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Halogenoalkanes Flashcards

1
Q

What is a nucleophile?

A

An electron-rich species that can donate a pair of electrons

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2
Q

What is the meaning of a nucleophile?

A

Means ‘nucleus/positive charge loving’ as they are attracted to positively charged species

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3
Q

What is the nucleophile in the formation of alcohols?

A

The hydroxide ion OH-

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4
Q

What is used to form an alcohol?

A

An aqueous solution of sodium hydroxide or potassium hydroxide with ethanol

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5
Q

Why is the reaction mixture to form an alcohol warmed?

A

It is very slow at room temperature

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6
Q

What kind of reaction is the formation of an alcohol?

A

Hydrolysis

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7
Q

What does the rate of reaction of the formation of an alcohol depend on?

A

The type of halogen in the halogenoalkane - the stronger the CX bond, the slower the rate of reaction

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8
Q

Which halogenoalkanes don’t react in the formation of an alcohol and which do?

A

Fluoroalkanes don’t react but iodoalkanes react very quickly

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9
Q

Apart from the hydroxide ion, what can the formation of an alcohol from a halogenoalkane also use as the nucleophile?

A

Water - but it is very slow

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10
Q

Why is the hydroxide ion a better nucleophile than water in the formation of an alcohol?

A

OH- carries a full negative charge and the O in H2O only carries a partial negative charge

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11
Q

What will happen when halogenoalkanes react with water?

A

H2O is a weak nucleophile but will eventually substitute for the halogen and an alcohol will be formed

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12
Q

What happens if we add silver nitrate solution in ethanol to the reaction of halogenoalkanes with water?

A

The silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate

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13
Q

What is the nucleophile in the formation of nitriles?

A

Cyanide ion CN-

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14
Q

What do we need to do in order to form a nitrile from a halogenoalkane?

A

An ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane

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15
Q

Give an example of the formation of a nitrile?

A

bromoethane + ethanol potassem cyanide = propanenitrile
CH3CH2Br + CN- = CH3CH2CN + Br-

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16
Q

What happens to the carbon chain in the formation of a nitrile?

A

It is extended by adding an extra carbon

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17
Q

How can the formation of nitriles be used by chemists?

A

To make a compound with 1 more carbon atom than the best available organic starting material

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18
Q

What is the nucleophile in the formation of primary amines?

A

Ammonia - NH3

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19
Q

What do we need to do in order to form a primary amine from a halogenoalkane?

A

An ethanolic solution of excess ammonia is heated under pressure with a primary halogenoalkane

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20
Q

Why is an excess of ammonia used in the formation of a primary amine?

A

The product is more reactive than ammonia so further substitutions could occur

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21
Q

Give an example of the formation of a primary amine?

A

CH3CH2Br + NH3 = CH3CH2NH2 + HBr

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22
Q

What do we need to do the formation of an alkene from a halogenoalkane?

A

The halogenoalkane is heated under reflux with ethanolic NaOH

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23
Q

What happens as a result of heating the halogenoalkane under reflux to form an alkene?

A

The C-X bond breaks heterolytically, forming an X- ion and leaving an alkene as an organic product

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24
Q

Give the equation for the formation of an alkene from a halogenoalkane?

A

Halogenoalkane + NaOH(ethanol) = alkene + water + NaX

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25
What kind of reaction is a nucleophilic substitution?
One in which a nucleophile attacks a carbon atom which carries a partial positive charge
26
What is replaced by the nucleophile in nucleophilic substitution?
An atom that has a partial negative charge
27
Why will halogenoalkanes undergo nucleophilic substitution?
Due to the polar C-X bond X has higher electronegativity than C
28
What is the mechanism when halogenoalkanes react with aqueous potassium hydroxide?
The OH- (nucleophile) displaces the Br- atom and donates 2 electrons to the carbon atom, forming an alcohol and bromine ion
29
What is the mechanism when halogenoalkanes react with ammonia?
- Lone pair on ammonia attacks partially positive carbon atom - C-X bond breaks -Second molecule of NH3 acts as a base and removes a proton - N-H bond breaks and lone pair is restored
30
What is an alkyl group?
CnHn2n+1
31
What are primary carbocations?
They have one alkyl group attached to the positively charged carbon (Secondary have 2 and tertiary have 3 alkyl groups)
32
How does the stability of carbocations increase as the number of alkyl groups attached increased?
It increases
33
What is being pushed from the alkyl group to the positive carbon atom?
Electrons
34
How is a dipole created in a primary (and other) carbocation?
CH3 becomes slightly positive and positive carbon atom becomes slightly negative
35
What does the alkyl group create?
A positive inductive effect
36
What does the electron pushing effect place more of on the carbocation as the number of alkyl groups increase?
More negative charge on the carbon as you go from primary - secondary - etc. Reduces overall positive charge on carbon
37
What is spread over more atoms as you go from primary - secondary - etc. carbocations?
Positive charge
38
What happens if the charge is spread over more atoms in carbocations?
The ion is more stable
39
In what 2 ways can nucleophilic substitution reactions occur?
Sn1 and Sn2
40
Which halogenoalkanes favour Sn1 reactions?
Tertiary halogenoalkanes
41
Which halogenoalkanes favour Sn2 reactions?
Primary halogenoalkanes
42
In tertiary halogenoalkanes, what is the C attached to the halogen attached to?
Bonded to 3 alkyl groups
43
What does the 1 mean in Sn1 reactions?
The rate of reaction depends on the concentration of only one reagent, the halogenoalkane
44
What is the rate of reaction determined by in Sn1 reactions?
The slowest step of the reaction
45
How many steps are in Sn1 mechanisms?
2
46
What is the 1st step of the Sn1 mechanism?
The C-X bond breaks heterolytically
47
How does the halogen leave the halogenoalkane after the 1st step of the Sn1 mechanism?
As an X- ion, leaving a tertiary carbocation
48
What is the rate determining step in the Sn1 mechanism?
The breaking of the C-X bond - it is the slowest step
49
What is the 2nd step of the Sn1 mechanism?
The tertiary carbocation is attacked by the nucleophile
50
What does the 2 mean in the Sn2 mechanism?
The rate of reaction depends on the concentration of both the halogenoalkane and the nucleophile ions
51
How many steps are in the Sn2 mechanism?
1
52
What are the stages of the Sn2 mechanisms?
- The nucleophile donates a pair of electrons to the + carbon atom to form a new bond - At the same time, the C-X bond is breaking and the halogen takes both electrons in the bond (leaves halogenoalkane as an X- ion)
53
What can be used to explain the different reactivities of halogenoalkane substitution reactions and why?
Bond energies - these reactions involve breaking the C-X bond
54
Which halogenoalkane has the weakest C-X bond?
The C-I bond requires the least energy to break so is the weakest
55
Which halogenoalkane has the strongest C-X bond?
C-F bond requires the most energy to break so is the strongest
56
Which halogenoalkanes are the least likely to undergo substitution reactions?
Fluoroalkanes
57
What can be used to measure the rate of hydrolysis of halogenoalkanes?
Acidified aqueous silver nitrate
58
What is set up initially in the experiment to measure the rates of hydrolysis?
3 test tubes in a 50 degrees celsius water bath, with a mixture of ethanol and acidified silver nitrate
59
What do we need to do to measure the rates of hydrolysis?
Add a few drops of chloroalkane, bromoalkane and an idodoalkane to each test tube and start a stop watch Time how long it takes for the precipitates to form
60
What happens when we react halogenoalkanes with aqueous silver nitrate solution?
The formation of a precipitate
61
What can the rate of the formation of the precipitate be used for?
To determine the reactivity of the halogenoalkane
62
What is the fastest nucleophilic substitution reaction in the rate of hydrolysis experiment?
Yellow silver iodide precipitate (C-I bond has lowest bond enthalpy so ions form the fastest)
63
What is the slowest nucleophilic substitution reaction in the rate of hydrolysis experiment?
White chloride precipitate (C-Cl bond has highest bond enthalpy so ions form slowest)
64
Why will a silver fluoride precipitate not form in the rate of hydrolysis experiment?
It is soluble - confirms fluoroalkanes are least reactive and iodoalkanes are most
65
What can we predict from the rate of hydrolysis experiment?
Any halogen below iodine in group 7 of the periodic table will form a silver halide precipitate even quicker
66
When comparing the rates of hydrolysis in primary, secondary and tertiary halogenoalkanes, what must remain the same?
The halogen involved and the overall molecular formula of the halogenoalkane
67
What reacts the quickest when comparing the rates of hydrolysis in primary, secondary and tertiary halogenoalkanes?
Tertiary halogenoalkanes
68
What reacts the slowest when comparing the rates of hydrolysis in primary, secondary and tertiary halogenoalkanes?
Primary halogenoalkanes

Chemistry (17 decks)

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