15) Haloalkanes

Question 1

Haloalkanes can be classified as…?

Answer 1

primary, secondary or tertiary

Question 2

What can you infer from the fact that halogen atoms are more electronegative than carbon atoms?

Answer 2

the carbon-halogen bond is polar

the e- pair is closer to the halogen

Question 3

Define nucleophile

Answer 3

electron pair donor
an atom/group of atoms that is attracted to an electron-deficient centre or atom, where it donates a pair of e- to form a new covalent bond

Question 4

What does a nucleophile do when reacting with a haloalkane?

Answer 4

replaces the halogen in a substitution reaction - producing a new compound with a different functional group

Question 5

Name 3 possible nucleophiles

Answer 5

:OH-
H2O:
:NH3

Question 6

Define nucleophilic substitution

Answer 6

a reaction in which a nucleophile is attracted to an electron-deficient carbon atom, and replaces an atom/group of atoms on the carbon atom

Question 7

Define hydrolysis

Answer 7

a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule - resulting in the molecule being split into two products

Question 8

Haloalkane -> alcohol

Answer 8

NaOH(aq)
heat under reflux
hydrolysis

Question 9

Where are the 2 curly arrows in the nucleophilic substitution mechanism of NaOH(aq) with a haloalkane?

Answer 9

From the :O(-)H to the partially positive carbon

From the C-halogen bond to the partially negative halogen atom

Question 10

What does the rate of hydrolysis of a haloalkane depend upon?

Answer 10

the strength of the carbon-halogen bond in the haloalkane (bond enthalpy)

Question 11

How can the rate of hydrolysis be measured?

Answer 11

carry out the reaction in the presence of aqueous silver nitrate and recording the time taken for the silver halide precipitate to form:
Cl - white ppt slowly
Br - cream ppt
I - yellow ppt rapidly
an ethanol solvent is used as haloalkanes are insoluble in water

Question 12

C-F C-Cl C-Br C-I
Bond enthalpy _ ->
Rate of hydrolysis _ ->

Answer 12

decreases

increases

Question 13

Define organohalogen compounds

Answer 13

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

Question 14

What are organohalogen compounds often used in?

Answer 14

pesticides

Question 15

Are organohalogen compounds broken down naturally in the environment?

Answer 15

no

Question 16

Where is the ozone layer found?

Answer 16

at the outer edge of the stratosphere

Question 17

What does ozone do to UV-B from the sun’s rays

Answer 17

absorbs most of it, allowing only a small amount to reach the Earth’s surface

Question 18

What is UV-B?

Answer 18

a form of biologically damaging ultraviolet radiation which can lead to genetic damage and an increased risk of skin cancer

Question 19

Give two equations to understand the breakdown and regeneration of ozone

Answer 19

O2 -UV-> 2O

O2 + O -> O3

Question 20

What does O represent in an equation?

Answer 20

an oxygen radical

Question 21

What were CFCs and HCFCs commonly used in/as?

Answer 21

air-conditioning units
refrigerants
aerosol propellants

Question 22

Why are CFCs stable compounds?

Answer 22

carbon-halogen bond

Question 23

CFCs are stable until…?

Answer 23

reaching the stratosphere where they break down forming chlorine radicals

Question 24

What are chlorine radicals thought to do?

Answer 24

catalyse the breakdown of ozone

Question 25

Name the process by which a CFC absorbs UV and is broken down into radicals

Answer 25

photodissociation

Question 26

Give the initiation, propagation (2) and overall equations for the breakdown of CF2Cl2 by UV
What is important to note about the 2nd propagation step?

Answer 26

initiation: CF2Cl2 -UV-> CF2Cl. + Cl.
propagation: Cl. + O3 -> ClO. + O2
ClO. + O -> Cl. + O2
overall: O3 + O -> 2O2
The chlorine radical is regenerated which can undergo the 1st propagation step again

Question 27

Name another radical, other than Cl. , which catalyses the breakdown of ozone and how it is formed

Answer 27

nitrogen oxide radical

formed naturally during lightning strikes and as a result of aircraft travel

Question 28

Give the 2 propagation steps for the breakdown of ozone by NO.

Answer 28

NO. + O3 -> NO2. + O2

NO2. + O -> NO. + O2