Alkenes

Alkenes are a homologous series which have carbon-to-carbon double bond. They have a general formula C n H 2n . Alkenes are named by taking the longest carbon chain and then specifying where the double bond begins. Eg: But-1-ene and but-2-ene But-1-ene || but-2-ene ||
 * [[image:ibchem4u/but-1-ene.png width="222" height="113"]]

Alkenes have physical properties similar to alkanes, they are //non-polar// or have very //low polarity//. They are //insoluble in polar solvents// such as water. They have //low boiling points//. Alkenes undergo combustion reactions where they react with excess oxygen to form carbon dioxide and water.

Due to presence of the double bond, alkenes are said to be **unsaturated**. This is because the double bond can be broken and other molecules added to the alkene molecule in reactions known as //addition reactions//. Once the double bond is broken and other molecules have reacted with the alkene molecule, it is said to be saturated because it can’t react anymore.

Reactions with hydrogen (hydrogenation) Alkenes react with hydrogen in the presence of nickel catalyst at temperature 180 o C, to produce an alkane. Eg: 
 * Addition reactions of alkenes **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Alkenes react with halogens to produce a di-substituted product. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: ethene reacts with bromine to produce 1,2-dibromoethane <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">
 * Reaction with halogens (halogenation) **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Hydrogen halides such as hydrogen chloride and hydrogen bromide react readily with alkenes to form a halogeno-alkane <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">
 * Reaction with hydrogen halides **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Water does not react readily with alkanes in normal conditions. However, concentrated sulphuric acid can be used to make a reaction happen. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The concentrated sulphuric acid reacts with the alkene to form a hydrogen sulphate addition product; this product reacts with water to give an alcohol and to regenerate the sulphuric acid. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">
 * Reaction with water **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Under high pressures the double carbon bond is broken and the alkene molecules add to each other to form one long molecule. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: under high pressure ethene polymerizes to form polyethene. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">
 * Reaction with themselves (addition polymerization) **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">In order to produce margarine, a mixture of vegetable oils need to be solidified. To do this Some of the carbon-carbon double bonds need to be broken, this is done by hydrogenation reactions.
 * The importance of alkenes in industry **
 * Production of margarine **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Ethanol has many industrial uses. To produce ethanol for industrial purposes, the reaction of ethanol with steam (using phosphoric acid (H 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">PO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">) as a catalyst) is used. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">C 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (g) + H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (g) C 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 5 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">OH (g)
 * Production of ethanol **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Alkene monomers are joined together to make a variety of addition polymers which are of vast importance in today’s industrialised societies. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Plastics for example are made of polymers such as polyethene which is used to make a variety of plastic products such as bags and containers.
 * Addition polymers **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Bromine water is usually used to distinguish between saturated and unsaturated compounds in the laboratory. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">That is why bromine water can be used to distinguish between an alkane (which is a saturated molecule) and and alkene (which is an unsaturated molecule due to the presence of carbon-carbon double bonds). <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">When bromine, which is orange/yellow in colour, is added to an alkene, the colour gradually disappears as the bromine reacts with the unsaturated alkene. eg: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Bromine reacts with the unsaturated ethene molecule to form bromoethane.
 * Test for unsaturation (Is it an alkane or an alkene??) **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">However if bromine solution is added to an alkane, the colour stays the same because alkane is a saturated molecule.

By the end of this lesson you should be able to: __**<span style="color: #0000ff; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Next up- Alcohols **__
 * use equations to show the reactions of alkenes with hydrogen and halgones.
 * use equations to show the reactions of symmetrical alkenes with hydrogen halides and water.
 * outline the polymerization of alkenes.
 * outline the economic importance of the reactions of alkenes
 * use bromine water to distinguish between an alkene and an alkane.
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