OZ - Reaction mechanisms *01 (and sustainability)

Question 1

what is a substitution reaction?

Answer 1

when a functional group in a compound is replaced by another functional group

Question 2

what is a nucleophile?

Answer 2

something that is attracted to the slightly +ve carbon atom

is usually -ve

Question 3

What does the relative reactivities of haloalkanes depend on?

Answer 3

The carbon halogen bond strength determines reactivity
- decreases down group

C-F bond the strongest (despite being the most polar) as it has the highest bond enthaply

Question 4

What can you test the relative reactivities of the haloalkanes?

Answer 4

react the haloalkanes with water in the presence of silver nitrate

1. see which reacts the fastest.
   - put a chloroalkane, bromoalkane, and iodoalkane in three different test tubes
   - add silver nitrate solution and some ethanol

Ag+ + X- —> AgX (s)

2. the silver halide forms a precipitate
3. the precipitate forms fastest with iodoalkane - so must be the most reactive

Question 5

what is bond fission?

Answer 5

breaking a covalent bond

Question 6

bond fission - heterolytic

Answer 6

two different substances are formed

• a positively charged cation
• a negatively charged anion

Question 7

bond fission - homolytic

Answer 7

two electrically uncharged radicals are formed

eg.

X-Y —> X* + Y*

Question 8

what are chain reactions?

Answer 8

When a product causes more reactions to take place

Question 9

chain reactions - initiation

Answer 9

free radicals are produced

Question 10

chain reactions - propagation

Answer 10

free radicals react with molecules and produce new radicals

these go on to react with more molecules, producing even more radicals

Question 11

chain reactions - termination

Answer 11

two radicals react together to form a stable molecule

Question 12

Radical mechanism for reaction of alkanes with halogen - initiation

Answer 12

1. sunlight provides enough energy to break the Cl-Cl bond - photodissociation
2. the bind splits equally - homolytic fission
3. the molecule form two highly reactive free radicals

Cl-Cl —UV—> Cl* + Cl*

Question 13

Radical mechanism for reaction of alkanes with halogen - propagation

Answer 13

1. Cl* attacks a methane molecule, forming a hydrogen halide and a methyl radical

Cl* + CH4 –> HCl + *CH3

2. the new methly radical, *CH3, can attack another Cl2 molecule

CH3 + Cl-Cl —–> CH3Cl + Cl
the new Cl* can attack another CH4 and so on until all the Cl2 and CH4 molecules are wiped out

Question 14

Radical mechanism for reaction of alkanes with halogen - termination

Answer 14

1. if two free radicals join together they make a stable molecule
2. there are lots of possible termination reactions
   eg.
   * CH3 + Cl* —–> CH3Cl

*CH3 + *CH3 —> C3H6

Question 15

what is ozone?

Answer 15

in the stratosphere

contains ozone molecules, O3

Question 16

How is Ozone formed?

Answer 16

when UV radiation from the sun hits oxygen molecules

O2 + hV —-> O + O

O + O2 —-> O3

Question 17

how is the ozone layer constantly being replaced?

Answer 17

O2 + O < —> O3

it is a reversible reaction
ozone is continuously being destroyed and replaced as UV hit the molecules

an equilibrium is set up so concentrations stay fairly constant

Question 18

Why is ozone important?

Answer 18

protects us from UV radiation
- UVB can damage DNA in cells and cause skin cancer (also sunburn)

however in small amounts UV needed
- vitamin D

Question 19

Ozone at ground level (troposphere) causes and effects

Answer 19

due to the effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons
- from vehicle engine and power stations

in industrialised areas ozone mixes with solid particles of carbon to create PHOTOCHEMICAL SMOG

• can cause respiratory problems
• damages plants and materials
• ozone itself is toxic to humans

Question 20

How is Ozone broken down in the stratosphere

Answer 20

by CFCs (chlorofluorocarbons) 
- haloalkanes that have all their hydrogens replaced by Cl and F atoms 

the C-Cl bond can be broken down by high energy UV in the stratosphere to form chlorine radicals
- these act as catalysts in the breakdown of ozone

Question 21

ease of photodissociation of haloalkanes

Answer 21

all haloalkanes have carbon and halogen atoms - UV can break these bonds
- the carbon-halogen bonds split homolytically to create two free radicals

the ease it is broken depends on the halogen

• carbon-iodine most likely to break because it has the lowest bond enthalpy
• carbon-fluorine lest likely to break because it has the highest bond enthalpy

Question 22

How is the ozone layer destroyed by homogeneous catalysis

Answer 22

Chlorine free radicals formed when CFCs are broken down by high energy UV in the stratoshpere
- these free radicals are catalysts
INITATION - CCl3F3 (g) + hv —> CCl2F3 (g) + Cl (g)

These radicals react with ozone to form an intermediate ClO* and an oxygen molecule
PROPAGATION :
Cl* + O3 —> O2 + ClO*
ClO* + O —-> O2 + Cl* (the Cl* radical is regenerates and goes onto destroy more ozone)

the reaction will only terminate if two radicals react together
TERMINATION: Cl* + Cl* —> Cl2

OVERALL REACTION: O3 + O —-> 2O2 ….. and Cl* is the catalyst

Question 23

what other free radical can destroy ozone?

Answer 23

NO from nitrogen oxides

eg. NO2 + hv —> NO* + O