# The collision theory of reactions

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Homogeneous catalysts
A homogeneous catalyst forms an intermediate compound before breaking down to the final product and reforming the catalyst again. This is why the enthalpy profile shows TWO humps.

CFCs act as homogeneous catalysts in the stratosphere, breaking down ozone. Cl atoms catalyse the reaction, forming the intermediate ClO:

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.

 do assignment 8, p.71

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

H2O(g) + O(g)  HO + HO

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

NO + O3  NO2 + O2

NO2 + O  NO + O2

SL A4: The CFC story

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…

CI 13.1 Halogenoalkanes

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.

Naming halogenoalkanes (haloalkanes)

(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.

Chemical reactions of halogenoalkanes

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

Shorthand is: CH3-Cl + hv  CH3 + Cl (occurs in stratosphere).
Heterolytic fission is more common in lab conditions using polar solvents such as ethanol or ethanol and water. The polar C-Hal bond can break, leaving a negative halide ion and positive carbocation.
CH3 CH3
CH3 C Cl + hv  CH3 C+ + Cl-

H H

2-chloro-2-methylpropane carbocation chloride ion

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

Importance of reaction conditions…for determining how bonds break
Eg. Bromoethane C-Br bonds break:

• 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.

Different halogens, different reactivity.

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.

 

C–Br H–O

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

Heterolytic fission results in IONS and not radicals.

Curly arrows show the movement of electrons (full headed arrows for a pair of electrons…unlike radical reactions).

_

O

H

Halogenoalkanes can give substitution reactions with hydroxide ions and other NUCLEOPHILES. Nucleophiles can donate a pair of electrons to a positively charged carbon atom to create new covalent bonds.

Some common nucleophiles:

 Name Formula Structure showing lone pairs Hydroxide ion OH- _ H__O Ethanoate ion CH3COO- _ CH3__C__O O Ethoxide ion C2H5O- _ CH3CH2__O Water molecule H2O O H H Ammonia molecule NH3 N H H H Cyanide ion CN- _ N C

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.

If X- represents a nucleophile, the nucleophilic substitution process is:
 

C–Hal  C__X + Hal-

X 

Water as a nucleophile
Nucleophiles may be neutral or have negative charge, so long as it has a lone pair of electrons which can form a bond to a carbon atom.

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

The resulting ion loses H+ to form an 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

The overall equation for the reaction of water with a genera; halogenoalkane R__Hal is:
R__Hal + H2O  + R__OH + H+ + Hal-

Ammonia as a nucleophile
A lone pair of electrons on the N (similar to water) attacks the halogenoalkane to produce an AMINE with an NH2 group:
R__Hal + NH3  R__NH3+ Hal- R__NH2 + H+ + Hal-

Using nucleophilic substitution to make halogenoalkanes
Halogenoalkane + OH alcohol
Halogenoalkanes can be made via the reverse reaction of making alcohols; the nucleophile is Hal-.
Eg. 1-bromobutane is made using a nucleophilic substitution reaction between butan-1-ol and Br- ions, in the presence of a strong acid.

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

The overall equation for the reaction is:
CH3CH2CH2CH2OH + H+ + Br-  CH3CH2CH2CH2Br + H2O
 Activity A4.2

 Problems for 13.1 pages 303- 304 questions 1- 9.