Haloalkanes

Question 1

What are haloalkanes and how to you name them?

Answer 1

1. Haloalkanes are compound containing the elements carbon, hydrogen and at least one halogen
2. A prefix is added to the name of the longest chain to indicate the identity of the halogen
3. When two or more halogens are present in a structure they are listed in alphabetical order
   - Aliphatic haloalkanes can also be named as primary, tertiary and secondary school

Question 2

What is the carbon-halogen bond like?

Answer 2

1. Haloalkanes have a carbon-halogen bond in their structure
2. Halogen atoms are more electronegative than carbon atoms
3. The electron pair in the carbon-halogen bond is therefore closer to the halogen atoms than the carbon atom
4. The carbon-halogen bond is polar
   (Why most halogenoalkanes dissolve in ethanol)

Question 3

Describe the reactivity of haloalkanes

Answer 3

1. In haloalkanes the carbon atom has a slightly positive change and can attract species containing a lone pair of electrons, species that donate a lone pair of electrons are known as nucleophiles
2. When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction
3. A new compound is produced containing a different functional group and the reaction mechanism is nucleophilic substitution
4. Primary haloalkanes undergo nucleophilic substitution reactions with a variety of different nucleophiles to produce a wide range of different compounds
5. Substitution is a reaction in which one atom or group of atoms is replaced by another atom or group of atoms

Question 4

What is a nucleophile? What are some examples?

Answer 4

1. A nucleophile is 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
2. Hydroxide ions (OH-), water molecules (H2O), ammonia molecules (NH3)

Question 5

What is hydrolysis?

Answer 5

It is a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule, and this results in the molecule being split into two products

Question 6

What happens in the hydrolysis of a haloalkane?

Answer 6

• The halogen atom is replaced by an -OH group, and this is an example of a nucleophilic substitution reaction
  1. The nucleophile OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
  2. This direction of attack by the OH- ion minimises repulsion between the nucleophile and the delta minus halogen atom
  3. A lone pair of electrons on the hydroxide ion is attracted and donated to the delta plus carbon atom
  4. A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
  5. The carbon-halogen bond breaks by heterolytic fission
  6. The new organic product is na alcohol and a halide ion is also formed

Question 7

How can haloalkanes also be converted into alcohols?

Answer 7

• Using aqueous sodium hydroxide

- The reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good yield of product

Question 8

How strong is the carbon-halogen bond?

Answer 8

1. In hydrolysis the carbon-halogen bond is broken and the -OH group replaces the halogen in the haloalkane
2. The rate of hydrolysis depends upon the strength of the carbon-halogen bond in the haloalkane (higher bond enthalpy stronger bond)

Question 9

Which carbon-halogen bond is the strongest?

Answer 9

1. The C-F bond is the strongest carbon-halogen bond and the C-I bond is the weakest, therefore less energy is required to break the C-I bond than other carbon-halogen bonds
   - Using bond enthalpies we can predict that:
2. Iodoalkanes react faster than bromoalkanes
3. Bromoalkanes react faster than chloroalkanes
4. Fluoroalkanrs are unreactive as a large quantity of energy is required to break the C-F bond

Question 10

How do you measure the rate of hydrolysis of primary haloalkanes?

Answer 10

• To compare rate of hydrolysis of 1-chlorobutane, 1-bromobutane, 1-iodobutane
• Place test tubes in water bath and add aqueous silver nitrate which has also been placed in water bath so that all tubes reach a constant temperature and observe for 5 mins and record the time taken for the precipitate to form
• As the reaction takes place halide ions, X- (aq) are produced which react with Ag+ (aq) ions to from a precipitate of the silver halide
• Rate of hydrolysis INCREASES as the strength of the carbon-halogen bond DECREASES

Question 11

What happens in the silver nitrate test?

Answer 11

1. The nucleophile in the reaction is water, which is present in the aqueous silver nitrate
2. Haloalkanes are insoluble in water, and the reaction is carried out int he presence of an ethanol solvent
3. Ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers

Question 12

What are some uses of organohalogen compounds?

Answer 12

1. Organohalogen compounds are molecules that contain at least one halogen atom joined to a carbon chain
2. They are used in many pesticides, general solvents, making polymers and flame retardents
3. They are rarely found in nature and as they are not broken down naturally in the environment, they have become the focus of some concern
4. Halogen-conating alkenes are used to make: PVC Poly(cholroethene) and poly(tetrafluoroethene)

Question 13

What is the ozone layer?

Answer 13

1. It is found at the outer edge of the stratosphere, at a height that varies from about 10 to 40km above the Earth’s surface
2. Only a tiny fraction of the gases making up the ozone layer is ozone, but this is enough to absorb most o the biologically damaging ultraviolet radiation (called UV-B) from the suns layer, allowing only a small amount to reach the Earth’s surface
3. UV-B radiation is the radiation most commonly linked to sunburn
4. It is feared that continued depletion of the ozone layer will allow more IV-B radiation to recite eh Earth’s surface and this could lead to increased genetic damage and a greater risk of skin cancer in humans

Question 14

How is ozone formed?

Answer 14

1. In the stratosphere, ozone is continually being formed and broken down by the action of UV radiation
2. Initially very high energy UV breaks oxygen molecules into oxygen radicals
3. A steady state is then set up into;vomg O2 and the oxygen radicals in which ozone forms and then breaks down
   - O2–> 2O
4. In this steady state, the rate of formation fo ozone is the sae as the rate at which it is broken down
   - O2 + O (equilibrium arrow) O3
5. Human activity, especially in the production and use of chlorofluorocarbons (CFCs) has upset this delicate equilibrium

Question 15

What are CFCs?

Answer 15

• CFCs and HCFCs were commonly used in air-conditoning units, aerosol and propellants as they are unareaction, non-almmabe and non tori
• CFCs are very stable because of the strength of the carbon-halogen bonds within their molecules
• They remain stable until they reach the stratosphere and here the CFCs begin to breakdown, forming chlorine radicals which are though to catalyse the breakdown of the ozone layer
• They were used to make expanded polystyrene packaging and as refrigerants, because they have a suitable volatility and so can be relate evaporated and recondensed

Question 16

How do CFCs deplete the ozone layer?

Answer 16

1. The stability of CFCs due to the strength of their carbon-halogen bonds, means that cFCS have long residence time in the troposphere
2. It may take them many years to reach the stratosphere
3. Once in the stratosphere UV radiation provides sufficient energy to break a carbon-halogen bond found in CFCs by homiletic fission to form radicals
4. The C-CL and has the lowest bond enthalpy and so is the bond that breaks

Question 17

What is photodissociation?

Answer 17

• As radiation initiates the breakdown, this is called photodissociation and the photodissociation of CF2Cl2 is:
• CF2Cl2 –> CF2Cl dot + Cl dot

Question 18

What happens to the chlorine radical formed?

Answer 18

1. It is a very reactive intermediate
2. It can reach with an ozone molecule, breaking down the ozone into oxygen and occurs in two steps
   - Propagation step 1: C dot + O3 –> ClO dot + O2
   - Propagation step 2: ClO dot + O –> Cl dot + O2
   - Overall: O3 + O –> 2O2
3. Propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone on propagation step 1
4. The two propagation steps repeat in a cycle over an dover gain in a chain reaction
5. It has been estimated that a single CFC molecule can promote the breakdown of 100,000 molecules of ozone

Question 19

What other things are depleting the ozone?

Answer 19

1. Other radicals also catalyse the breakdown of ozone
2. Nitrogen oxide radicals are formed naturally during lightning strikes, and also as a result of aircraft travel in the stratosphere
3. Nitrogen oxide radicals cause the break down of ozone by a mechanism similar to that involving chlorine radicals
   - Propagation step 1: NO dot + O3 –> NO2 dot + O2
   - Propagation step 2: NO2 dot + O –> NO dot + O2
4. The overall equation is the same as with chlorine radicals, showing that the radicals act as a catalyst for the process
   - O3 + O –> 2O2

Question 20

What alternatives to CFCs have been suggested?

Answer 20

1. Chemists have been involved in finding alternatives to these chemicals which have much less harmful effect on the Earth’s atmosphere
2. Alternative to CFCs include HCFCs which have a C-H bond making them more degradable in the atmosphere
3. Other alternatives include hydrocarbons and HFCs, which contain no chlorine atoms
4. CO2 is used as an alternative blowing agent for expanded polystyrene

Question 21

What is meant by the term hydrolysis?

Answer 21

The splitting/breaking of a C-X/bonds by adding/with water

Question 22

What is the simplest ionic equation with the precipitate formed of iodine is silver nitrate?

Answer 22

Ag+ + I- –> AgI

Question 23

Why would you not do the silver nitrate test with 1-fluorobutane?

Answer 23

The AgF is soluble and so no precipitate would form