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Advanced Level Inorganic Chemistry Periodic Table Revision Notes Part 9. Group 7/17 The Halogens

9.10 Ozone, CFC's and halogen organic chemistry links

What is the connection between halogen compounds such as CFCs are the destruction of the ozone layer? Ozone layer depletion in terms of the free radical reactions involving CFCs chlorine atoms. At the end of the page are links to the chemistry of organic halogen compounds.

PLEASE NOTE KS4 Science GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES are on a separate webpage

INORGANIC Part 9 Group 7/17 Halogens sub-index: 9.1 Introduction, trends & Group 7/17 data * 9.2 Halogen displacement reaction and reactivity trend  * 9.3 Reactions of halogens with other elements * 9.4 Reaction between halide salts and conc. sulfuric acid * 9.5 Tests for halogens and halide ions * 9.6 Extraction of halogens from natural sources * 9.7 Uses of halogens & compounds * 9.8 Oxidation & Reduction - more on redox reactions of halogens & halide ions * 9.9 Volumetric analysis - titrations involving halogens or halide ions * 9.10 Ozone, CFC's and halogen organic chemistry links * 9.11 Chemical bonding in halogen compounds * 9.12 Miscellaneous aspects of halogen chemistry

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots * All 11 Parts have their own sub-indexes near the top of the pages

9.10 Ozone, CFC's and halogen organic chemistry links

CFCs, Ozone and Free Radicals

• CFCs - what is so good about them? (before we get into the problems they cause!)

  • A CFC is a covalently bonded relatively small molecule of carbon, chlorine and fluorine atoms (chlorofluorocarbon).

• If enough energy is supplied by heat or by visible/uv electromagnetic radiation, or the is weak enough, a covalent bond can break in two ways. This illustrated with the molecule chloromethane CH₃Cl.

• The bond breaks unevenly where the electron bond pair can stick with one fragment and a positive and negative ion form.

  • e.g. CH₃Cl ==> CH₃⁺ + Cl^-   (called heterolytic bond fission)

  • shows what happens to the molecule, or

• The bond breaks evenly, where the bonding pair of electrons are equally divided between two highly reactive fragments called free radicals.

  • Free radicals are characterised by having an unpaired electron not involved in a chemical bond.

  • The ^. means the 'lone' electron on the free radical, which is not part of a bond anymore, and wants to pair up with another electron to form a stable bond - that's why free radicals are so reactive!

  • e.g.  CH₃Cl ==> CH₃^. + ^.Cl    (called homolytic bond fission)

  •   shows what happens to the molecule.

  • Homolytic bond fission can occur by molecules hit by uv photons i.e. ultraviolet electromagnetic radiation of quite high energy - great enough to cause homolytic bond fission.

• The chemistry of free radicals is important in the current environmental issue of ozone layer depletion.

• Chlorofluorocarbons (CFC's for shorthand) are organic molecules containing carbon, fluorine and chlorine

• e.g.  dichlorodifluoromethane has the formula CCl₂F₂ (shown above).

• They are very useful low boiling organic liquids or gases, until recently, extensively used in refrigerators and aerosol sprays e.g. repellents.

• They are relatively unreactive, non-toxic and have low flammability, so in many ways they are 'ideal' for the job they do.

• However it is their chemical stability in the environment that eventually causes the ozone problem but first we need to look at how ozone is formed and destroyed in a 'natural cycle'. This presumably has been in balance for millions of years and explains the uv ozone protection in the upper atmosphere - the stratosphere.

• Ozone is formed in the stratosphere by free radical reactions.

  • 'ordinary' stable oxygen O₂ (dioxygen) is split (dissociates) into two by high energy ultraviolet electromagnetic radiation (uv photon energy 'wave packets from Planck's Equation E = uv) into two oxygen atoms (which are themselves radicals) and then a 'free' oxygen atom combines with an oxygen molecule (dioxygen) to form ozone (trioxygen).

    • O₂ + uv ==> 2O^. then O^. + O₂ ==> O₃ 

  • The ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons. The oxygen atom radical can do several things including ...

    • O₃ + uv ==> O₂ + O^. 

  • This last reaction is the main uv screening effect of the upper atmosphere and the ozone absorbs a lot of the harmful incoming uv radiation from the Sun.

  • If the ozone levels are reduced more harmful uv radiation reaches the Earth's surface and can lead to medical problems such as increased risk of sunburn and skin cancer and it also accelerates skin aging processes.

  • There is strong evidence to show there are 'holes' in the ozone layer with potentially harmful effects, so back to the CFC problem for some explanations and solutions!

• The chemically very stable CFCs diffuse up into the stratosphere and decompose when hit by ultraviolet light (uv) to produce free radicals, including free chlorine atoms, which themselves are highly reactive free radicals.

  • e.g. CCl₂CF₂ ==> CClF₂^. + Cl^.  (note the C-Cl bond is weaker than the C-F bond, so breaks first)

• The formation of chlorine atom radicals is the root of the problem because they readily react with ozone and change it back to much more stable ordinary oxygen.

  • O₃ + Cl^. ==> O₂ + ClO^. bye bye ozone! and no uv removed in the process!

  • and then: ClO + O ==> Cl + O₂ , which means the 'destructive' Cl atom free radical is still around!

  • The two reactions above involving chlorine atoms are known as a catalytic cycle because the chlorine atoms from CFC's etc. act as a catalyst in the destruction of ozone. 

• Therefore many countries are banning the use of CFCs, but not all despite the fact that scientists predict it will take many years for the depleted ozone layer to return to its 'original' O₃ concentration and alternatives to CFC's are already being marketed.

  • BUT at least the ozone layer is recovering thanks to some world-wide co-operation and the work of chemists in developing less environmentally harmful alternatives.

• Alternatives to CFCs i.e. HFCs and HCFCs)

  • The idea is to use replacement compounds that are less harmful to the ozone layer.

  • The molecules listed below contain C-H bonds and are broken down in the lower troposphere before they reach the ozone layer in the stratosphere.

  • Hydrochlorofluorohydrocarbons (a HCFC is composed of hydrogen, chlorine, fluorine and carbon atoms)

    • e.g. CH₃CFCl₂ 1,1-dichloro-1-fluoroethane

  • Hydrofluorocarbons (a HFC is composed of hydrogen, fluorine and carbon atoms)

    • e.g. CH₂FCF₃ 1,1,1,4-tetrafluoroethane

  • Alkanes (composed of hydrogen and carbon atoms)

    • e.g. butane CH₃CH₂CH₂CH₃

    • but they are flammable!

  • However, all of these molecules are greenhouse gases and will contribute to global warming!

CHLOROALKANES (halogenoalkanes)

• Alkanes are usually not very reactive unless burned! BUT they will react with reactive chemicals like chlorine when heated or subjected to uv light to form chlorinated hydrocarbons.
  • Despite the reactivity of chlorine you still need something extra to initiate the reaction.
  • A substitution reaction occurs and a chloro-alkane is formed e.g.
  • a hydrogen is swapped for a chlorine and the hydrogen combines with a chlorine atom
  • ethane + chlorine ==> chloroethane + hydrogen chloride
  • C₂H₆ + Cl₂ ==> C₂H₅Cl + HCl
  • + Cl₂ ==> + HCl
  • Chloro-alkanes are useful solvents in the laboratory or industry but though their vapours can be harmful.

Halogen - alkene addition reaction

Used as a test for alkenes: Hydrocarbons are colourless. Bromine dissolved in water or trichloroethane solvent forms an orange (yellow/brown) solution. When bromine solution is added to both an alkane or an alkene the result is quite different. The alkane solution remains orange - no reaction. However, the alkene decolourises the bromine as it forms a colourless dibromo-alkane compound - see equations below.

Ex 1. .... or

ethene + bromine ==> 1,2-dibromoethane

Ex 2. .... or

propene + bromine ==> 1,2-dibromopropane

Ex 3.

cyclohexene + bromine ==> 1,2-dibromocyclohexane

A level Organic Halogen Compound page links

• The structure and nomenclature of halogenoalkanes/haloalkanes/alkyl halides (3 linked pages)

• Aromatic Halogen Compounds

• Acyl/acid chloride structure

• Mechanisms of the reactions of halogenoalkanes/haloalkanes/alkyl halides

• Mechanisms of the reactions of acyl/acid chlorides

• Iodination of ketones e.g. propanone :

PLEASE NOTE KS4 Science GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES are on a separate webpage

WHAT NEXT?

INORGANIC Part 9 Group 7/17 Halogens sub-index: 9.1 Introduction, trends & Group 7/17 data * 9.2 Halogen displacement reaction and reactivity trend  * 9.3 Reactions of halogens with other elements * 9.4 Reaction between halide salts and conc. sulfuric acid * 9.5 Tests for halogens and halide ions * 9.6 Extraction of halogens from natural sources * 9.7 Uses of halogens & compounds * 9.8 Oxidation & Reduction - more on redox reactions of halogens & halide ions * 9.9 Volumetric analysis - titrations involving halogens or halide ions * 9.10 Ozone, CFC's and halogen organic chemistry links * 9.11 Chemical bonding in halogen compounds * 9.12 Miscellaneous aspects of halogen chemistry

A level Inorganic Chemistry Group 7 Halogens Periodic Table Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

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