Haloalkaes

Question 1

What are halo alkanes

Answer 1

Compounds containing carbon, hydrogen and at least one halogen atom

Question 2

Primary halolkanes

Answer 2

One carbon attatched to the carbon adjoining the halogen

Question 3

Secondary halo alkane

Answer 3

2 carbons attatched to carbon atom adjoining the halogen

Question 4

Tertiary halolkanes

Answer 4

3 carbons attatched to the carbon atom ajoining the halogen

Question 5

Carbon - halogen polar bond in halolkanes

Answer 5

Halogens are (generally) more electronegative than carbon - so POLAR
The polar positive carbon is electron dificient - can be attatcked by a nucleophile

Question 6

Neucelophiles

Answer 6

Elecron pair donators
OH- , CN- and NH3 are all
React with halolkanes

Water - nucleophile but reacts SLOWLY

Question 7

Hydrolysis

Answer 7

Involves water or an aqueous solution of a hydroxide
Reagent = OH-( e.g. NaOH)
Heat under reflux

Halo alkane — alcohol

Question 8

What happens in hydrolysis of halo alkanes - nucleophilic substitution

Answer 8

Nucleophile approaches carbon (attatched to halogen)on OPPOSITE side
Direction of OH- ion minimises repulsion between nuc and slightly negative halogen
A lone pair of electrons on OH- ion donated to slightly positive carbon
Bond between oxygen of OH- and Carbon atom
The carbon- halogen bond breaks by HETEROLYTIC fission
Halide ion and alcohol produced

Question 9

Water as nucleophile for hydrolysis ?

Answer 9

Slower reaction

Produces H+ ion too

Question 10

Strength of carbon- halogen bond in halolkanes

Answer 10

Weaker bond = faster reaction
As the bonding electrons get further from nucleus - more shielding- bond enthalpies LOWER
Iodoalkanes react faster than bro alkanes

Question 11

Testing strength of carbon halogen bond with silver nitrate ions

Answer 11

Ag+ + X- —- AgX

The silver ions react with halide ions ONCE THEY form , giving a silver halide precipitate

Faster precipitate = faster substitution reaction = more reactive halo alone
AgI precipitate

Question 12

Chlorofluorocarbons

Answer 12

Halo alkanes
Chlorine, fluorine and carbon
Stable, volatile, non flammable , non toxic

Question 13

What is the ozone layer

Answer 13

Outer edge of atmosphere- 10-40km from earths surface
O3
Absorbs UV Ray’s from sun
Formed naturally, when an oxygen is broken down into 2 free radicals by UV

Question 14

Ozone formation

Answer 14

O2 +UV—– O+O

O2+O— O3

Question 15

Ozone depletion equation

Answer 15

O3+ UV– O2+ O

Question 16

Destruction of the ozone layer

Answer 16

Holes forming
CFC’s in atmosphere absorb UV and split to form chlorine free radicals
Free radicals catalyse destruction of ozone- destroy ozone molecule adn then regenerated to destroy more ozone

Question 17

Step 1 of destruction of ozone layer with CFCS

Answer 17

Chlorine free radicals produced
Photo dissociation
Homolytic fission
UV provides sufficient energy to break bonds

Question 18

How do you work out in CFC’s which forms free radicals

Answer 18

Whichever bond has the lowest enthalpy - so it breaks

Question 19

Step 2 of ozone layer destruction ( with chlorine radicals)

Answer 19

These radicals are catalysts - react with ozone to form an intermediate( e.g. Cl0dot) and an O2 molecule
PROPAGATION

CL DOT + o3— o2 + Clo dot
Cl0dot + O (breakdown of oxygen from UV) — o2 + cl2

Question 20

Overall reaction f destruction of ozone by CFC

Answer 20

O3+O— 2O2

Question 21

Nitrogen oxide - destroying ozone

Answer 21

NO’ free radicals from nitrogen oxides can destroy ozone too, Nitrogen oxides are produce by car + air craft engines and thunderstorms

R+ O3– RO + o2
RO+ o– R+ o2

Question 22

Alternatives to CFC’S

Answer 22

HCFC’s — breaks down ozone
HFC’s— no chlorine

ALTERNATIVES,green house gases