Haloalkanes

Question 1

how are haloalkanes formed

Answer 1

• alcohols undergoing substitution reactions with halide ions in presence of acid
• free radical subtiturion of alkanes
• electrophilic addition of alkenes with X2 or HX

Question 2

equation for the formation of hydrogen halide

Answer 2

NaX (s) + H2SO4 (aq) —-> HX (aq) + NaHSO4 (aq)
- hydrogen halide produced in situ whilst alcohol is heated under reflux

Question 3

define nucleophile

Answer 3

e- pair donor that is attracted to regions of +ve charge

Question 4

general equation for reaction between alcohol and hydrogen halide

Answer 4

alcohol + hydrogen halide (HX) —–> haloalkane + water

Question 5

what 2 type of reactions occur between an alcohol and hydrogen halide (forward and backward direction)

Answer 5

• nucleophilic substitution (forward)
• hydrolysis (backward)

Question 6

why are nucleophiles able to attack haloalkanes and what type of reaction is this

Answer 6

• C-X bond is polar
• nucleophilic substitution

Question 7

general equation for reaction between OH- (nucleophile) and haloalkane

Answer 7

• haloalkane + OH- —-> alcohol + (X-) + (H+ )
  e.g. CH3CH2Br + H2O —-> C2H5OH + (Br-) + (H+)
• reflux
• OH- may come from compound like KOH or H2O

Question 8

2 methods of producing alcohols from haloalkanes

Answer 8

1) NaOH + haloalkane
solvent: ethanol
- heat under reflux
2) H2O + haloalkane
solvent: ethanol
- warm to around 60C using water bath

Question 9

how can bond strength of C-X be compared

Answer 9

• by measuring the rate of formation of the AgX ppt when AgNO3 (solution contains water) is added during hydrolysis of haloalkanes by water
• ethanol used as solvent
• this is the identifying reaction of haloalkanes

Question 10

why is the C-X bond in haloalkanes polar

Answer 10

• halogen is more electronegative than C
• C-F bond is the most polar as F is the most electronegative element

Question 11

why does reactivity of the C-X bond increase down group 7

Answer 11

• polarity and bond enthalpy decrease as you go down the group
  C-F
  C-Cl
  C-Br
  C-I

Question 12

what can bond enthalpy of C-X determine

Answer 12

• the relative rate of reactivity of haloalkanes

Question 13

what’s the most important factor that affects the reactivity of haloalkanes during hydrolysis

Answer 13

bond enthalpy

Question 14

why does bond enthalpy of C-X decrease down the group

Answer 14

• halogen atom becomes larger
• C-X bond becomes longer and weaker as there is less attraction between the nuclei and shared e-
• weaker bonds broken more readily so iodoalkanes react fastest

Question 15

what are CFCs and its alternatives

Answer 15

• chlorofluorocarbons
• used to be regularly released into atmosphere
• HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) being used as alternatives, however they’re greenhouse gasses

Question 16

what are HCFCs

Answer 16

• hydrochlorofluorocarbons
• used to be regularly released into atmosphere

Question 17

what’s photodissociation (in terms of CFCs)

Answer 17

the breakdown of CFCs into radicals by UV radiation producing chlorine radicals by homolytic fission of C-Cl bond

Question 18

how is the ozone layer being depleted

Answer 18

• depleted by the breakdown of ozone, O3, in a reaction catalysed by radicals

Question 19

steps in the depletion of ozone by Cl*

Answer 19

1) CF2Cl2 —> CF2Cl* + Cl*
2) Cl* + O3 —> ClO* + O2
ClO* + O —> Cl* + O2
3) overall = O3 + O —> 2O2

Question 20

how are NO* radicals produced

Answer 20

• lightning strikes
• also catalyses breakdown of ozone

Question 21

steps in depletion of ozone by NO*

Answer 21

1) NO + O3 —> NO2* + O2
2) NO2* + O —> NO* + O2
overall: O3 + O —> 2O2

Question 22

properties of CFCs

Answer 22

• non toxic
• non-flammable
• low reactivity
• volatile

Question 23

by which process are haloalkanes formed from alkanes

Answer 23

FREE RADICAL SUBSTITUTION