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Doc Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

5. ALKENES – unsaturated hydrocarbons – their chemical reactions

The alkenes are a series of hydrocarbon molecules (made of carbon and hydrogen atoms). They are referred to as 'unsaturated' hydrocarbons because they have a carbon – carbon C=C double bond and other atoms can add to them via simple addition reactions. The physical properties and chemical reactions of alkenes with hydrogen (to form alkanes), bromine to form a dibromoalkanes (used as a test for alkenes), polymerisation (self–addition of alkene molecules to form polymers like polyethene and with oxygen (combustion, burning) are fully described with word and symbol equations. These notes on alkenes are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry and KS4 science courses.

Index of KS4 Science GCSE/IGCSE Chemistry Oil & Organic Chemistry Pages: 1. Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3. ALKANES – saturated hydrocarbons and combustion : 4. Pollution, carbon monoxide, nitrogen oxides, what makes a good fuel?, climate change–global warming : 5. Alkenes – unsaturated hydrocarbons : 6. Cracking – a problem of supply and demand, other products : 7. Polymers, plastics, uses and problems : 8. Introduction to Organic Chemistry – Why so many series of organic compounds? : 9. Alcohols – Ethanol – properties, reactions, biofuels : 10. Carboxylic acids and esters : 11. Condensation polymers, Nylon & Terylene, comparing thermoplastics, fibres and thermosets : 12. Natural Molecules – carbohydrates – sugars – starch : 13. Amino acids, proteins, enzymes & chromatography : 14. Oils, fats, margarine and soaps : 15. Vitamins, drugs–analgesic medicines & food additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free radicals : 17. Extra notes, ideas and links on Global Warming and Climate Change : Multiple Choice and Gap–Fill Quizzes: m/c QUIZ on Oil Products (GCSE/IGCSE easier–foundation–level) : m/c QUIZ on Oil Products (GCSE/IGCSE harder–higher–level) : IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry : and 3 Easy linked GCSE/IGCSE Oil Products word–fill worksheets

ALL my Advanced Level Organic Chemistry revision notes

click me! A Level notes on the structure and naming of ALKENES click me!

 
5a. The ALKENE hydrocarbons series (unsaturated)
  • Alkenes are a family of hydrocarbons containing a carbon...carbon double bond (>C=C<) as well as single bonds.

    • Note that the name ends in ...ene eg , ethene, propene, butene etc.

    • All alkenes have single C–H bonds, and from propane onwards, C–C single bonds as well as the characteristic functional group C=C bond.

    • A hydrocarbon, e.g. an alkene, can only consist of carbon and hydrogen atoms.

      • If another type of atom (element) is present in the molecule it cannot be a hydrocarbon e.g. alcohols, esters and carboxylic acids contain oxygen atoms.

  • Alkenes have the general formula CnH2n where n = 2, 3, 4 etc. giving the formulae C2H4, C3H6, C4H8 etc.

    • The three formula quoted above match the three names above.

    • As with naming all organic molecule series in alkenes eth.. means 2 carbon atoms in the chain, prop... means 3 and but.. means 4 etc.

    • In the general formula n = number of carbon atoms in the alkene molecule (n = 2, 3, 4 etc.) and from the general formula you can deduce the number of hydrogen atoms, hence the complete molecular formula for ANY alkene.

  • Alkenes are called unsaturated molecules because two atoms can join onto half of the carbon – carbon double bond when it opens up. In other words they potentially have spare bonds to link up with other atoms.

    • The word unsaturation implies the fact that the carbon atoms are not bonded to the maximum number of atoms they can be.

    • ie 2 atoms can join onto the two atoms of the carbon–carbon double bond in alkenes.

      • C=C + X–Y ==> X–C–C–Y

        • though when studying and writing the structural/displayed formula equations, make sure each carbon atom forms a total of 4 bonds, not 2 or 3!

  • The first three in the alkenes series are shown in the section below and are colourless smelly gases.

  • Alkenes are extremely reactive and important compounds in the chemical industry and are converted into very useful compounds e.g. plastics and alcohols.

  • You can demonstrate cracking in the laboratory by heating paraffin grease over an aluminium oxide catalyst at 400–700oC, and collecting the smaller gaseous hydrocarbon molecules over water – easily shown to be flammable!

    • This experiment needs to be done as a teacher demonstration – most carefully!

    • Any hydrocarbon liquids collected in the bottle should decolourise bromine water – the test for alkenes.

  • Examples of alkene structure

  • (1) is the molecular formula: a summary of the totals of each atoms of each element in one molecule

  • (2) is are 'shorthand' or 'condensed' versions of the full structural formula or displayed formula (3)

  • (3) is called the full structural formula or displayed formula:

    • The displayed/structural formula shows how all the atoms are linked with the covalent bonds (the dashes –) ie the C–C bonds and the C–H atom bonds.

    • Note that carbon must form four bonds (C–C single bond or a C=C double bond) and hydrogen forms one bond (C–H).

    • It is the presence of the covalent carbon = carbon double bond (C=C) which makes alkenes unsaturated hydrocarbon molecules.

    • More notes on molecules and covalent bonding

Examples of the molecular formula and molecular structure of ALKENES
(1)doc b oil notes, (2)doc b oil notes, (3)doc b oil notes

the C=C is referred to as the carbon – carbon  'double bond'

ethene
(1)doc b oil notes, (2a)doc b oil notes, (2b)doc b oil notes, (3)doc b oil notes propene
(1)doc b oil notes, (2)doc b oil notes butene

The covalent bonding diagram for the alkene ETHENE

(c) doc b Two atoms of carbon (2.4) combine with four atoms of hydrogen (1) to form ethene C2H4 (only the outer shell of carbon's electrons are shown).

simplified 'dot and cross' electronic diagram for the covalently bonded ethene molecule

Electronically, hydrogen (1 outer electron) becomes like helium (2 outer electrons, full outer shell) and carbon (2.4, 4 outer electrons) becomes like neon (2.8, full outer shell of 8 electrons), so ALL the hydrogen and carbon atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

With only four hydrogen atoms in the ethene molecule, two carbon atoms must share four electrons to form a double covalent bond (C=C).

The molecule can be shown as (c) doc b with one carbon = carbon double bond and four carbon – hydrogen single covalent bonds.

 
Advanced Chemistry Page Index and Links

5b. Three important reactions of alkenes

(1) Addition of bromine: A test to distinguish between ALKANE and ALKENE hydrocarbons
doc b oil notes
  • 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.
  doc b oil notes doc b oil notes doc b oil notesarrow doc b oil notes.... or

CH2=CH2 + Br2 ==> Br–CH2CH2–Br

doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

ethene + bromine ==> 1,2–dibromoethane

colour of mixture changes from orange to colourless

  • Alkenes are unsaturated molecules, atoms can add to them via the C=C double bond, so a reaction occurs.

  • The double bond opens up and new carbon – bromine bonds (C–Br) are formed.

  • Alkanes are saturated – no double bond – and atoms cannot add – so no reaction.

doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes.... or

 

CH3CH=CH2 + Br2 ==> CH3–CHBr–CH2Br

doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

propene + bromine ==> 1,2–dibromopropane

2nd example of bromine addition to a double bond.

The decolourisation of bromine is a simple and effective chemical test for an alkene – an unsaturated hydrocarbon.

This reaction is NOT given by alkanes because they do NOT have a carbon –carbon double bond.

(2) Alkenes can add hydrogen to form a saturated alkane molecule
CH3–CH=CH2 + H2 ==> CH3–CH2–CH3

doc b oil notes  doc b oil notes H2 doc b oil notes doc b oil notes

propene + hydrogen ==> propane

  • Alkenes will react with hydrogen gas over a nickel catalyst.

  • The reaction process is used to make margarine from vegetable oils.

(3) Addition of water to make alcohols

CH2=CH2 + H2O ==> CH3–CH2–OH

+ H2O ==>

ethene +water ===> ethanol

eg the alcohol ethanol can be made by passing ethene gas and water vapour over an acid catalyst at 300oC.

This is an example of an addition reaction and a hydration reaction because it involves the addition of water to another molecule.

see Alcohols, Ethanol, manufacture, reactions, biofuels & alternative fuels

Advanced Chemistry Page Index and LinksFor (4) Polymer formation – Polymerisation See Part 7. Polymers–Plastics section

and Part 11. More on addition polymers and condensation polymers

5c. More on ALKENESunsaturated hydrocarbons – a quick summary

  • These cannot be obtained directly from crude oil and must be made by cracking (see section 6 cracking notes).
  • The unsaturated hydrocarbons form an homologous series called alkenes with a general formula CnH2n
    • Unsaturated means the molecule has a C=C double bond to which atoms or groups can add.
  • Alkene examples: Names end in ...ene
    • ethene
      • C2H4 or or
    • propene
      • C3H6 or or or
    • butene
      • or
  • The alkenes are more reactive than alkanes because of the presence of the carbon = carbon double bond. The alkenes readily undergo addition reactions in which one of the carbon = carbon double bonds breaks allowing each carbon atom to form a covalent bond with another atom such as hydrogen or bromine.
  • Examples of addition reactions are: with hydrogen under pressure and in the presence of a nickel catalyst to form an alkane
    • + H2 ==>

      • Advanced Chemistry Page Index and Linksethene + hydrogen ==> ethane

    • + H2 ==>
      • propene + hydrogen ==> propane
  • Alkenes react by 'addition' with bromine and decolourises the orange bromine water because the organic product is colourless, and this is a simple test to distinguish an alkene from an alkane.
  • Vegetable oils contain unsaturated fats and can be hardened to form margarine by adding hydrogen on to some of the carbon=carbon double bonds using a nickel catalyst. The process is called hydrogenation,
  • Alkenes can add to themselves by addition polymerisation to form 'plastic' or polymeric materials.
  • Alkenes readily burn, just like alkanes, to give carbon dioxide and water.
    • e.g. ethene + oxygen ==> carbon dioxide + water
    • C2H4 + 3O2 ==> 2CO2 + 2H2O
    • However, they are NOT used as fuels for two reasons.
      1. They are far too valuable for use to make plastics, anti–freeze and numerous  other useful compounds.
      2. They burn with a more smokey flame than alkanes due to less efficient and more polluting combustion.
  • Alkenes are isomeric with cycloalkanes e.g. the molecular formula C6H12 can represent hexene or cyclohexane
    • hexene CH3–CH2–CH2–CH2–CH=CH2  or  cyclohexane  
    • and note that ....
      • hexene is an unsaturated hydrocarbon with a double bond,
      • the isomeric cyclohexane does not have a double bond and is a saturated hydrocarbon,
      • so a simple bromine test could distinguish the two similar colourless liquids,
        • because only the hexene would decolorize the bromine water test reagent.

Advanced Chemistry Page Index and Links


Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier–foundation–level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder–higher–level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

ALSO gap–fill ('word–fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap–fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

ALL my Advanced Level Organic Chemistry revision notes

click me! A Level notes on the structure and naming of ALKENES click me!


Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences Chemistry revision notes for GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising AQA GCSE Science AQA GCSE Chemistry, Edexcel GCSE Science, Edexcel GCSE Chemistry, OCR 21st Century Science Chemistry, OCR Gateway Science chemistry, WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10)

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