Chapter 15 Haloalkanes

Question 1

What are haloalkanes

Answer 1

compounds containing carbon hydrogen and at least one halogen
fluoro/ chloro/ bromo/ iodo/

Question 2

Reactivity of haloalkanes

Answer 2

C-halogen bond is polar as halogen atoms are more electronegative than carbon atoms (electron pair is closer to halogens than carbon)

Carbon atom therefore is slightly positive & can attract species containing a lone pair of electrons

Question 3

What is a nucleophile?

Answer 3

species that donate a lone pair of electrons
–> an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond

e.g. hydroxide ions, water molecules and ammonia molecules

Question 4

Haloalkane + nucleophile reaction?

Answer 4

nucleophile replaces halogen in substitution reaction
–> new compound has a different functional group
–> nucleophilic substitution

Question 5

Haloalkane + nucleophile reaction?

Answer 5

nucleophile replaces halogen in substitution reaction
–> new compound has a different functional group
–> nucleophilic substitution

Question 6

Hydrolysis of haloalkane

Answer 6

halogen atom is replaced by hydroxyl group (nucleophilic substitution)
1. nucleophile OH approaches carbon on opposite side of haloalkane
2.lone pair of electrons from hydroxide ion is attracted and donated to delta positive carbon atom (C-halogen polar bond)
3. New bond is formed between oxygen atom of hydroxide ion and C atom
4. Carbon-halogen bond breaks by heterolytic (towards halogen)
5. New Alcohol is made and a halide ion is formed

Question 7

What is hydrolysis?

Answer 7

chemical process involving water/ aqueous solution of hydroxide causing molecule to be split into two products

Question 8

How else can haloalkanes be converted to alcohols

Answer 8

using aqueous sodium hydroxide and heated under reflux

haloalkane + sodium hydroxide –> alcohol + sodium halide

Question 9

Hydrolysis and carbon-halogen bond strength

Answer 9

C-Halogen bond is broken and hydroxyl group replaces halogen during hydrolysis
–> rate of hydrolysis depends upon strength of C-halogen bond in haloalkane

C-F bond is strongest whilst C-I is weakest (less energy is required to break C-I bond)
–> can conclude that fluoroalkanes are unreactive

Question 10

Measuring rate of hydrolysis of primary haloalkanes

Answer 10

C4H9X + H2O –> C4H9OH + (H+) + X-
- carry out in presence of aqueous silver nitrate (halide ions react with silver ions to form precipitate
Ag+ (aq) + X-(aq) –> AgX(s)

Question 11

Results of hydrolysis of haloalkanes

Answer 11

chloroalkane forms a white precipitate very slowly
bromoalkane forms a cream precipitate faster than chloro but slower than iodo
iodoalkane forms a yellow recpipitate rapidly

chloro > bromo > iodo (carbon halogen bond strength)

Question 12

Hydrolysis of primary/ secondary and tertiary haloalkanes

Answer 12

Tertiary haloalkane is hydrolysed first as a tertiary haloalkane reacts by a two step mechanism
–> 1. C-halogen bond is broken by heterolytic fission, forming a tertiary carbocation & halide ion
–>2. hydroxide ion attacks carbocation to form product

Primary haloalkane will react via a one-step mechanism

Question 13

Organohalogen compounds

Answer 13

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

Question 14

Ozone layer and UV-B

Answer 14

• found at the outer edge of the stratosphere (10-40km above surface)
• only tiny fraction of gases making up ozone layer is ozone but enough to absorb most biologically damaging UV radiation (UV-B) from sun’s rays

Question 15

What is UV-B radiation linked to?

Answer 15

radiation linked to sunburns and harmful effects
–>depletion of ozone layer = more UV-B reaching earth = bad for living organisms due to increased genetic damage and greater risk of skin cancer

Question 16

Formation of Ozone O3

Answer 16

ozone is continually formed and broken by UV radiation(initial very high UV breaks oxygen molecules into oxygen radicals O:

O2–> 2O

Steady state is set up involving O2 and oxygen radicals (rate of formation of ozone= rate at which it is broken down

O2 + O <=> O3

Question 17

CFCs

Answer 17

Chlorofluorocarbons and HCFCs (used as refrigerants, air con units and aerosol propellants
–> very stable due to strength of carbon halogen bond
–> CFCs remain stable until the stratosphere where they break down into chlorine radicals (these catalyse the breakdown of ozone layer)

Question 18

How does a CFC deplete the ozone layer

Answer 18

UV radiation in stratosphere is enough to break carbon-halogen bond by homolytic fission to form radicals

Photodissociation:
e.g. CF2Cl2 –> CF2Cl* + Cl*

Question 19

Propagation steps of photodissociation

Answer 19

1. Cl* + O3 –> ClO* + O2
2. ClO* + O –> Cl* + O2
   Step 2 regenerates a chlorine radical (can remove another molecule of ozone in step 1)

Overall reaction:
O3 + O –> 2O2

Question 20

Other substances responsible for ozone depletion

Answer 20

Nitrogen oxide radicals (form naturally during lightning strikes and aircraft travel)

Propagation 1: NO* + O3 –> NO2* + O2
2. NO2* + O –> NO* + O2

Overall reaction: O3 + O –> 2O2