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AQA A Level Organic Chemistry > 3. Halogenoalkanes > Flashcards

3. Halogenoalkanes Flashcards

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
Overall equation for nucleophilic substitution reaction with chloroethane and sodium hydroxide?
CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl
26
Overall equation for nucleophilic substitution reaction with chloroethane and potassium cyanide?
CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl
27
Overall equation for nucleophilic substitution reaction with bromoethane and ammonia?
CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
28
Does bond polarity determine the rate of substitution?
No
29
What is the rate of substitution dependent on?
Bond enthalpy - the strength of the carbon-halogen bond
30
What is bond enthalpy?
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
31
What happens to the carbon-halogen bond as the halogen atom becomes larger?
It becomes longer, weaker and easier to break - and the corresponding halogenoalkanes react more quickly
32
What type of halogenoalkanes react the most quickly?
Iodoalkanes
33
Why are iodoalkanes the halogenoalkanes that react the most quickly?
The C-I is the longest, weakest and easiest to break - as iodine is at the bottom of the group and the largest
34
What are the reagents of nucleophilic substitution for when an alcohol is produced?
NaOH₍ₐᵩ₎ or KOH₍ₐᵩ₎
35
What are the conditions of nucleophilic substitution for when an alcohol is produced?
Warm under reflux
36
What does reflux do?
Helps the reaction to go to completion
37
General equation for nucleophilic substitution with hydroxide?
R-X + OH⁻ → R-OH + X⁻
38
What are the reagents of nucleophilic substitution for when a nitrile is produced?
KCN in aqueous ethanol
39
What are the conditions of nucleophilic substitution for when a nitrile is produced?
Boil under reflux
40
General equation for nucleophilic substitution with cyanide?
R-X + CN⁻ → R-CN + X⁻
41
What are the reagents of nucleophilic substitution for when an amine is produced?
Excess ammonia in ethanol
42
What are the conditions of nucleophilic substitution for when an amine is produced?
Heat in a sealed tube
43
General equation for nucleophilic substitution with ammonia?
R-X + 2NH₃ → R-NH₂ + NH₄X
44
What is an elimination reaction?
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
What is formed in an elimination reaction?
A double bond
46
What does a halogenoalkane have to react with in order for an elimination reaction to occur?
OH⁻
47
What do hydroxide ions act as in nucleophilic substitution and elimination respectively?
* nucleophilic substitution - act as a nucleophile | * elimination - proton acceptor (base)
48
What is a proton acceptor also known as?
A base
49
What do the curly arrows specifically show during an elimination reaction?
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
Overall, what essentially happens in an elimination reaction?
A hydrogen atom and the halogen atom are removed from the halogenoalkane, a double bond is formed and results in an alkene
51
What is the reaction mechanism in elimination of haloalkanes dependent on?
1. the type of halogenoalkane (1°, 2° or 3°) | 2. the reaction conditions
52
What type of halogenoalkanes are particularly susceptible to nucleophilic substitution?
Primary
53
What type of halogenoalkanes are particularly susceptible to elimination?
Tertiary
54
What type of halogenoalkanes are particularly susceptible to both nucleophilic substitution and elimination?
Secondary
55
What does the reaction between a halogenoalkane and a strong base usually result in?
A mixture of substitution and elimination products
56
What happens if halogenoalkanes are boiled with an ethanolic solution of KOH?
Elimination will occur (OH- acts as a base)
57
What does ethanolic mean?
Dissolved in ethanol
58
What happens if halogenoalkanes are mixed with an aqueous solution of KOH at room temperature?
They will undergo nucleophilic substitution
59
What category of alcohols do secondary halogenoalkanes produce?
Secondary
60
Why can secondary and tertiary halogenoalkanes result in more than one product from elimination?
The hydrogen is always lost from a carbon atom adjacent to the carbon atom attached to the halogen
61
What two types of reaction could occur when 1-chlorobutane reacts with potassium hydroxide?
* nucleophilic substitution | * elimination
62
Symbol for ozone?
O₃
63
Where are small amounts of ozone present?
At ground level and just above in the troposphere
64
Where are large concentrations of ozone present?
In the stratosphere (upper atmosphere)
65
What is the appearance of ozone?
Pale blue gas
66
Is ozone toxic?
Yes
67
Equations for formation of ozone?
1. O₂ → 2 O• | 2. •O + O₂ → O₃
68
Equation for break down of ozone?
O₃ → O₂ + O•
69
What does ozone absorb?
Harmful UV radiation
70
What does the natural formation and breakdown of ozone reduce?
The concentration of UV radiation
71
What can UV radiation cause?
Skin cancer
72
What is the problem with the current rates that ozone is being broken down and formed?
* usually break down and formation occur at equal rates | * but currently it is being broken down faster than it is being formed
73
Why should the concentration of ozone in the atmosphere usually stay constant?
The natural formation and break down of ozone occur at equal rates
74
What do CFCs stand for?
Chloroflurocarbons
75
What can chloroalkanes and chlorofluroalkanes be used as?
Solvents
76
What are CFCs used in?
* coolants (in fridges) | * solvents (degreasing circuit boards, dry cleaning solvents)
77
What are chloroflurocarbons?
Halogenoalkanes containing chlorine and fluorine atoms but not hydrogen atoms
78
How do chlorine radicals affect the decomposition of ozone?
They catalyse it
79
Why can even small quantities of chlorine radicals significantly destroy the ozone layer?
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
What has the decomposition of ozone due to chlorine radicals resulted in?
Formation of a hole in the ozone layer
81
How does the rate of breakdown of ozone molecules by chlorine radicals compare to by oxygen radicals?
Chlorine radicals break down ozone 1500 times faster
82
What was the result of the ozone layer depletion?
Chemists supported legislation to ban CFCs completely
83
What is the alternative to CFCs?
HFCs - hydroflurocarbons
84
Why are HFCs a good alternative to CFCs?
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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