The collision theory of reactions

Cl + O₃  O₂ + ClO  intermediate

ClO + O  Cl + O₂

O₃ + O  O₂ + O₂  overall change

A single Cl atom can catalyse the reaction of many ozone molecules through a catalytic cycle.

Industry uses mostly heterogeneous catalysts. However homogeneous catalysts can be more specific and controllable.

E
^{Rhodium (aq)}
g. Methanol ethanoic acid

Conversion is 99% with soluble rhodium compounds.

 Do problems for 10.5 p.243 questions 1 and 2.
SL: Other ways ozone is removed

Radicals such as hydroxyl and nitrogen monoxide can destroy ozone, as well as chlorine and bromine.

In general: X + O₃  XO + O₂

XO + O  X + O₂

Overall reaction: O + O₃  O₂ + O ₂

Hydroxyl radicals (HO) form in the stratosphere when water molecules react with oxygen. The reaction with ozone is:

HO + O₃  HO₂ + O₂

HO₂ + O  HO + O₂

The reformed HO radicals can react with more ozone, in a catalytic cycle.

Nitrogen monoxide (NO) forms nitrogen dioxide and dioxygen when it reacts with ozone. NO and NO₂ are relatively stable radicals which can be collected in ordinary ways.

1. 
   Write an equation to show the formation of HO radicals from O atoms and water.
   

2. 
   Write equations to show how nitrogen monoxide can destroy ozone in a catalytic cycle.
   

NO₂ + O  NO + O₂

In the early 1970s there was concern about jet aircraft releasing

NO in their exhausts. Levels were not significant at the time. In 1974 CFCs became a concern…

 read about it p.71-74 and do assignments 9 and 10.

Are man-made compounds with one or more halogen atoms (F, Cl, Br, I) attached to a carbon atom. The attached halogen changes the chemical properties of alkane chains…they are very unreactive, and so have been very useful to humans.

(similar rules to naming alcohols, just add the halogen as a prefix):

• 
  halogens are in alphabetical order.
  
• 
  lowest numbers possible are used.
  

CH₃CH₂CH₂Cl is 1-chloropropane

CH₃CHClCH₂Cl is 1,2-dichloropropane

CH₃­CHBrCH₂CH₂Cl is 3-bromo, 1-chlorobutane.

CH₃CHICHBrCH₂Cl 2-bromo,1-chloro,3-iodobutane

2-bromo, 3-chloro, 1-iodopentane

CH₂ICHBrCHClCH₂CH₃
Physical properties of halogenoalkanes

• 
  immiscible with water
  
• 
  The bigger the halogen atom /the larger the number of halogen atoms the higher the boiling point.
  
• 
  Larger halogen atoms (Br or Cl) cause greater environmental damage than smaller halogen atoms (F); this is important when designing replacements for CFCs.
  

Carbon –halogen (C-Hal) bonds can break either homolytically or heterolytically.

Homolytic Fission forms radicals eg when a halogenoalkane absorbs radiation of the right frequency.

H H
H C Cl + hv  H C + Cl

H H

H H

(negatively charged substances may react with the positive carbocation causing a substitution reaction).

• 
  Heterolytically, forming ions when dissolved in a polar solvent (say a mixture of ethanol and water) BUT
  
• 
  Homolytically, in the gas phase at high temp. or when dissolved in a non-polar solvent, such as hexane.
  
• 
  
  

All reactions with halogenoalkanes involve breaking the C-Hal bond. The C-F bond is the strongest (bond enthalpy 467 kJmol^-1) and therefore the hardest to break, whereas the C-I bond is relatively weaker (228 kJmol^-1) and therefore easier to break. C-Hal bonds get weaker, and so more reactive, down group 7.

Chloro compounds are fairly unreactive and remain in the troposphere long enough to reach the stratosphere, where they react with and destroy the ozone layer.
Substitution reactions of halogenoalkanes
Halogenoalkanes can hydrolysed by hydroxide ions to form alcohols.
Eg. Bromobutane forms butanol:
CH₃–CH₂–CH₂–CH₂–Br + OH^-  CH₃–CH₂–CH₂–CH₂–OH + Br^-

The C-Br bond is polar The oxygen atom on OH^- is –vely charged.

^^ 

The partial positive charge on the carbon atom attracts the negatively charges oxygen of the hydroxide ion. A lone pair of electrons on the O atom forms a bond with the C atom as the C^__Br bond breaks.

H H H H H H H H _

H^__C^__C^__C^__C^__Br H^__C^__C^__C^__C^__O^__H + Br

H H H H H H H H

O

The carbon atom attacked by the nucleophile may be part of a carbocation and carry a full positive charge, or it may be part of a neutral molecule (as in the above example with bromobutane) and carry a partial positive charge as a result of bond polarisation.

Eg. Water has 2 lone pairs of electrons on the O atom. First it attacks the halogenoalkane (bromobutane in this case):

H H H H H H H H _

H^__C^__C^__C^__C^__Br H^__C^__C^__C^__C^__O^__H + Br

H H H H H H H H H

O

H H

H H H H + H H H H

H^__C^__C^__C^__C^__O H^__C^__C^__C^__C^__O^__H + H⁺

H H H H H H H H H

Ist step: H⁺ ions bond to O atom on the alcohol:

H H H H H H H H +

H^__C^__C^__C^__C^__O H^__C^__C^__C^__C^__O^__H

H H H H H H H H H

H⁺

This gives the C atom to which the O is attached a greater partial positive charge. It is now more readily attacked by Br^- ions, forming bromobutane.
H H H H + H H H H

H^__C^__C^__C^__C^__O H^__C^__C^__C^__C^__Br + H₂O

H H H H H H H H H

_

Br