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A level Chemistry > Chapter 15 - Haloalkanes > Flashcards

Chapter 15 - Haloalkanes Flashcards

1
Q

like alcohols, can be ?

A

classified as primary, secondary or tertiary

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2
Q

nucleophiles include?

A
  • Hydroxide ions
  • H2O
  • Ammonia (lone pair on the N)
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3
Q

what happens when a haloalkane reacts w a nucleophile?

A

the nucleophile replaces the halogen in a substitution reaction.

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4
Q

haloalkanes can b converted to alcohols using?

A

aqueous sodium hydroxide. The reaction is very slow at room temp so the mixture is heated under reflux to obtain a good yield of product

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5
Q

relative reactivities?

A
  • iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes (most to least)
  • depends on bond enthalpy value
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6
Q

hydrolysis of haloalkanes procedure?

A

1) set up 3 test tubes, each containing 1cm3 of ethanol and 2 drops of each haloalkane
2) stand test tubes in a water bath @ 50 degrees
3) place a test tube containing 0.1moldm-3 silver nitrate in the water bath and allow all tubes to reach a constant temp
4) add 1cm3 of the silver nitrate solution quickly to each test tube. Immediately start a stop clock
5) record time taken for precipitate to form, calculate rate using 1/ time

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7
Q

as radiation inititates the breakdown in CFCs this process is called?

A

photodissociation

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8
Q

are CFCs responsible for all ozone depleting reactions?

A
  • no. other radicals also catalyse the breakdown of ozone

- NO radicals are formed naturally during lightning strikes and as a result of aircraft travel in the stratosphere

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9
Q

nitrogen radical overall equation?

A

is the same as w the chlorine radicals , showing that the radicals act as a catalyst for the process
O3 + O -> 202

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10
Q

haloalkanes are?

A

sat.

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11
Q

can be classed as ?

A

primary, secondary, tertiary

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12
Q

what is a primary haloalkane?

A

only has 1 C (alkyl gr) attached to the C that is bonded to the halogen

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13
Q

why does the polarity of the C-X bond ⬇ down G7?

A

The electroneg of the halogen decreases down the gr

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14
Q

how do we assess reactivity?

A
  • compare relative rates of hydrolysis of C-X bond
  • add AgNO3 solution
  • the speed of silver halide precipitate formation indicated the speed of hydrolysis
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15
Q

which haloalkane is hydrolysed fastest and why?

A

iodoalkane - smallest bond enthalpy

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16
Q

nucleophiles are?

A 
Nucleo = +
philes = liking

are attracted to e- deficient areas - usually delta + areas

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17
Q

rate of hydrolysis is faster when?

A

heated w an (aq) alkaline solution, rather than H2O

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18
Q

CFCs = ?

A

Chlorofluorocarbons e.g. dichlorodifluromethane

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19
Q

why were CFCs used sm?

A

they are inert and non toxic

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20
Q

CFCs?

A
  • Are gases at room temp
  • after use, they are able to diffuse thru layers of the atmosphere
  • in the stratosphere, CFCs become exposed to UV radiation
  • UV light causes C-Cl bond in the CFCs to undergo homolytic fission, forming Cl free radicals
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21
Q

N radicals?

A
  • NO * + O3 -> NO2 * + O2
  • NO2 + O -> NO* + O2
  • overall : O3 + O -> 2O2
  • This occurs naturally when lighting strikes- high temp around the lightning bolt causes the reaction
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22
Q

what makes haloalkanes reactive?

A

they are polar molecules. They r often used to synthesise other chemicals

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23
Q

GF?

A

CnH2n+1X

X =F, Cl, Br, I

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24
Q

Uses - Dichlorodifluoromethane

A

used as refrigerants and aerosol propellants (CCl2F2)

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25
Q

Uses - which polymers are haloalkanes?

A

PVC & Teflon

26
Q

Uses - Bromotrifluoromethane ?

A

flame retardants (BrCF3)

27
Q

Uses - Carbontetrachloride?

A

dry cleaning solvent - CCl4

28
Q

polarity of the C-X bond?

A
  • decreases down the G

- C-F is the most polar bond, but these are the least reactive of the haloalkanes

29
Q

reactivity of the C-X bond?

A
  • the strength of the C-X bond is more important
  • the C-I bond requires least energy to break so iodoalkanes are the most reactive
  • Reactivity of haloalkanes ⬆ down G7
30
Q

Hydrolysis definition?

A

the breaking of a bond by the reaction of H2O or OH-

31
Q

Reagent and Condition for hydrolysis?

A

R: NaOH (aq), or K(OH) & AgNO3(aq) when looking at RATE
C: hot/ reflux and in ethanol

32
Q

what is a nucleophile?

A

a particle w a lone pair of e-s that can be donated to form a covalent bond (an electron pair donor) e.g. H2O, OH-, NH3

33
Q

a substitution reaction is?

A

a reaction where one atom or group of atoms is swapped for a diff atom or group of atoms

34
Q

nucleophilic substitution - what happens?

A

the halogen is swapped for the OH

35
Q

e.g. of nucleophilic substitution - bromoethane and sodium hydroxide?

A

CH3CH2Br +NaOH -> CH3CH2OH + NaBr

36
Q

describe what happens in nucleophilic substitution?

A
  • the lone pair of e-s on the O of the nucleophile is donated to the e- definicient C, delta +, forming a Covalent bond
  • the C-Br bond breaks heterolytically and both e-s are passed to the Br forming Br-. (w a lone pair)
37
Q

which haloalkane would hydrolyse the fastest?

A
  • iodoalkanes
  • the C-I bond has the lowest bond enthalpy so will break w least energy
  • 2nd fastest is bromoalkanes
  • slowest: choloroalkanes
38
Q

what are the essential reagents and conditions for the hydrolysis of haloalkanes?

A
  • heat in water bath @ 50 degrees C to heat flammable ethanol and keep constant temp
  • haloalkanes need to be dissolved in ethanol - allows mixing w AgNO3(aq)
  • AgNO3(aq) to supply the H2O (aq) for hydrolysis and to detect the halide ion by formation of AgX(s)
39
Q

what would u see when carrying out an experiment to find out rate of hydrolysis?

A
  • precipitate for yellow AgI(s) forms instantly, followed by cream precipitate of AgBr (s) and the slow formation of a white precipitate of AgCl (s) - C-I bond is the weakest so iodoalkanes hydrolyse fastest
40
Q

what is the formula for atomic Oxygen?

A

O or O radical

41
Q

Ozone molecule formula?

A

O3

42
Q

O, O2, O3 all exist in?

A

the stratosphere

43
Q

what does Ozone do?

A

forms a layer of ozone gas in the stratosphere where it filters out short wavelength UV radiation that is harmful to humans and can cause skin cancer

44
Q

how is the amount of ozone in the stratosphere kept constant?

A
  • Ozone in the stratosphere is continually being formed and broken down by the action of uv radiation
  • V high energy uv radiation sspits O2 into O radicals: O2 -> O-
  • An equil is then set up and ozone is formed and broken down by the same rate: O3 -> (REVERSIBLE) O- + O2
45
Q

If no interference by mankind,?

A

the amounts of O2, O-, O3 would remain unchanged, hwvr for many yrs we used CFCS

46
Q

Why is ozone destroyed in the stratopshere?

A

as a consequence of the use of CFCs

47
Q

what r CFCs?

A

Chlorofluorocarbons, they break down in the presence of uv light to form chlorine radicals

48
Q

when CFCs reach the stratosphere?

A

uv light breaks the C-Cl bonds forming Cl free radicals

49
Q

what kind of bond breaking occurs when the C-Cl bond breaks?

A

homolytically - each atom takes 1 e- from the bond forming a radical w one unpaired e-

50
Q

the equation for ozone decomposing in the stratosphere is?

A

O3 + O- -> 2O2 - this reaction is catalysed by the presence of gaseous Cl radicals

51
Q

the O in the 2nd step form continuously in the stratosphere when?

A

uv light reacts w O3 and O2 molecules

52
Q

the role of Cl and ClO radicals was proven conclusively by who?

A

an American-led team of scientists who flew into the hole in the ozone layer recording concs of O3 and ClO radicals

53
Q

Which other radical destroys ozone?

A

NO - nitrogen monoxide radical

54
Q

2 types of catalyst ?

A

homogeneous or heterogeneous

55
Q

homogeneous catalysts?

A

catalysts and reactants in the SAME state - think HOMOsexual

56
Q

heterogeneous catalyst?

A

catalyst n reactant are in diff states e.g. NO and CO in exhaust fumes are gases but the Pt in a catalytic converter is a solid catalyst

57
Q

what does hydrolysis of a haloalkane produce?

A

alcohol and halide ions

58
Q

?breakdown of ozone 3 steps?

A
  1. CCl2F2 -> Cl- + CClF2-
  2. O3 + Cl- -> ClO- + O2
  3. ClO- + O- -> O2 + Cl-
59
Q

alternatives to CFCs- HCFCs?

A
  • Hydrofluoroalkanes

- these r non flammable and non toxic but still damage the ozone layer but only 1/10th as much as CFCs

60
Q

alternatives to CFCs- ozone friendly products?

A

hydrocarbon refrigerants & propellants, like butane do not damage the ozone layer, are flammable.

A level Chemistry (39 decks)

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