ALKENES - structure & chemical properties

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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. These revision notes on the chemical properties reactions and uses of alkenes should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

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5a. The ALKENE hydrocarbons series (unsaturated)
  • Alkenes are a family of hydrocarbons containing at least one covalent carbon...carbon double bond (>C=C<) as well as the single bonds formed between carbon and carbon (C-C) and carbon and hydrogen (C-H).

    • The C=C double bond is called the functional group of the homologous series we call alkenes.

    • Note that the name of all alkene hydrocarbons 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 are another homologous series of organic compounds with the general formula CnH2n where n = 2, 3, 4 etc. giving the formulae C2H4, C3H6, C4H8 etc.

    • A homologous series is a family of compounds which have a general formula (CnH2n for alkenes) and have similar chemical properties because they have the same functional group.

      • In the general formula n = number of carbon atoms in the alkane 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 with a carbon chain containing one double bond only.

      • This general formula only applies to hydrocarbon alkenes with one C=C double bond.

      • The carbon - carbon double bond (>C=C<) is referred to as the functional group of the alkenes.

      • The functional group is a group atoms common to all members of a homologous series that confer a particular set of characteristic chemical reactions on each member of the series.

      • This means you can not only predict the formula of an alkene, but you can also predict their possible chemical reactions and the outcome i.e. the products.

    • The three alkene 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.

    • See section 8. or more on homologous series and the variety of organic compounds

  • 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 and alkanes cannot do this.

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

      • Alkenes have two fewer hydrogen atoms than alkanes.

      • Alkanes are described as saturated because they have no C=C double bond and atoms cannot add to them. like in the bromine water test described below.

    • 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 or gases in the inverted test tube 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 and names
(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'

(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 and alkenes structure and naming (c) doc b(butene has 2 possible structures)

better shown as but-1-ene and but-2-ene

These two different molecular structures for the same molecular formula are called isomers.

(1) alkenes structure and naming (c) doc b, (2)alkenes structure and naming (c) doc b pentene

The full displayed formula for the first four members of the homologous series of ALKENES

These diagrams show ALL the covalent bonds (C-H, C-C and C=C) in alkene molecules

The formulae can also be written as: CH2=CH2, CH3CH=CH2, CH3CH2CH=CH2 and (i) CH3CH2CH2CH=CH2

Note: Just like butene above, there is another molecule of pentene where the double bond is in another alternative position in the carbon chain: (ii) CH3CH2CH=CHCH3

that is, as well as (i) there is also (ii)

Like with butene, this is another example of what we call isomers, molecules of different structure with the same molecular formula, in this case C5H10. You might not need to know the word isomer, but you may need to know these different structures exist, and, be able to spot them with a spot of atom counting!

alkenes structure and naming (c) doc b      another form of C5H10 and below, two examples of C6H12

alkenes structure and naming (c) doc band alkenes structure and naming (c) doc b

The point I'm making here is that they all have the carbon - carbon double bond (>C=C<) somewhere in the molecule making them an alkene molecule!

other alkenes

names not important

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). So two electrons from each carbon atom are shared to form the carbon = carbon double bond. The other carbon electrons pair up with a hydrogen electron to make the single carbon - hydrogen bonds.

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.

is the full 'dot and x' electronic diagram for ethene.

Advanced Chemistry Page Index and Links

5b. Three important addition reactions of alkenes

IMPORTANT comparison: Unsaturated alkenes are much more reactive than alkanes because of the reactivity of the carbon = carbon double bond (saturated alkanes have no double bonds).

In these addition reactions unsaturated alkenes form a new bonds either side of the original double bond and therefore form a saturated compound (no longer has a C=C double bond)

You can think of the double bond as half opening up and a bond breaking in the added molecule and two new bonds form e.g.

(1) Addition of bromine: A test to distinguish between ALKANE and ALKENE hydrocarbons
doc b oil notes What is a simple test for an alkene?

Hydrocarbons are colourless. Bromine dissolved in water or trichloroethane solvent forms an orange (yellow/brown) solution.

When orange bromine solution (bromine water) 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 the word and balanced symbol equations below.


ethene + bromine ====> 1,2–dibromoethane

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

CH2=CH2 + Br2 ====> Br–CH2CH2–Br  or best

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

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.

This double bond makes alkenes much more reactive than alkanes, the bromine water test for alkenes is just one example.

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

propene + bromine ====> 1,2–dibromopropane

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.... or best

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

2nd example of bromine addition to a double bond giving a saturated dibromo compound.

The decolourisation of bromine is a simple and effective chemical test for an alkene – an unsaturated hydrocarbon. The same reaction happens with chlorine (just but Cl instead of Br)

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

a butene + bromine ====> a dibromobutane

+ Br2 ====>

+ Br2 ====>

The addition of bromine to the two butenes giving two slightly different dibromobutanes. Note again you have gone from an unsaturated alkene (can add atoms to it) to a saturated derivative of an alkane (cannot add atoms to it)
(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 corresponding saturated alkane is formed.

  • Addition of hydrogen is 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 (steam) 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 for more details of the process

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 – and a quick summary too

  • Alkenes 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 which is relatively easily partially broken.
    • Therefore,  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.
    • In alkanes all the C–C and C–H single bonds are very strong and not easily broken to allow the alkane molecule to readily undergo a chemical change
  • 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 (not hydrocarbons) and can be hardened to form margarine by adding hydrogen on to some of the carbon=carbon double bonds using a nickel catalyst.
  • 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 if combustion is complete e.g.
    • complete oxidation = complete combustion
    • alkene hydrocarbon + oxygen ===> carbon dioxide + water
    • ethene + oxygen ====> carbon dioxide + water
      • C2H4 + 3O2 ====> 2CO2 + 2H2O
    • propene + oxygen ====> carbon dioxide + water
      • C3H6 + 41/2O2 ====> 3CO2 + 3H2O
      • or   2C3H6 +9O2 ====> 6CO2 + 6H2O
    • 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 smoky flame than alkanes due to less efficient, and more polluting incomplete combustion, so the heat energy release is lower than for alkanes.
    • Free unburned carbon might be released or only half-burned to carbon monoxide. The hydrogen will always be oxidised to water.
      • So, for example, pentene might partially burn as follows ..
      • C5H10  +  5O2  ==>  2C  +  CO  + 2CO2  +  5H2O
      • ... giving two complete combustion products and two incomplete combustion products, smokey!
  • 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, a typical alkene with a double bond, and, readily decolourises bromine water
      • the isomeric cyclohexane does not have a double bond and is a saturated hydrocarbon, and does not react with bromine water.
      • so a simple bromine test could distinguish the two similar colourless liquids,
        • because only the hexene would decolorize the bromine water test reagent.

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