4.2.2 Haloalkanes

Question 1

Which bond in haloalkanes is polar?

Answer 1

• difference in electronegativity between halogen and carbon causes the carbon-halogen bond to be polar

Question 2

What is the trend in electronegativities of halogens?

Answer 2

• electronegativity decreases as you go down group 7
• polarity decreases down the group

Question 3

What is a nucleophile?

Answer 3

• a molecule or ion with a lone pair available for bond formation
• an electron pair donor
• they will attack a region of low electron density (d+) in another molecule

Question 4

What are common nucleophiles?

Answer 4

-:OH
H2O
:NH3
NC:- (cyanide ion)

Question 5

What is the method for nucleophilic substitution in haloalkanes?

Answer 5

• heat under reflux with aqueous hydroxide ions
  C3H7Cl + OH- -> C3H7OH + Cl-

Question 6

What is the mechanism for nucleophilic substitution in haloalkanes?

Answer 6

See notes

Question 7

What is the method for the rate of hydrolysis of primary haloalkanes?

Answer 7

• add haloalkanes of the same chain length to test tubes and place them in a water bath at 50°C
• half-fill separate test tubes with ethanol, water and aqueous silver nitrate and place in water bath
• add ethanol, water and AgNO3 to each haloalkane and measure time taken for precipitate to appear
• calculate relative rate of hydrolysis using rate = 1/time

Question 8

What are the results of the rate of hydrolysis of primary haloalkanes?

Answer 8

Rate of hydrolysis increases ->
1-fluorobutane, 1-chlorobutane, 1-bromobutane, 1-iodobutane
High polarity. Low polarity
High bond enthalpy. Low bond enthalpy

Question 9

What is the explanation for the rate of hydrolysis of primary haloalkanes?

Answer 9

• reactivity of haloalkanes in nucleophilic substitution depends on polarity and bond enthalpy (bond enthalpy is the most important factor)
• C-Cl bond is the most polar and so most attractive, but because of high bond enthalpy, they have the lowest rate of hydrolysis
• C-I is the least polar and least attractive, C-I has the highest rate of hydrolysis because the bond is weaker and easier to break

Question 10

How are halogen radicals produced from CFCs?

Answer 10

• CFCs are chlorofluorocarbons (all Hs replaced by Cl and F)
• halogen radicals are produced from action of ultraviolet radiation on CFCs in the upper atmosphere

Question 11

What is the effect of halogen radicals being produced on the ozone layer?

Answer 11

• catalyses break down of the ozone layer - earths protective layer that stops damaging UV light
• ozone can be removed and the rate of this is very slow but increased by pollutants

Question 12

What is the equation for the production of halogen radicals?

Answer 12

C2F2Cl2 -> Cl. + C2F2Cl.

Question 13

What is the equation for ozone destruction by CFCs?

Answer 13

Cl. + O3 -> ClO. + O2
ClO. + O -> Cl. + O2
Overall: O3 + O -> 2O2

Question 14

What is the mechanism for destruction of the ozone layer by other radicals (e.g. NO.)?

Answer 14

R. + O3 -> RO. + O2
RO. + O -> R. + O2
Overall: O3 + O -> 2O2