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Chemistry A level > haloalkanes > Flashcards

haloalkanes Flashcards

1
Q

How do you classify a haloalkane as primary secondary or tertiary?

A

Remove the halogen from the carbon on which it was bonded

Look at that carbon:
If it’s bonded to 1 other Carbon it’s primary

Bonded to 2 other Carbons it’s secondary

Bonded to 3 other carbons it’s tertiary

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2
Q

What are the boiling points of 1-chloroalkanes in comparison to alcohols and alkanes?

A

Higher than alkanes, lower than alcohols

Because alkanes are non polar molecules so only have weak London forces, so lowest boiling points

Chloroalkanes are polar molecules so have permanent dipole-dipole forces which are stronger than London forces

Alcohols can form hydrogen bonds which is the strongest type of intermolecular force, so highest boiling points

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3
Q

Why are haloalkanes generally more reactive than alkanes?

A

The C-Halogen bond (except C-Flourine has a higher bond enthalpy) is weaker than a C-H bond, so breaks more easily allowing the haloalkanes to react

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4
Q

What is meant by the term electronegativity?

A

The measure of attraction of a bonded atom for the shared pair of electrons in a covalent bond

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5
Q

What is the consequence for the carbon-halogen bond in haloalkanes?

A

Because the halogens have a higher electronegativity than Carbon the C-halogen bond is polar, with the C being delta positive and halogen being delta Negative

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6
Q

Would an electron rich or electron poor species be attracted be attracted to the carbon in a haloalkane?

A

Electron rich because the carbon has a delta negative charge

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7
Q

What is a hydrolysis reaction?

A

A chemical reaction involving water or an aqueous solution of an hydroxide, that causes the breaking of a bond in a molecule

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8
Q

Describe a hydrolysis reaction in a haloalkane?

A

It’s a substitution reaction, the halogen in the haloalkane is replaced by an OH group, and a halogen hydride is formed

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9
Q

Describe the preparation of an alcohol by hydrolysis of a haloalkane?

A

Reagent: aqueos NaOH
Conditions: heat under reflux

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10
Q

What does reflux mean?

A

The continual boilin and condensing of a mixture back to the original container to ensure that the reaction takes place without the contents of the flask boiling dry

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11
Q

How does the apparatus used for reflux allows reflux?

A

As the mixture boils, the vapour rising will be condensed back down by the water cooled condenser, without escaping through the top

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12
Q

Why do organic reactions often require heating under reflux whereas if you were making a salt, you would just heat the reaction mixture in a beaker?

A

Organic compounds have simple molecular structures, so have relatively low boiling points, compared to ionic compounds such as salts

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13
Q

What type of mechanism is a hydrolysis reaction of alkanes?

A

nucleophilic substiution

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14
Q

What is a nucleophile?

A

an electron pair donor

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15
Q

Give 3 examples of nucleophiles?

A

H2O, NH3, OH-, they all have a lone pair that can be donated to form a new covalent bond

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16
Q

What does a curly arrow show in a reaction mechanism?

A

The movement of an electron pair

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17
Q

Where does the curly arrow start and finish?

A

Starts where the electron pair originates eg lone pair or covalent bond

Ends where the electron pair goes eg. to an atom to form a new covalent bond, or to an atom on breaking a bond heterolytically, resulting in a lone pair

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18
Q

How would you draw the hydrolysis mechanism of a halogen with a hydroxide ion, eg. Bromoethane

A

Draw bromomethane, Label the carbon in which the halogen is bonded as delta positive and the halogen as delta negative

Draw an OH- ion away from bromomethane as with a lone pair of electrons

Draw a curly arrow from the lone pair on the OH- to the carbon, and a curly arrow from the covalent bond in C-Br to the Br

Products formed are a bromide ion, and the ethane but with OH instead of Br

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19
Q

In the hydrolysis of bromoethane with an OH- ion, what is the nucleophile?

A

The OH- ion

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20
Q

In the hydrolysis of bromoethane with an OH- ion, what part of the bromoethane attracts the nucleophile?

A

Delta positive Carbon

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21
Q

Order of reactivity of Haloalkanes fastest to slowest?

A

iodoalkane
bromoalkane
chloroalkane

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22
Q

Explain the order of reactivity the haloalkanes?

A

The C-I bond is weaker than the C-Br bond, which is weaker than the C-Cl bond. So C-I bond will break the easiest so it’s the most reactive

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23
Q

Describe experiment to test the reactivity of haloalkanes?

A
  1. first reaction is nucleophilic substitution of the haloalkane using water as the nucleophile
  2. further reaction then takes place between the halide ion formed and the silver ions in the silver nitrate solution, to form a solid silver halide
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24
Q

In the experiment to test the reactivity of haloalkanes why was a hot water bath needed?

A

To speed up the rate of reaction

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25
Q

In the experiment to test the reactivity of haloalkanes explain 2 roles of the aqueos silver nitrate solution?

A

As it aqueos it provides the water required in the hydrolysis reaction

And the silver ions from the AgNO3 trap the halide ion released on reaction by forming a precipitate, allowing us to see that the hydrolysis reaction has taken place

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26
Q

In the experiment to test the reactivity of haloalkanes what was the purpose of the ethanol used?

A

As a co-solvent. Both water and the haloalkane dissolve in ethanol, so the reactants are allowed to mix, increasing the rate of reaction

27
Q

In the experiment to test the reactivity of haloalkanes how was the rate of each haloalkane compared?

A

By timing how long it took to produce a visible quantity of the silver halide precipitate, The less time taken, the faster the reaction

28
Q

In the experiment to test the reactivity of haloalkanes what was done the ensure the test was fair?

A

The same volumes of AgNO3 solution and ethanol used, so the concentrations were the same

Same quantities of haloalkanes used

Same hot water bath, with same time heating

29
Q

In the experiment to test the reactivity of haloalkanes why was it necessary to carry out the hydrolysis using water rather than NaOH?

A

If OH- ions were present, the Ag+ ions in the silver nitrate will react to form a brown precipitate, removing Ag+ ions from reacting with the halides

30
Q

In the experiment to test the reactivity of haloalkanes why did the AgNO3 (aq), not form a precipitate immediately after it was added?

A

There were no halide ions present, as the halogen was still covalently bonded in the haloalkane until hydrolysis occured

31
Q

In the experiment to test the reactivity of haloalkanes describe how you would monitor reaction rates and explain why different haloalkanes react at different rates (model answer)

A

Reagents: aqueos silver Nitrate

Reaction conditions: Ethanol as co-solvent and warming in hot water bath

Observations: Time the formation of the silver halide precipitate

Equation for hydrolysis reaction: CH3CH2X +H2O = CH3CH2OH + H+ + X-

Equation for precipitation reaction:
Ag+ + X- = AgX

Trend in reactivity:
Iodoethane reacts fastest, chloroethane reacts the slowest

Factor affecting reactivity:
The strength of the C-Halogen bond

Explanation of the reactivity trend: C-I bond is the weakest, C-Cl bond is the strongest

32
Q

Use of TFE?

A

To make the polymer Teflon, used in non stick cookware

33
Q

Use of Freon-12?

A

Refrigerant, or aerosol propellant

34
Q

Use of TCE?

A

Solvent or anaesthetic

35
Q

Use of Chloroform?

A

Used to be anaesthetic, now as a precursor to other organic products

36
Q

Use of Halothane?

A

General anaesthetic

37
Q

Use of Halon 1301?

A

In fire extinguishers

38
Q

Use of PVC?

A

insulation and waterproof clothing

39
Q

Use of teflon?

A

Non stick cookware

40
Q

Use of HCHC-22?

A

Propellant and refrigerant

41
Q

Use of DDT?

A

Insectide

42
Q

Where is the ozone layer found in the atmosphere?

A

Outer edge of stratosphere, approx. 10-40km from earth’s surface

43
Q

Equation which shows the ozone layer being formed

A

O2 =(UV light) 2O

Oxygen radicals then react with Oxygen molecules to from the ozone layer

44
Q

Write equation showing the breaking down of the ozone layer

A

O3 =(UV) O2+O

45
Q

If the ozone layer is constantly forming and breaking down explain how a steady concentration of ozone can be reached?

A

If the rate of formation is equal to the rate of breakdown

46
Q

What does the ozone layer protect us from?

A

From harmful UV(-B) radiation, the ozone layer absorbs the UV radiation instead to break down the ozone

47
Q

What dangers do we face without the ozone layer?

A

Genetic damage, skin cancer

48
Q

What CFC stand for and give an example?

A

Chloroflurocarbon eg. Freon-12, (CCl2F2)

49
Q

What does HCFC stand for and give an example?

A

Hydrochloroflurocarbon eg. HCFC-22 (HCClF2)

50
Q

What were HCFC’s and CFC’s used for?

A

Refrigerants and aerosol propellants

51
Q

Why do HCFC’s and CFC’s persist in the lower atmosphere (troposphere)?

A

C-F and C-Cl bonds are strong, so the molecules do not break down easily

52
Q

What produces chlorine radicals?

A

Photodissociation of CFC’s in the stratosphere

53
Q

Write an equation for photodissociation?

A

CF2Cl2 →(UV) CF2Cl• + Cl•

54
Q

In photodissociation why does the C-Cl bond break in preference to the C-F bond?

A

C-Cl bond is weaker

55
Q

In photodissociation what type of bond breaking is this?

A

Homolytic

56
Q

What is the step in the radical mechanism in which the radicals are produced?

A

Initiation

57
Q

Write equations to show how Chlorine radicals break down the ozone?

A

Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2
O3 + O → 2O2 overall

58
Q

What role does the Cl radical take in the breaking down of the ozone?

A

A catalyst, as it is recovered unchanged at the end

59
Q

What other compound can break down the Ozone?

A

Nitrogen Monoxide

60
Q

Where does NO in the stratosphere come from?

A

Lighting strikes and high flying planes

61
Q

Write equations to show the breaking down of the Ozone from NO?

A

NO• + O3 → NO2• + O2
NO2• + O → NO• + O2
O3 + O → 2O2 overall

62
Q

What step is the breaking down of the ozone in the radical mechanism?

A

Propagation as one new radical is produced as one is used up

63
Q

What type of catalyst does NO act as?

A

Homogenous, because the catalyst and the reactants are in the same phase

Chemistry A level (16 decks)

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