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Organic Chemistry Year 12 > 4.2.2 Haloalkanes > Flashcards

4.2.2 Haloalkanes Flashcards

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1
Q

What are haloalkanes

A

Compounds containing carbon, hydrogen, and at least one halogen

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2
Q

What is the rule to remember when naming haloalkanes

A

If more than one halogen present, name alphabetically
(e.g. 2-chloro-1-iodopropane)

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3
Q

What are haloalkanes structured as

A

Primary, secondary, tertiary
- dependent on position of halogen, and how many carbons are attached to it

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4
Q

What are the 3 ways to make haloalkanes

A

Free radical substitution, electrophilic addition, nucleophilic substitution

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5
Q

Free radical substitution to make haloalkanes

A

Alkane + halogen ===> haloalkanes
(Need UV light)

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6
Q

Electrophilic addition to make haloalkanes

A

Alkene + hydrogen halide ===> haloalkanes
(room temperature)

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7
Q

Nucleophilic substitution of haloalkanes

A

Alcohol + hydrogen hapkido ===> haloalkane

(Hear under reflux)
(Or can use sodium halide and H2SO4)

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8
Q

Why is the halogen-carbon bond polar

A

Halogens are more EN than Carbon

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9
Q

What is the nature of the C atom attached to the halogen in haloalkanes

A
  • X-C bond is polar
  • so the electron pair is closer to the halogen
  • so the carbon atom attached to it is slightly positive
  • (so can attract species containing lone pair of e-)
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10
Q

What is a nucleophile

A

An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donated a pair of electrons to form a new (dative) covalent bond

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11
Q

What are examples of nucleophiles

A

:OH-
H2O:
:NH3

  • must have lone pair of electrons to donate
  • can be charged or neutral
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12
Q

What is nucleophilic substitution

A

When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction

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13
Q

What is a substitution reaction

A

Reaction in which one atom or a group of atoms is replaces by another atom or group of atoms

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14
Q

What undergoes nucleophilic substitution

A

Primary haloalkanes to form a primary product, variety of

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15
Q

What is hydrolysis

A

Chemical reaction involving water/an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
- means molecule is split into 2 products
- e.g. in haloalkanes, where the -OH group replaces the halogen atom

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16
Q

What is the mechanism of hydrolysis nucleophilic substitution

A

1) the nucleophile, OH-, approaches the a C atom attached to the halogen on the opposite side of the molecule containing the C atom
2) this direction AS this minimises the repulsion between the OH- nucleophile attacking and the the slightly negative H atom
3) a lone pair of electrons on the OH- ion is attracted and is donated to the slightly positive C atom
4) a new bond forms between the O of the OH- and the C atom
5) the C-X bond breaks via heterocyclic fission
6) new product formed is an alcohol and a halide ion

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17
Q

Where can you get the OH- ion from in hydrolysis

A

Aqueous NaOH
- slow at room temp, so heated under reflux to maintain a good yield
- the Na will later react with a halide

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18
Q

explain how you would draw the reaction mechanism for a haloalkane and OH- ion

A

1) draw the haloalkane with the δ+ and δ- charges on the C and X
2) draw the :OH- below
3) draw a line coming from the : lone pair on the ion to the C attached to the halogen, making sure the arrow is on the opposite side of the halogen
4) draw an arrow coming from the C-X bond to the halogen
5) draw your products, being the alcohol and halide ion

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19
Q

how would you draw the nucleophilic substitution of water and a haloalkane

A

1) draw a line coming from the :O-H-H bond pair to the C attached to the C-X bond, making sure arrow is on opposite side of the halogen (also add on δ+ and δ- charged too)
2) draw the alcohol formed but with a + on the one side of the O attached to the OH, and on the other side, draw the H bonded
3) draw a line coming from the O-H bond to the O attached to the OH
4) add + halide ion on here
5) then, draw the resulting alcohol, + halide ion, + H+ ion

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20
Q

what do you need to remember when reacting water with a haloalkane

A
  • need a solvent, such as ethanol, to allow the haloalkane and water to mix
  • it would form a hydrogen halide instead, instead of sodium halide as before
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21
Q

what is the rate of hydrolysis of haloalkanes dependent on

A

dependent on the strength of the C-X bond, as this is what gets broken

22
Q

how does the strength of bonds affect the rate of reaction, therefore the rate of enthalpy

A
  • the stronger the bond (C-F is strongest)
  • the higher its enthalpy
  • so the harder it is to break
  • so it is the least reactive
23
Q

why does bond strength decrease as you go down group 7

A
  • the size of the halogen atom increases
  • so the C-X bond length increases
  • so the C-X bond enthalpy decreases
  • so weaker, and requires less energy to break (hence the C-I bond is the most reactive, as is the weakest)
24
Q

how can you measure the hydrolysis of haloalkanes

A
  • haloalkanes react with water
  • haloalkane + H2O ===> alcohol + H+ + X-
  • the X- ion (halide ion) reacts with aqueous silver nitrate to produce a precipitate of silver halide
  • Ag+ + X- ===> AgX(s)
  • you can measure the rate that the precipitate is produced
25
Q

why do you add ethanol when measuring rate of hydrolysis of haloalkanes

A
  • haloalkanes are insoluble in water
  • but you need to get the nucleophile, H2O:, from the aqueous AgNO3
  • so you add ethanol as solvent
  • means that you will end up with single solution, and not 2 layers
26
Q

explain the steps in measuring the rate of hydrolysis of haloalkanes

A

1) set up 3 test tubes, each with 1cm^3 of ethanol and drops of haloalkanes (making sure they’re all the same structure, primary, and carbon length, butane), from Cl to I
2) stand all in water bath at 60 degrees
3) add test tubes of 0.1 moldm^-3 of AgNO3 to water bath too
4) add 1cm^3 into each tube, and start the stop clock
5) observe for 5 minutes, and see time for precipitate to form

27
Q

what should be your results for testing the rate of hydrolysis of haloalkanes

A

CHLORO= white, SLOW
BROMO= cream, MIDDLE
IODO= yellow, FAST

28
Q

what are organohalogen compounds

A

molecules that contain at least 1 halogen atom joined to a carbon chain

29
Q

what are the uses of organohalogen compounds

A

pesticides, solvents, making polymers, flame retardants, refrigerants, aerosol cans, air conditioning

30
Q

explain organohalogen compounds in nature

A
  • rarely found in nature
  • not broken down naturally in the environment
31
Q

where is the ozone layer found

A

the outer edge of the stratosphere

32
Q

what is the role of the ozone layer

A

contains ozone - O3
- which absorbs most of the biologically damaging UV radiation, UVB , from sunrays, only allowing a small amount to reach the surface

33
Q

what will a depletion of O3 result in

A

increase in sunburns and skin cancer (caused by the genetic damage of UV)

34
Q

explain how ozone is continually broken down and made in the stratosphere

A
  • due to the actions of UV radiation

1) high energy UV breaks O2 molecules into O radicals
- O2 ===> 2O (UV on arrow)
2) a STEADY STATE is set up involving O2 and the O radicals, in which O3 forms and breaks
- O2 + O === O3

35
Q

what is the equilibrium of ozone production, and what has human activity done to this

A

rate of formation of O3 is equal to the rate at which O3 is broken down
- a STEADY STATE
- however, human activity, such as CFCs, has upset this equilibrium

36
Q

what does CFC stand for

A

chlorofluorocarbons

37
Q

what are CFCs used for

A
  • refrigerants
  • air conditioning units
  • aerosol propellants
38
Q

why are CFCs very stable

A

have strong C-X bonds in their molecules:
- so have a long residence time in the troposphere (don’t react very well)
- takes years to reach the stratosphere

39
Q

what happens to CFCs once they reach the stratosphere

A
  • the UV radiation provides enough energy to break the C-X bonds in CFCs, via homolytic fission
  • and forms radicals
  • the C-Cl bond will always be the one to break, as C-Cl bond has lower enthalpy than C-F
40
Q

what is the first step in the free radical mechanism of CFCs

A

PHOTODISSOCIATION - is the initiation step, as the radiation initiated the breakdown

  • the C-Cl bond breaks via homolytic fission, and forms a Cl radical and a CFC radical
41
Q

how would you draw the first step in CFC mechanism

A

1) draw the CFC
2) you CAN show the fish hook arrows in the C-Cl bond, going to each atom, but not needed
3) draw arrow, going to the 2 radicals formed

42
Q

what do the Cl. radicals act as in the propagation step of CFC mechanism

A

act as an intermediate, as are very reactive

43
Q

what are the equations for the propagation of the CFC mechanism

A

1) Cl. + O3 ===> ClO. + O2
2) ClO. + O ===> Cl. + O2
OVERALL: O + O3 ===> 2O2

  • repeats in cycle in a chain reaction
44
Q

why can the Cl. radical be seen as a catalyst in the CFC reaction mechanism

A
  • it is not used up in the reaction (same used up as produced)
  • but it lowers the AE and increases the ROR
45
Q

what is the other radical responsible for catalysing the breakdown of O3

A

nitrogen oxide radicals

46
Q

where are NO radicals found

A
  • naturally in lightning strikes and thunderstorms
  • as a result of aircraft travel in the stratosphere
47
Q

what is the equation for making NO radicals

A

N2 + O2 ===> 2NO.

(need high temp on arrow, as both N≡N and O=O have high bond enthalpies, so require a lot of energy to break)

48
Q

explain the steps of NO. reacting with O3

A

1) NO. + O3 ===> NO2. + O2
2) NO2. + O ===> NO. + O2
OVERALL: O3 + O ===> 2O2

49
Q

what is the montreal protocol

A

an international agreement phasing out CFCs (and bromine containing compounds), and finding alternatives where possible

50
Q

even though the montreal protocol exists, why will CFCs still take time to phase out

A
  • still until 2060s and 2070s:
  • CFCs are very stable in lower atmosphere, so have a long life (C-F and C-Cl have strong, high bond enthalpies, need lots of energy to break)
  • CFCs take a long time to reach the stratosphere (years)
  • CFCs are still being released
  • Cl radicals can catalyse O3 depletion many times before being terminated
51
Q

reasons why CFCs are good for aerosols specifically

A
  • volatile
  • non-flammable
  • non-toxic
  • low reactivity
52
Q

what is a biodegradable alternative to using CFCs in aerosol cans

A

hydrochlorofluorocarbon
- (contains H too in structure)

Organic Chemistry Year 12 (7 decks)

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