Chapter 15

Question 1

What are haloalkanes?

Answer 1

Compounds which contain carbon, hydrogen and at least one halogen.

Question 2

What do you do when naming haloalkanes?

Answer 2

A prefix is added to the name of the longest chain to indicate the identity of the halogen. When there is more than one halogen present in the structure they are ordered alphabetically.
F - Fluoro Cl- Chloro Br- Bromo I- Iodo

Question 3

Describe the electronegativity of halo alkanes?

Answer 3

The halogen atoms are more electronegative than the carbon atoms so the pair of electrons in the bond are closer to the halogen atoms and the carbon atom where this bonds between carbon hydrogen is polar. The carbon atom has a slight positive charge so can attract species containing a lone pair of electrons.

Question 4

What are nucleophiles?

Answer 4

There are spaces which can donate a lone pair of electrons to form a new covalent bond. They are negative ions which are attracted to an electron deficient carbon atom. These include hydroxide ions, water molecules and ammonia molecules.

Question 5

What happens when a halo alkanes reacts with a nucleophile?

Answer 5

The nucleophile replaces the halogen in a substitution reaction. A new compound is produced containing a different functional group. The reaction mechanism is nucleophilic substitution.

Question 6

What type of halo alkanes undergo nucleophilic substitution?

Answer 6

Primary Haloalkanes

Question 7

What is hydrolysis ?

Answer 7

It is a chemical reaction which involves water or aqueous hydroxide ions which causes the breaking of a bond in a molecule resulting in 2 products.

Question 8

What does hydrolysis of a haloalkane produce?

Answer 8

A compound where the halogen atom is replaced with an OH group to form an alcohol this reaction occurs under reflux.

Question 9

Describe the mechanism of nucleophilic substitution?

Answer 9

1. The nucleophile OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the delta minus halogen atom to minimise repulsion.
2. A lone pair of electrons on the OH- ion is donated to the delta plus carbon atom. A new bond forms between the oxygen of the OH- and the carbon atom.
3. The carbon- halogen bond breaks due to heterolytic fission.
4. The new organic products is an alcohol and a halide ion.

Question 10

What can halo alkanes be converted using into alcohols?

Answer 10

Can you sodium hydroxide which is heated under reflux in order to obtain a good yield.

Question 11

What does the rate of hydrolysis depend on?

Answer 11

The bond strength

Question 12

What happens in hydrolysis of haloalkanes?

Answer 12

The carbon-halogen bond breaks and the OH group replaces the halogen in the haloalkane.

Question 13

Which haloalkanes react the fastest?

Answer 13

Iodoalkanes react the fastest (C–I bond is the weakest, less energy is required to break the bond)

Question 14

Which haloalkanes react the slowest?

Answer 14

Fluoroalkanes react the slowest (C – F bond is the strongest, more energy is required to break it). With Fluoroalkanes being unreactive.

Question 15

In order to state the haloalkanes which reacts the fastest to the slowest?

Answer 15

Iodoalkanes, bromoalkanes, chloroalkanes, fluoroalkanes

Question 16

What is the general equation for hydrolysis of primary haloalkanes?

Answer 16

CH3CH2CH2CH2X +H2O —> CH3CH2CH2CH2OH + H+ + X-

Question 17

What do halide ions react with to form a precipitate of silver halide?

Answer 17

Silver ions

Ag+ (aq) + X- (aq) –> AgX

Question 18

What is a bridging solvent and why is it used?

Answer 18

Approaching solvent allows water and the halo alkanes to mix as halo alkanes are insoluble in water so the reaction is carried out in the presence of an ethanol solvent. The nucleophile is water.

Question 19

Describe the experiment of the hydrolysis of primary haloalkanes?

Answer 19

1. Set up 3 test tubes
   Tube 1: 1 cm³ of ethanol and 2 drops of 1-chlorobutane
   Tube 2: 1 cm³ of ethanol and 2 drops of 1-bromobutane
   Tube 3: 1 cm³ of ethanol and 2 drops of 1-iodobutane
2. Stand the test tubes in a water bath of 60°C.
3. Place a test tube of 0.1 moldm³silver nitrate in a water bath and allow all tubes to reach a constant.
4. Add 1 cm³ of silver nitrate into each test tube and start the timer.
5. Observe the test tubes for 5 minutes and record the time it takes for precipitates to form.

Question 20

State the results found in the hydrolysis of the primary haloalkanes?

Answer 20

1-chlorobutane - white precipitate forms very slowly
1- bromobutane - cream precipitate forms slower than iodo and faster than chloro
1-iodobutane - yellow precipitate forms rapidly

Question 21

Describe the rate of hydrolysis as the carbon halogen bond strength decreases?

Answer 21

As the carbon halogen bond strength decreases the rate of hydrolysis increases.

Question 22

Give the uses of organohalogen compounds

Answer 22

General solvents, dry cleaning solvents, making polymers, flame retardant, refrigerants.

Question 23

What is the ozone layer?

Answer 23

It is found that the outer edge of the stratosphere
(10- 40 km above the Earth surface). Ozone absorbs the biological damaging ultraviolet radiation (UV – B) from the sun rays allowing only a small amount to reach the surface. Continue depletion of the ozone layer will allow more UV-B radiation to reach the surface.

Question 24

What does exposure to UV-B cause?

Answer 24

Sunburn, skin cancer and genetic damage.

Question 25

Describe the breakdown of ozone?

Answer 25

Initially very high energy UV breaks oxygen molecules into oxygen radicals, O via homolytic fission. A steady state is then set up involving oxygen and oxygen radicals in which ozone is being constantly formed and broken down. In this steady state the rate of those formed is the same as the rate of ozone breakdown.

Question 26

What does UV radiation cause for ozone?

Answer 26

In the stratosphere ozone is continually being formed and broken down by the action of UV radiation.

Question 27

What has disrupted the equilibrium of ozone formation and breakdown?

Answer 27

Human activity, especially in CFCs.

Question 28

What are CFCs?

Answer 28

These are chlorofluorocarbons which contain chlorine, fluorine and carbon. CFCs and HCFCs were the most common compounds found in refrigerants, air-conditioning unit and has aerosol propellants.

Question 29

What did Thomas Midgley discover?

Answer 29

That due to the strength of carbon – halogen bonds CFCs are very stable, chemically inert, non-toxic, low flammability.

Question 30

What did Mario Molina and Frank Sherwood Rowland do?

Answer 30

They looked into the impact of CFCs on the earths atmosphere. They concluded that CFCs remain stable until they reach the stratosphere where they break down into chlorine radicals.

Question 31

What do chlorine radicals do?

Answer 31

They catalyse the breakdown of the ozone layer.

Question 32

What happens when CFCs reach the stratosphere?

Answer 32

The UV radiation provides a sufficient amount of energy to break the carbon- halogen bond in CFCs by homolytic fission to form radicals. As C– Cl bond has the lowest enthalpy that bond breaks first (C-F bond is too strong to break).

Question 33

What is the initiation of breakdown due to radiation called?

Answer 33

Photodissociation

Question 34

Tell me about the chlorine radical?

Answer 34

It is a very reactive intermediate which can react with ozone molecules breaking it down into oxygen. In the second propagation step the chlorine radicals is regenerated as it can then attack and remove another ozone molecules in propagation.

Question 35

How many molecules of ozone can 1 CFC molecule break down?

Answer 35

100,000 molecules of ozone

Question 36

Where are nitrogen oxide radicals formed?

Answer 36

They are naturally formed during lightning strikes and aircraft travel in the stratosphere. Nitrogen radicals cause a breakdown of ozone in a similar way to chlorine.

Question 37

State the general equation for any radical, X in the stratosphere?

Answer 37

X* + O3 ---> XO* + O2 XO* + O ---> X* + O2 overall equation : O3+O ---> 2O2

Question 38

What is the Montréal protocol?

Answer 38

It was an agreement to introduce steps of the complete removal of CFCs.

Question 39

What has been developed to remove refrigeration and air-conditioning units?

Answer 39

Coolants have been developed using hydrocarbons, ammonia or CO2.

Question 40

What has been used to replace aerosol form products?

Answer 40

Pump action spray dispensers have been produced. Where aerosols are still being used the propellant is likely to be a hydrocarbon i.e. butane.

Question 41

What are HFCs?

Answer 41

They are hydrofluorocarbons. Which contain no chlorine atoms but are very expensive to manufacture.

Question 42

What a HCFCs?

Answer 42

They are hydrochlorofluorocarbons which break down more easily than CFCs as they contain chlorine atoms some could reach the stratosphere. But as they break down more easily than CFCs they're more likely to break into chlorine radicals before reaching the stratosphere.

Question 43

What are BFRs?

Answer 43

These are brominated flame retardant which are organobromine compounds used in the electronics industry for circuit boards, outer coverings and cables to reduce flammability of the product.