15) Haloalkanes Flashcards

1
Q

Haloalkanes can be classified as…?

A

primary, secondary or tertiary

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2
Q

What can you infer from the fact that halogen atoms are more electronegative than carbon atoms?

A

the carbon-halogen bond is polar

the e- pair is closer to the halogen

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3
Q

Define nucleophile

A

electron pair donor
an atom/group of atoms that is attracted to an electron-deficient centre or atom, where it donates a pair of e- to form a new covalent bond

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4
Q

What does a nucleophile do when reacting with a haloalkane?

A

replaces the halogen in a substitution reaction - producing a new compound with a different functional group

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5
Q

Name 3 possible nucleophiles

A

:OH-
H2O:
:NH3

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6
Q

Define nucleophilic substitution

A

a reaction in which a nucleophile is attracted to an electron-deficient carbon atom, and replaces an atom/group of atoms on the carbon atom

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7
Q

Define hydrolysis

A

a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule - resulting in the molecule being split into two products

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8
Q

Haloalkane -> alcohol

A

NaOH(aq)
heat under reflux
hydrolysis

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9
Q

Where are the 2 curly arrows in the nucleophilic substitution mechanism of NaOH(aq) with a haloalkane?

A

From the :O(-)H to the partially positive carbon

From the C-halogen bond to the partially negative halogen atom

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10
Q

What does the rate of hydrolysis of a haloalkane depend upon?

A

the strength of the carbon-halogen bond in the haloalkane (bond enthalpy)

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11
Q

How can the rate of hydrolysis be measured?

A

carry out the reaction in the presence of aqueous silver nitrate and recording the time taken for the silver halide precipitate to form:
Cl - white ppt slowly
Br - cream ppt
I - yellow ppt rapidly
an ethanol solvent is used as haloalkanes are insoluble in water

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12
Q

C-F C-Cl C-Br C-I
Bond enthalpy _ ->
Rate of hydrolysis _ ->

A

decreases

increases

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13
Q

Define organohalogen compounds

A

molecules that contain at least one halogen atom joined to a carbon chain

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14
Q

What are organohalogen compounds often used in?

A

pesticides

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15
Q

Are organohalogen compounds broken down naturally in the environment?

A

no

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16
Q

Where is the ozone layer found?

A

at the outer edge of the stratosphere

17
Q

What does ozone do to UV-B from the sun’s rays

A

absorbs most of it, allowing only a small amount to reach the Earth’s surface

18
Q

What is UV-B?

A

a form of biologically damaging ultraviolet radiation which can lead to genetic damage and an increased risk of skin cancer

19
Q

Give two equations to understand the breakdown and regeneration of ozone

A

O2 -UV-> 2O

O2 + O -> O3

20
Q

What does O represent in an equation?

A

an oxygen radical

21
Q

What were CFCs and HCFCs commonly used in/as?

A

air-conditioning units
refrigerants
aerosol propellants

22
Q

Why are CFCs stable compounds?

A

carbon-halogen bond

23
Q

CFCs are stable until…?

A

reaching the stratosphere where they break down forming chlorine radicals

24
Q

What are chlorine radicals thought to do?

A

catalyse the breakdown of ozone

25
Name the process by which a CFC absorbs UV and is broken down into radicals
photodissociation
26
Give the initiation, propagation (2) and overall equations for the breakdown of CF2Cl2 by UV What is important to note about the 2nd propagation step?
initiation: CF2Cl2 -UV-> CF2Cl. + Cl. propagation: Cl. + O3 -> ClO. + O2 ClO. + O -> Cl. + O2 overall: O3 + O -> 2O2 The chlorine radical is regenerated which can undergo the 1st propagation step again
27
Name another radical, other than Cl. , which catalyses the breakdown of ozone and how it is formed
nitrogen oxide radical | formed naturally during lightning strikes and as a result of aircraft travel
28
Give the 2 propagation steps for the breakdown of ozone by NO.
NO. + O3 -> NO2. + O2 | NO2. + O -> NO. + O2