Chapter 15

Question 1

Aliphatic haloalkanes

Answer 1

Halogen is joined to a straight/branched carbon chain

Question 2

How are halogens classified?

Answer 2

Primary, secondary and tertiary ; dependent on the number of carbon groups directly attached to each halogen

Question 3

Are haloalkanes polar?

Answer 3

Yes (unless all of the atoms attached to the carbon are halogesn then it cancels out as it is symmetrical) ; halogen atoms are more electronegative than carbon atoms therefore the electrons are attracted closer to the halogen ; halogen is δ-

Question 4

What are nucleophiles?

Answer 4

Species that donate a lone pair of electrons ; they have a slight negative charge and are attracted to the slight delta positive charge of the carbon attached to the halogen. Nucleophiles are attracted to electron deficient carbon stoms where it donates a pair of electrons to form a new covalent bond.

Question 5

What do nucleophiles include?

Answer 5

Hydroxide ions
Water molcukes
Ammonia molecules

Question 6

How do haloalkanes react with nucleophiles?

Answer 6

Nucleophile replaces the halogen in a substitution reaction ; a new compound is produced containing a different functional group. This is called nucleophilic substitution

Question 7

Hydrolysis?

Answer 7

Chemical reaction involving water or a solution of a hydroxide which causes rhe breaking of a bond in a molecule - resulting in the molecule split into two products

Question 8

Products of hydrolysis of haloalkane

Answer 8

Halogen replaced with an -OH group ; nucleophilic substitution

Question 9

Describe the nucleophilic substitution of an -OH group

Answer 9

OH- is a nucleophile which approaches the slightly positively charged carbon atom from the opposite side of the halogen atom
Direction of attack by OH- ion minimises repulsion between the nucleophile and the delta negative halogen
Lone pair of electrons on the OH- ion is attracted and donated to the delta positive carbon atom
New bond js formed between the oxygen atom of the hydroxide (which contains the lone pair) and the carbon atom
Carbon-halogen bond breaks by heterolytic fission with a new alcohol formed and a halide ion

Question 10

How are haloalkanes converted to alcohols?

Answer 10

Using aqueous NaOH - reaction is very slow at room temperature so the kixture is heated under reflux to obtain a good yield of the alcohol ; left with sodium HALIDE

Question 11

Rate of hydrolysis?

Answer 11

Depends on the strength if the carbon-halogen bond in the haloalkane ; bond enthalpies decreases GOING DOWN THE HALOGEN GROUP
C-F bond is strongest and C-I is weakest ; as electronegativity decreases so does the eneergy requiree to break the C-X bond and thus the rate of hydrolysis increases (time taken decreases)

Question 12

What conclusions can we make from the rate of hydrolysis graph down Group 7?

Answer 12

Iodoalkanes react faster than all of the other haloalkanes

Fluoroalkanes require the most energy to break their bond so they are unreactive

Question 13

What other factor affects rate of hydrolysis?

Answer 13

Tertiary haloalkanes are hydrolysed the fastest while hydrolysis in primary haloalkane is the slowesst - main reason lies with the reaction mechanism ; increased stability of the tertiary carbocation compared with that of the primary carbocation

Question 14

Organohalogens

Answer 14

Compounds are molciles that contain at least 1 halogen atom joined to a carbon chain

Question 15

Practical uses of organohalogens

Answer 15

In many pesticides, dry cleaning solvents, making polymers, flame retardants and refrigerants

Question 16

Ozone layer

Answer 16

Outer edge of stratosphere - only a tiny fraction of gases in ozone are ozone but this is enough ti absorb the UV-B damaging rays from the sun only allowinf a small amount to reach the Earth’s surface ; ozone depletion is causinf mire UV-B radiation to reach the surface causing more sunburn and increased risk of cancer (genetic damage)

Question 17

What is happening continually in stratosphere?

Answer 17

Ozone being formed and broken sown by UV Radiation
O2 -> 2O (O2 gas into 2 oxygen radicals)
Steady state is set up involing O2 and the ozone radicals in which rate of formation of ozone = rate at which it is broken down
O2 + O ->

Question 18

Chlorofluorocarbons

Answer 18

Disrupt this equilibrium of breaking down and forming the ozone layer ; CFCs are very stable due to strength of the C-X bonds
Only contain carbon fluorine and chlorine

Question 19

What happens to CFCs in stratosphere?

Answer 19

They break down forming chlorine radicals which catalyse depletion of ozone layer

Question 20

How do CFCs deplete the ozone layer?

Answer 20

CFCs have long residence time in troposphere due to strength of C-X bond - once in strayosphere UV radiation provides enough energy to break a C-X bond by homolytic fission to form radicals.
C-Cl bond has lowest enthalpy thus it breaks

Question 21

Photodissociation (CF2Cl2)

Answer 21

CF2Cl2 -> CF2Cl. + Cl.

Question 22

Propagation steps

Answer 22

Chlorine radical is very reactive and thus reacts with an ozone molecule breaking down ozone into oxygen
1) Cl. + O3 -> ClO. + O2
2) ClO. + O -> Cl. + O2
Overall ewuation is O3 + O -> 2O2
REGENERATES CL. WHICH CAN ATTACK AND REMOVE ANOTHER MOLECULE OF OZONE IN STEP 1 ; REPEAT OVER AND OVER IN A CHAIN REACTION FOR UP TO 100000 molcules of ozone

Question 23

Are CFCs only responsible?

Answer 23

NO. Radicals are also formed naturally during lightning strikes and aircraft travel
They also cause breakdown
1) NO. + O3 -> NO2. + O2
2) NO2. + O -> NO. + O2
Overall equation is 
O3 + O -> 2O2
RADICALS ARE CATALYSTS