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as level chemistry > 2.6 halogenoalkanes > Flashcards

2.6 halogenoalkanes Flashcards

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1
Q

the C-Halogenoalkane bond is ____?

A

polar

δ+ δ-
C - X

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2
Q

why is the C-halogenoalkane bond said to be polar?

A
  • the halogen atom is more electronegative than the carbon it is bonded to
  • this results in an electronegativity difference
  • this leads to a permanent dipole as the bonding electrons are drawn towards the more electronegative element
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3
Q

the carbon in a C-Halogenoalkane is said to be _____ ______and is susceptible to _____ ______?

A

electron deficient
nucleophilic attacks

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4
Q

what is a nucleophile?

A

a species that has a lone pair of electrons that can be donated to an electron-deficient species

(opposite of an electrophile)

(that has an area of high electron density)

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5
Q

nucleophile examples:

A
  • hydroxide ions
  • (any species with a lone pair) e.g water
  • ammonia
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6
Q

what are the 2 reactions of halogenoalkanes?

A
  1. nucleophilic substitution reactions MECHANISM REQUIRED
  2. elimination reactions
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7
Q

what happens in a nucleophilic substitution reaction?

A
  • nucleophiles (e.g OH-) are attracted to the δ+ carbon
  • the lone pair on the nucleophile attacks the δ+ on the C atom in the halogenoalkane
  • as the carbon makes a bond with the incoming nucleophile, it simultaneously starts to break its bond with the halogen atom - sometimes referred to as a transition state
  • the carbon makes a full bond to the ex-nucleophile. the halogen atom is now a halide ion
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8
Q

what does nucleophilic substitution reactions form?

A

(from a haloalkane) to an alcohol

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9
Q

what are the conditions for nucleophilic substitution?

A
  • aqueous sodium hydroxide (NaOH)
  • reflux/warm
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10
Q

why don’t you want to boil halogenoalkanes in an open flame?

A

they are flammable and it would boil the alcohol made off

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11
Q

what are some key points about nucleophilic substitution?

A
  • label your dipoles between the C and X atom on the haloalkane
  • your initial curly arrow needs to come from the LONE PAIR on the nucleophile (also with a full negative charge) and needs to end at the carbon delta plus on the haloalkane
  • the breaking of the C-X bond must start with an arrow from the centre of the C-X bond and end up on the X atom
  • don’t forget the charge on the new halide ion formed as negative
  • the very reason the halogenoalkane has a dipole is why the reaction occurs
  • you should include the lone pair of electrons on the oxygen of the hydroxide ion
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12
Q

what happens in the elimination reaction? what does it form?

A
  • an elimination reaction is one where you start with a larger molecule, that ends up being broken down into smaller products
  • it involves the loss of a small molecule to produce a double bond
  • we say that the smaller product has been eliminated
  • when we perform an elimination reaction on a halogenoalkane, we get an alkene, a water molecule and a halide ion
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13
Q

what are the conditions for the elimination reaction?

A
  • reflux
  • sodium hydroxide in ethanol (ethanolic sodium hydroxide)
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14
Q

what does the elimination reaction form?

A

an alkene

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15
Q

bond polarity of C-X bond (increases/decreases) as you go up group 7?

A

increases

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16
Q

why does bond polarity of C-X increase as you go up group 7?

A

bc more electronegative so bigger dipole

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17
Q

what does it mean for C-F (in terms of nucleophiles) because bond polarity increases as you go up group 7?

A

this mean mean that a C-F bond is more likely to be attacked by a nucleophile and break

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18
Q

bond enthalpy (increases/decreases) up group 7?

A

increases

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19
Q

because bond enthalpy increases up group 7, C - __ bond is more likely to break

A

C-I

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20
Q

between bond enthalpy and bond polarity, which is the more important factor when looking at rates at which different halogenalkanes react with nucleophiles? why?

A

bond enthalpy

the primary reason for this is due to the bond length. bond lengths increase down the group and become easier to break

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21
Q

what are the steps of testing for halogenoalkanes?

A
  1. add aqueous NaOH (sodium hydroxide)
  2. acidify with HNO3 (nitric acid)
  3. add AgNO3 (silver nitrate)
  4. observe colour of precipitate
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22
Q

why is the first step of testing for haloalkanes to add aqueous NaOH?

A
  • creates an ion (e.g Br-) by nucleophilic substitution
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23
Q

why is the second step of testing for haloalkanes to acidify with HNO3?

A
  • AgOH is insoluble (produces a white ppt)
  • it eliminates OH-
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24
Q

what is the precipitate colour of chlorides during the test of halogenoalkanes?

A

white
AgCl

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25
Q

what is the precipitate colour of bromides during the test of halogenoalkanes?

A

cream
AgBr

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26
Q

what is the precipitate colour of iodides during the test of halogenoalkanes?

A

yellow
AgI

27
Q

what is the further test for halogenoalkanes?

A
  • add diulte ammonia —> Cl redissolved
  • add conc ammonia —> Cl and Br redissolved
28
Q

why don’t we test for fluoride ions?

A

as its too soluble so it doesnt produce an insoluble ppt

29
Q

why don’t we use hydrochloric acid instead of nitric acid in the test for haloalkanes?

A

Cl- —> interfere with test

30
Q

why add NaOH solution first in the test for haloalkanes?

A

to create halide ion (e.g Br-)

31
Q

what are some examples of the uses of halogenoalkanes?

A
  1. solvents - something that dissolves a solute e.g dichloromethane
  2. anaesthesia - a chemical given to patients in hospital to prevent pain during surgery e.g desflurane
  3. refrigerants - a chemical used in a cooling system e.g air con or refrigerators

LEARN 1

32
Q

what are some problems with using halogenoalkanes for these uses? (see previous flashcard)

A
  • many solvents are very toxic to humans (carcinogenic) and cannot be poured down the sink - instead are gathered to be chemically treated after use in the lab. green chemistry is an active area of research to find more environmentally friendly solvents
  • many organic solvents are extremely flammable
  • refrigerants can help made up of chlorofluorocarbons (CFCs) which damage the ozone layer
33
Q

what are chlorofluorocarbons (CFCs)? info:

A
  • these are compounds made up of chlorine, fluorine and carbon
  • these compounds destroy ozone
  • their use has been heavily regulated since the 1970s
  • CFCs are inert and not broken down at lower altitudes, but if let into the environment they travel into the stratosphere and the C-Cl bonds get homolysed by UV light
  • the resulting chlorine free radicals attack ozone molecules (O3)
  • this leads to a thinning of the ozone layer, letting more harmful UV light to damage cell DNA which can cause mutations known as cancer
  • the C-C and C-F do not break as they have greater bond strengths
34
Q

how do CFCs destroy the ozone layer?

A
  • CFCs are inert and not broken down at lower altitudes, but if let into the environment they travel into the stratosphere and the C-Cl bonds get homolysed by UV light
  • the resulting chlorine free radicals attack ozone molecules (O3)
  • this leads to a thinning of the ozone layer, letting more harmful UV light to damage cell DNA which can cause mutations known as cancer
  • the C-C and C-F do not break as they have greater bond strengths
35
Q

what is the mechanism between ozone and chlorine free radicals?

A

O3 + Cl• —> ClO• + O2
ClO•O3 —> 2O2 + Cl• (regenerated)

overall:
2O2 —> 3O2 (net effect)

36
Q

what are CFC alternatives?

A
  • HFCs / HCFCs
  • the carbon-chlorine bond is much weaker than the carbon-fluorine bond
  • HCFCs are hydrochlorofluorocarbons. they all contain at least one hydrogen atom and this causes them to be much less stable in the lower atmosphere than CFCs
  • fewer of the HCFC molecules reach the stratosphere where they can deplete the ozone layer
  • one adverse property is that HCFCs are potent greenhouse gases. HFCs are hydrofuorocarbons and contain no chlorine and as the carbon-fluorine bond is strong they are unlikely to form radicals which can destroy the ozone layer
37
Q

until recently chlorofluorocarbons (CFCs) were used in many commercial products but nowadays hydrofluorocarbons (HFCs) are preferred.
explain why HFCs are preferred to CFCs [5]

A
  • C-Cl weaker C-F
  • less energy needed to break C-Cl
  • less free radicals (Cl•)
  • less likely to deplete O3
  • less UV hits earth surface
38
Q

what is reflux?

A

a technique involving the condensation of vapour and the return of this condensate to the system from which it originated
(continuous evaporation and condensation of reactions)

  • reflux condenser sometimes referred to ad a Liebig condenser
  • involves heating and evaporation, then condensation back into the original flask it came from
  • used to heat mixture
  • connected to distillation condensor

adv:
- when boils, it evaporates but then condenses and can fall back
- can heat it without loosing it to evaporation

(leave top open/no bung bc otherwise pressure build up)

39
Q

what is distillation?

A

a process of separating the component or substances from a liquid mixture by selective evaporation and condensation

is used when 2 liquids with different boiling points

40
Q

when is fractional distillation used?

A

to separate 2 substances that have similar boiling points

41
Q

in distillation, where does water go in and where does it come out?

A

water in at bottom, out at top

bc heat rises

42
Q

what is the difference in apparatus between fractional distillation and distillation?

A
  • glass beads placed in the column in fractional distillation (to increase surface area within column)
43
Q

how are the glass beads an advantage to fractional distillation?

A
  • glass beads in column in fractional distillation encourage evaporated vapour to lose heat and condense
  • f.d have a large surface area which encourages the vapour to dissipate (lose heat) and condense
  • substances with a slightly lower b.p will therefore condense on the beads and fall back into the heated mixture
44
Q

why do we use an oil bath (not water) to heat the mixture in fractional distillation

A

oil can heat things past 100°

45
Q

why is the liquid not boiled in a naked flame in fractional distillation ?

A

organic compounds are flammable

46
Q

what could be done to the flask containing the distilled liquid to ensure it stays in liquid form?

A
  • use an ice bath
47
Q

why is the thermometer placed where it is in fractional distillation? (at the place exactly as the gas goes into the condenser)

A

to take the temperature of the gas being turned into liquid

48
Q

what are halogenoalkanes?

A

part of a homologous series where one or more hydrogen atoms in the alkane have been replaced by a halogen atom

49
Q

halogenoalkanes contain a carbon to halogen ____ bond?

A

polar

50
Q

e.g 2-bromobutane produce both but-2-ene and but-1-ene.
do we need to know which one is more likely to form?

A

no
just that both are a posibility

51
Q

when is reflux used?

A

when the reaction is slow, therefore the reaction needs to be heated for a significant amount of time

this process also maximises yield

52
Q

electronegativity (increases/decreases) as the size of the halogen increases?

A

decreases

meaning the C-Cl bond is the most polar (out of Cl, Br, I), with the carbon atoms being the most δ+

53
Q

how can the rate of hydrolysis be measured?

A

by recording the rate at which the halide ion, X- forms a precipitate (AgX) with Ag+ ions

this can be done using a colorimeter

54
Q

fluoroethane vs iodoethane
which reaction is faster?

A

iodoethane faster

bc even tho fluoroethane has stronger attraction between C δ+ and OH- bc F δ-,

bond enthalpy dictates more than bond polarity

so iodoethane faster as C-I weaker/lower bond enthalpy

55
Q

why are halogenoalkanes used as solvents?

A
  • as they are polar compounds but insoluble in water, halogenoalkanes are able to mix with polar and non-polar organic compounds which can be used as solvents

often used in dry cleaning and degreasing

56
Q

why are halogenoalkanes used as refrigerants?

A
  • halogenoalkanes have a b.p close to room temperature so are easily vapourised
  • when a liquid evaporates, it takes in heat, cooling the surroundings down
  • halogenoalkanes are also non-toxic and non-flammable
  • all these properties make them ideal refrigerant la
  • CFCs were used extensively in the past for this reason, but now we know they have adverse environmental effects
57
Q

why is chlorine most likely to form free radicals in the atmosphere?

A

the C-Cl bond is weaker than the C-F and C-H bonds, so it breaks when subjected to UV light
the chlorine free radical created stars a chain reaction

58
Q

what is produced when a halogenoalkane reacts with an aqueous alkali, like KOH?

A

an alcohol is produced

the OH- ion acts as a nucleophile and a nucleophilic substitution reaction takes place

59
Q

what is the condition required for a halogenoalkane to produce an alcohol rather than an alkene when it reacts with KOH?

A
  • the KOH needs to be aqueous
  • if the KOH is ethanolic, an alkene will be produced
60
Q

why are there tight regulations surrounding the use of halogenoalkanes?

A

halogenoalkanes are toxic and have a negative impact on the environment, so their use is regulated

61
Q

if the halogen in a particular halogenoalkane is changed, the hydrolysis reaction will occur at a different rate. due to:
1. electronegativity
2. bond strength

A
  1. electronegativity: chlorine is the most electronegative so that the C-Cl bond is most polar and the carbon is the most δ+. this is the carbon that is attacked by the nucleophile
  2. bond strength: chlorine is the smallest halogen so that the C-Cl bond is the strongest. in hydrolysis, this bond is broken

these two factors act in opposite directions, but in practice, the iodocompound is hydrolysed most quickly. this means that the effect of the bond strength outweighs the effect of the charge on the carbon

62
Q

halogenoalkanes are hydrolysed by aqueous sodium hydroxide. state and explain which of 1-fluoropropane, 1-chloropropane and 1-bromopropane is hydrolysed most rapidly?

A

C3H7Br is hydrolysed most rapidly
because the C-Br bond is the weakest bond

63
Q

for the experiment to determine the enthalpy change of combustion for e.g methanol, we use a spirit burner and water with a thermometer.
why can the apparatus not be used to determine the enthalpy change of combustion of ethene?

A

ethene is a gas/not a liquid

64
Q

suggest a reason why there is still concern about ozone depletion [1]

CFCs have been largely banned and have been replaced

A
  • some CFCs still being used / CFCs take a very long time to reach the ozone layer / other substances deplete the ozone layer

as level chemistry (16 decks)

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