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A-level Chemistry Yr 12 Mine > C15 - Haloalkanes > Flashcards

C15 - Haloalkanes Flashcards

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1
Q

What is Hydrolysis?

A

A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. This results in a molecule being split into two products.

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2
Q

What is the result of hydrolysis?

A

The molecule being split into two products.

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3
Q

What are CFC’s?

A

Chlorofluorocarbons molecules which contain only chlorine, fluorine and carbon.

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4
Q

What are some benefits of CFC’s?

A

They are very stable, volatile, non-flammable and non-toxic.

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5
Q

What does volatile mean?

A

A substance easily evaporated at normal temperatures.

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6
Q

What is the Ozone?

A

Ozone (O3) in the upper atmosphere which absorbs ultraviolet radiation.

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7
Q

What are the dangers of ultraviolet radiation?

A

Causes sun burn and skin cancer.

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8
Q

How is the ozone formed?

A

When oxygen molecules (O2) is broken into two oxygen radicals by ultraviolet radiation. The oxygen radicals react with oxygen molecules to form O3, which is the Ozone.

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9
Q

How do CFC’s form holes in the atmosphere?

A

CFC’s in the upper atmosphere absorb UV radiation and split to form chlorine (halogen) free radicals. These free radicals catalyse the destruction of ozone, destroying ozone molecules and are then regenerated to destroy more.

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10
Q

Explain the break down process for the ozone layer and CFC’s with equations.

A
  1. CF2Cl2 —> · CF2Cl + Cl ·
    Chlorine free radicals are formed when the C-Cl bonds in CFC’s are broken by ultraviolet radiation.
  2. Cl · + O3 —> O2 + ClO ·
    ClO · + O —> O2 + Cl ·
    These free radicals are catalysts. They react with the ozone to form an immediate ClO · and oxygen molecules.
    The O radical comes from the breakdown of oxygen by ultraviolet radiation.
    The Chlorine free radical is regenerated and attacks another ozone molecule again.
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11
Q

Why are CFC’s stable?

A

Because of the strength of the carbon-halogen bonds.

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12
Q

So what is the overall reaction for radical (chlorine and other radicals)breaking down the ozone?

A

O3 + O —> 2 O2 and Cl · / other radicals is the catalyst.

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13
Q

What is another free radical (other than chlorine) which breaks down the ozone?

A

Nitrogen Oxides (NO ·)

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14
Q

How is nitrogen oxides produced?

A

Car and Aircraft engines, thunderstorms.

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15
Q

How do NO · free radicals affect the ozone?

A

The same way as chlorine radicals.

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16
Q

What is the universal reaction equations for radical and the ozone for it to break down?

Let the radical be R

A

R + O3 —> RO + O2
RO + O —> R + O2

Overall Reaction - O3 + O —> 2 O2

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17
Q

What did scientific evidence for CFC’s ozone depletion cause?

A

Government legislation against CFC use.

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18
Q

Why do C-Cl bonds in CFC’s only create free radicals instead of C-F or C-H bonds?

A

C-F and C-H bonds are too strong to be broken by UV light. C-Cl bonds are weak enough to broken by UV Radiation.

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19
Q

What alternatives uses instead for CFC’s?

A

HFC’s - don’t contain chlorine so don’t effect the ozone.
HCFC’s - still effect the ozone but much less then CFC’s.
Carbon Dioxide and Ammonia - used instead of nitrogen in aerosols.

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20
Q

What are Haloalkanes?

A

An Alkane with at least one halogen atom in place of a hydrogen atom.

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21
Q

What is the difference between a primary, secondary and tertiary haloalkanes?

A

Primary - the Halogen is bonded to a carbon atom which is bonded to ONE other carbon atom.

Secondary - the Halogen is bonded to a carbon atom which is bonded to TWO other carbon atom.

Tertiary - the Halogen is bonded to a carbon atom which is bonded to THREE other carbon atom.

22
Q

What is the carbon-halogen bond in the haloalkane?

A

Polar

23
Q

Why is the carbon-halogen bond in the haloalkane polar?

A

Halogens are normally more electronegative than carbon. So the carbon becomes electron deficient δ+ and halogen δ− a partial negative charge.

24
Q

What is a nucleophile?

A

An electron pair donor.

A lone pair of electrons ready to be donated to an electron deficient atom to form a covalent bond.

25
Q

Why are there partial charges on the carbon and halogens?

A

The covalent bond is closer to the halogen than the carbon due to it have a higher electronegativity.

26
Q

Carbon having a δ+ means that…

A

They are electron deficient and can be attacked by a nucleophile.

27
Q

What are the 4 types of Nucleophiles you need to know?

A

:OH-
:CN-
:NH3
H2O: (Water)

28
Q

What are the two things a nucleophile can be?

A

A negative ion or an atom with a lone pair of electrons.

29
Q

What is created from haloalkanes being hydrolysed?

A

Alcohols

30
Q

What type of reaction happens when haloalkanes are hydrolysed to make alcohols?

A

Nucleophilic substitution reaction.

31
Q

What must you use in the hydrolysis of haloalkanes?

A

A warm aqueous alkali, like sodium hydroxide or potassium hydroxide.

32
Q

What is the general equation for haloalkanes being hydrolysed to make alcohols with Warm Aqueous Alkali?

Let X be a halogen
R for an R group

A

Reflux
R-X + OH- ———> R-OH + X-

33
Q

What is Reflux?

A

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

34
Q

What is electronegativity?

A

A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.

35
Q

What is nucleophilic substitution?

A

A reaction in which a nucleophile is attracted to an electron-deficient carbon atom and replaces an atom or group of atoms on it.

36
Q

Why is water not normally used for hydrolysis of haloalkanes?

A

It is a weak nucleophile, so it is a much slower reaction.

37
Q

What is the general equation for haloalkanes being hydrolysed to make alcohols with Water?

Let X be a halogen
R for an R group

A

R-X + H2O —> R-OH + H+ + X-

38
Q

What would be the products of they hydrolysis of bromoethane with water?

CH3CH2Br + H2O —> ?

A

C3H5OH + H+ + Br-

39
Q

What would be the products for the hydrolysis of 1-bromobutane with Sodium Hydroxide?

CH3CH2CH2CH2Br + NaOH —> ?

A

CH3CH2CH2CH2OH + NaBr
(Butan-1-ol)

40
Q

Explain step by step the process of to draw a nucleophilic substitution reaction for the hydrolysis of a haloalkanes and an -OH group.

A
  1. The Nucleophile, OH-, approaches the carbon atom to the halogen on the opposite side of the molecule from the halogen atom.
  2. This direction of attack by the OH- ion minimises repulsion between the nucleophile and the δ− halogen atom.
  3. A lone pair of electrons on the hydroxide ion is attracted and donated to the δ+ carbon atoms.
  4. A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
  5. The carbon-halogen bond breaks by heterolytic fission.
  6. The new organic product is an alcohol. A halide ion is also formed.
41
Q

What effects how quickly a haloalkanes are hydrolysed?

A

The bond enthalpy of the carbon-halogen bond.

42
Q

What is the trend between bond Enthalpy and how quickly haloalkanes are hydrolysed and why?

A

Faster hydrolysis as the bond enthalpy decreases. As the weaker carbon-halogen bonds break more easily so they react faster.

43
Q

What is the trend for halogens and bond enthalpy?

A

Decreases down the group.

44
Q

For Iodoalkanes, Bromoalkanes, Chloroalkanes and Fluoroalkanes which reacts the fastest to the slowest?

A
  1. Iodoalkanes
  2. Bromoalkanes
  3. Chloroalkanes
  4. Fluoroalkanes
45
Q

Why are fluoroalkanes unreactive?

A

As a large quantity of energy is required to break the C-F bond.

46
Q

Explain the experiment to compare the reactivity of Chloroalkanes, Bromoalkanes and Iodinealkanes.

A
  1. Set up three test tubes each containing a different haloalkane and ethanol.
  2. Stand the test tubes in a water bath at 60°C till there all this temperature
  3. Add silver nitrate solution to each test tube.
  4. Start a stopwatch to see the time for the reaction to start.
    5.Iodinealkane will form a pale yellow precipitate rapidly.
    Brominealkane will form a cream precipitate faster than chloroalkane but slower than a iodinealkane.
    Chloroalkane a white precipitate (no change).
47
Q

What is the equation when silver ions react with the halide ions?

X = the halogen

A

Ag+ (aq) + X- (aq) —> AgX (s)

48
Q

Explain the observation of the experiment for haloalkanes and silver ions?

A

Chloroalkane reacts the slowest as the C-Cl bond is the strongest.

Iodoalkane reacts fastest as the C-I bond is the weakest

49
Q

Why is ethanol added to the haloalkanes before the silver nitrate solution is added?

A

Haloalkanes are insoluble in water as they can not form hydrogen bonds with the water molecules.

Ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers.

50
Q

What is the nucleophile in the reaction of the ethanol, haloalkane and aqueous silver nitrate solution?

A

Water in the aqueous nitrate solution.

A-level Chemistry Yr 12 Mine (15 decks)

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