Chem Unit 2 (Haloalkanes)

Question 1

Haloalkane is?

Answer 1

Alkane where one or more of the hydrogen atoms have been reeplaced by a halogen atom (e.g. CL, Br or F)

Question 2

Naming Haloalkanes

Answer 2

1) carbon -stem name
2) Br, Cl, F in alphabetical order (2-di, 3-tri)
3) add methyl groups and positions

Question 3

Free radical substitution (chloroalkanes)

Answer 3

1) start with a hydorcarbon e.g. CH4 +Cl2 = CH3Cl + Cl2 (take away one hydorgen atom, add HCl (see if there are any chlorine left to add on to new haloalkane) Keep repeating with each new haloalkane.

Question 4

Formation/Breakage of ozone

Answer 4

present in small amounts at trosophere
broken down/ formed in large concentrations in stratosphere
02 > 0. + 0. (UV radiation breaks down oxygen into two free radicals)
0. + 02 > 03 (oxgyen radical collides with oxygen atom = ozone)
03 > 02 + 0. (UV breaks down ozone too)

Question 5

Why is UV radiation bad?

Answer 5

UV damages biological life on earth (skin cancer)
Natural breaking down of ozone stops more reaching the earth’s surface
Overall its constant rate of breaking down = rate of forming

Question 6

What is the problem with there being other radicals that can break down ozone?

Answer 6

Problem: radicals like chlorine radicals are in compounds that are made in large quantiites becuase they sre useful (CFC’s)
When these diffuse into the stratosphere, the UV radiation breaks the C-Cl bond to produc e achlorine radical which can react with ozone.to produce oxygen
Cl. + 03 > Cl0. + 02
This can react with anoterh ozone molecule
Cl0. + 03 > 202 + Cl.

Question 7

add 2 equations to show ozone > oxygen

Answer 7

203 > 302

Question 8

Why is chlorine breaking down ozone a problem?

Answer 8

Becuasee it ahs a lower activation energy than chlorine, so breaks down ozone way faster than oxygen breaks down and FORMS ozoen so the rate is not constant (destroying the ozone)

Question 9

chlorine radical is a cat…?

Answer 9

catalyst so speeds up the rate fo reaction without being sued up itself

Question 10

Order of reactivity for haloalkanes?

Answer 10

C-F (unreactive- too much energy needed)
C-Cl
C-Br
C-I

Question 11

What are CFC’s and 3 uses of them?

Answer 11

gaseous haloalkanes

blowing foam plastics, AC, refridgerators

Question 12

Do CFC’s occur naturally?

Answer 12

No

Question 13

CFC’s naming?

Answer 13

C- 1) 2) 3)

1) Number of carbon atoms minus 1 (if 0 leave)
2) Number of hydrogen atoms plus 1
3) Number of fluroine atoms

Question 14

Why were CFC’s considered so useful?

Answer 14

stable- didn’t react n aerosal cans/ or with blow-foam plastics/ low boiling point/loads were released into atmosphere

Question 15

What is the problem with CFC’s being stable so they diffuse into stratosphere instead of trosophere?

Answer 15

problem: uv radiation breaks down the C-Cl bond into a chlorine radical (photodissocaition)
CF2Cl2 > (uv) C. F2Cl + Cl. (now reacts with ozone)

Question 16

Initiation step of haloalkanes?

Answer 16

Cl2 > Cl. + Cl.

Question 17

Homolytic fission?

Answer 17

when a covalent bond breaks each atoms gets one electron (unpaired)
makes it very reactive
radical= atom with at least one unpaired electron

Question 18

First propagation step:

Answer 18

hydrocarbon + radical > hydrocarbon -hdrogen (as a radical- donated an electron) + HCL/HF/HBr (radical has gained an electron and joined with the donated hydrogen)
e.g CH4 + Cl. > CH3. + HCl

Question 19

Second Propagation step:

Answer 19

The hydrocarbon radical reacts with Cl2/F2/Br2 to a haloalkane with one atom from Cl2/F2 and the other one becomes a radical.

Question 20

If the reactant is a haloalkane and a radical..

Answer 20

haloalkane radical and a HCl/HBr ect

Question 21

If the reactant is a haloalkane radical and a Cl2 / br2 ect?

Answer 21

haloalkane and a free radical

Question 22

Termination step:

Answer 22

free radicals react with each other (rarely radical haloge atoms on their own)

• unpaired electrons cancel each other out
  1) Look at a haloalkane radical product you’ve formed and react with a radical to make product they’re asking for

Question 23

Overall Equation:?

Answer 23

1) Look at last haloalkane product you’ve formed (asked in the question) (could be from termination step or a propagation step) and the product they want and balance the equation.

Question 24

use La Chatelier’s principle to show how an increase in termperature causes increase in yield of ozone?

Answer 24

As its an endothermic reaction, equilbrium shifts to absorb energy (moves to the right) > yield increases

Question 25

Nucleophilic substitution? properties of carbon-halogen bod?

Answer 25

polar (permanent dipole-dipole)
Halogen atom is $-
hydropgen atom, is $+
makes them able to do substitution

Question 26

What is a nucleophile?

Answer 26

molecule/ion with a lone pair of electrons that donates to froma a covalent bond

Question 27

What are nucleophiles attracted to?

Answer 27

The electron-deficient carbon atom ($+) C-H bond (donates a pair of electrons)
Br is $- (C-Br bond)- attracts more elctrons

Question 28

Conditions for Nucleophilic substitution with hydroxide ions (OH) ?

Answer 28

hydylosis- NaOH in aqueous solution or water

refuxed at low temperature

Question 29

As the size of molecule increases>bond strenth of carbon-halogen bond decreases so..?

Answer 29

hydrolysis is faster

Question 30

What is hydrolysis?

Answer 30

spllitting moleculs using water

Question 31

What is nucleophilic substitution?

Answer 31

replacing a halogen-carbon bond with a nucelophile-carbon bond

Question 32

OH nucelophile has to be in the same position as the …?

Answer 32

Br (halogen atom)

Question 33

For nucleophilic substitution for ammoia conditions:?

Answer 33

• excess ammonia

- sealed container as ammonia is quite volatile

Question 34

Why do we use an excess of ammonia for nucleophilic subsitution?

Answer 34

To ensure the main product is a primary amine rather than a secondary or tertiary amine.

Question 35

Conditons fro nucleophilic substitution by cyanide ions?

Answer 35

Aquerous or ethanol (usually ethanol)
Reagent: KCN
excess CN- would need to be removed

Question 36

Elimination reaction conditions?

Answer 36

• strong concentrated base (dissolved in enthanol)

- high temeorature

Question 37

Competition between nucleophilc subsituiin and elimiantion?

Answer 37

Aqueous/low temp- nucleophilic substituion

Concentrated/high temp- elimination