Haloalkanes

Question 1

What is a Nucleophile! and some EXAMPLES

Answer 1

1. species which is a ELECTRON PAIR DONOR!
2. Hydroxide Ions, Water molecules, ammonia molecules….! (due to lone pairs and -ve charge)

Question 2

Describe Hydrolysis Nuceophilic Substisution!

Answer 2

• Halogen atom replaced by -OH group!
  1. nucleophile, OH~, approaches carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom (Figure 3).
  2. This direction of attack by the ion minimises repulsion between the nucleophile and halogen
  3. A lone pair of electrons on the hydroxide ion is donated to the carbon atom.
  4. New bond 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 an alcohol + halide ion

Question 2

Define Nucleophilic Substitution!

Answer 2

1. Where nucleophile will REPLACE the previous functional group, forming that newer nuclephile’s functional group!

Question 3

Haloalkane conversion to alcohol reactions?

Answer 3

• Via aqueous sodium hydroxide.
• Reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good yield of product.

Question 4

Affecting hydrolysis rate of haloalkanes…?

Answer 4

• C-H bond is broken and the -OH nucleophile, replacing the halogen in the haloalkane.
• The rate of hydrolysis depends upon the strength of the carbon-halogen bond in the haloalkane

Question 5

How to measure hydrolysis rate of Primary Haloalkanes!

Answer 5

• In the presence of aqueous silver nitrate. As the reaction takes place halide ions are produced which react with Ag ions to form precipitate of the silver halide.
• Haloalkanes insoluble in water, so reaction carried out in presence of an ethanol solvent, allowing water and the haloalkane to mix and produce a single solution, rather than two layers

Question 6

Bond enthalpy relation to Haloalkane reaction rate?

Answer 6

1. C—F bond is the strongest carbon-halogen bond and the C—I bond the weakest.
2. Less energy is required to break the C—I bond than other carbon-halogen bonds.
3. Hence, iodoalkanes react faster than bromoalkanes, bromoalkanes react faster than chloroalkanes
4. BUT fluoroalkanes are unreactive as a large quantity of energy is required to break the C—F bond.

Question 7

Define Organohalogen!

Answer 7

• THESE are molecules that contain at least one halogen atom joined to a carbon chain!

Question 8

What is the Ozone Layer?

Answer 8

• found at the outer edge of the stratosphere!
• small majority is made of ozone, but enough to absorb most damaging ultraviolet radiation (UV-B) from the Sun’s rays, allowing only a small amount to reach the Earth’s surface.
• Feared that continued depletion of the ozone layer will allow more UV-B radiation to reach the surface, leading to increased genetic damage and a greater risk of skin cancer

Question 9

What occurs in ozone layer?

Answer 9

1. Ozone continually being formed and broken down by the action of (UV) radiation
2. In the steady state, the rate of formation of ozone is the same as the rate at breaking down (reversible reaction and homolyctic fission!!)

Question 10

What causes OZONE LAYER depletion?

Answer 10

1. CFCs (chloroflurocarbons) have a long residence time in the troposphere, taking many years to reach the stratosphere.
2. Once in stratosphere UV radiation provides sufficient energy to break a C-Halogen bond in CFCs by homolytic fission forming radicals.
3. The C—Cl bond has the lowest bond enthalpy and so is the bond that breaks, process is called photodissociation

Question 11

Reactions that occur in Ozone Layer, causing depletion?

Answer 11

1. Homolyctic Fission = CF2Cl2&raquo_space; CF2Cl* + Cl*
2. propagation step 1: Cl* + O3&raquo_space; ClO* + O2
   propagation step 2: ClO* + O&raquo_space; Cl* + O2 The overall equation: O3 + O&raquo_space; 2O2 !!

Question 12

Where can CFCs and HCFCs be found?

Answer 12

1. Refigerants
2. Ari-Conditioning Unis
3. Aerosol Propellants!

Question 13

How else can OZONE LAYER be depleted?

Answer 13

1. Nitrogen oxide radicals are formed naturally during lightning strikes, and aircraft travel in the stratosphere.
2. Nitrogen oxide radicals cause the breakdown of ozone by a mechanism similar to that involving chlorine radicals.

propagation step1: NO* + O3&raquo_space; NO2* + O2
propagation step 2: NO2* + O&raquo_space; NO* + O2