

Boil under reflux

40
Q

General equation for nucleophilic substitution with cyanide?

A

R-X + CN⁻ → R-CN + X⁻

41
Q

What are the reagents of nucleophilic substitution for when an amine is produced?

A

Excess ammonia in ethanol

42
Q

What are the conditions of nucleophilic substitution for when an amine is produced?

A

Heat in a sealed tube

43
Q

General equation for nucleophilic substitution with ammonia?

A

R-X + 2NH₃ → R-NH₂ + NH₄X

44
Q

What is an elimination reaction?

A

One in which the organic molecule loses two species from adjacent carbon atoms without replacement, resulting in the formation of a double bond between the two carbon atoms

45
Q

What is formed in an elimination reaction?

A

A double bond

46
Q

What does a halogenoalkane have to react with in order for an elimination reaction to occur?

A

OH⁻

47
Q

What do hydroxide ions act as in nucleophilic substitution and elimination respectively?

A
  • nucleophilic substitution - act as a nucleophile

* elimination - proton acceptor (base)

48
Q

What is a proton acceptor also known as?

A

A base

49
Q

What do the curly arrows specifically show during an elimination reaction?

A
  1. The OH⁻ ion using its lone pair of electrons to form a bond with one of the hydrogen atoms on a C atom adjacent to the C involved in the C-X bond, forming a water molecule
  2. The electron pair from the C-H bond now becomes part of the carbon-carbon double bond, forming an alkene
  3. The halogen takes the pair of electrons in the C-X bond, forming a halide ion
50
Q

Overall, what essentially happens in an elimination reaction?

A

A hydrogen atom and the halogen atom are removed from the halogenoalkane, a double bond is formed and results in an alkene

51
Q

What is the reaction mechanism in elimination of haloalkanes dependent on?

A
  1. the type of halogenoalkane (1°, 2° or 3°)

2. the reaction conditions

52
Q

What type of halogenoalkanes are particularly susceptible to nucleophilic substitution?

A

Primary

53
Q

What type of halogenoalkanes are particularly susceptible to elimination?

A

Tertiary

54
Q

What type of halogenoalkanes are particularly susceptible to both nucleophilic substitution and elimination?

A

Secondary

55
Q

What does the reaction between a halogenoalkane and a strong base usually result in?

A

A mixture of substitution and elimination products

56
Q

What happens if halogenoalkanes are boiled with an ethanolic solution of KOH?

A

Elimination will occur (OH- acts as a base)

57
Q

What does ethanolic mean?

A

Dissolved in ethanol

58
Q

What happens if halogenoalkanes are mixed with an aqueous solution of KOH at room temperature?

A

They will undergo nucleophilic substitution

59
Q

What category of alcohols do secondary halogenoalkanes produce?

A

Secondary

60
Q

Why can secondary and tertiary halogenoalkanes result in more than one product from elimination?

A

The hydrogen is always lost from a carbon atom adjacent to the carbon atom attached to the halogen

61
Q

What two types of reaction could occur when 1-chlorobutane reacts with potassium hydroxide?

A
  • nucleophilic substitution

* elimination

62
Q

Symbol for ozone?

A

O₃

63
Q

Where are small amounts of ozone present?

A

At ground level and just above in the troposphere

64
Q

Where are large concentrations of ozone present?

A

In the stratosphere (upper atmosphere)

65
Q

What is the appearance of ozone?

A

Pale blue gas

66
Q

Is ozone toxic?

A

Yes

67
Q

Equations for formation of ozone?

A
  1. O₂ → 2 O•

2. •O + O₂ → O₃

68
Q

Equation for break down of ozone?

A

O₃ → O₂ + O•

69
Q

What does ozone absorb?

A

Harmful UV radiation

70
Q

What does the natural formation and breakdown of ozone reduce?

A

The concentration of UV radiation

71
Q

What can UV radiation cause?

A

Skin cancer

72
Q

What is the problem with the current rates that ozone is being broken down and formed?

A
  • usually break down and formation occur at equal rates

* but currently it is being broken down faster than it is being formed

73
Q

Why should the concentration of ozone in the atmosphere usually stay constant?

A

The natural formation and break down of ozone occur at equal rates

74
Q

What do CFCs stand for?

A

Chloroflurocarbons

75
Q

What can chloroalkanes and chlorofluroalkanes be used as?

A

Solvents

76
Q

What are CFCs used in?

A
  • coolants (in fridges)

* solvents (degreasing circuit boards, dry cleaning solvents)

77
Q

What are chloroflurocarbons?

A

Halogenoalkanes containing chlorine and fluorine atoms but not hydrogen atoms

78
Q

How do chlorine radicals affect the decomposition of ozone?

A

They catalyse it

79
Q

Why can even small quantities of chlorine radicals significantly destroy the ozone layer?

A

Chlorine radicals catalyse the break down of ozone - and the process repeats itself

e.g. O₃ + •Cl → O₂ + •ClO then O₃ + •ClO → 2O₂ + •Cl

80
Q

What has the decomposition of ozone due to chlorine radicals resulted in?

A

Formation of a hole in the ozone layer

81
Q

How does the rate of breakdown of ozone molecules by chlorine radicals compare to by oxygen radicals?

A

Chlorine radicals break down ozone 1500 times faster

82
Q

What was the result of the ozone layer depletion?

A

Chemists supported legislation to ban CFCs completely

83
Q

What is the alternative to CFCs?

A

HFCs - hydroflurocarbons

84
Q

Why are HFCs a good alternative to CFCs?

A

C-F bond stronger. So need high UV to break bonds to form free radicals

AQA A Level Organic Chemistry (16 decks)

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