Summary of Chemistry Textbook - Section 11.4 Reactions of Alkenes

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  Summary of Chemistry Textbook: Section 11.4  –  Reactions of Alkenes Addition reactions of alkenes -   Alkanes   substitution reactions -   Alkenes   addition reaction -   In an addition reaction, two substances react together to form a single product (the reactants add together) -   A double carbon bond has a bond enthalpy of 612 kJ mol -1  and the single carbon bond has a bond enthalpy of 348 kJ mol -1  -   Therefore less energy required to break one half of a double bond than is needed to break a single bond in an alkane, making alkenes much more reactive than alkanes -   The double bond can be converted to a single bond, allowing extra atoms or groups of atoms to be added to the molecule o   E.g. ethene reacts readily with chlorine to produce 1,2-dichloroethane o   H 2 C=CH 2(g)  + Cl 2     CH 2 ClCH 2 Cl (l)  -   Although ethane reacts with chlorine only in sunlight or at high temperatures, the high reactivity of ethene’s double bond means that chlorine readily reacts with ethene at room temperature -   The addition of chlorine occurs ‘across the double bond’ so that one chlorine atom attaches to each carbon atom. -   Alkenes also undergo an addition reaction with hydrogen   hydrogenation = formation of a saturated hydrocarbon  –  an alkane -   Nickel, platinum and palladium all used as catalysts in hydrogenation -   Hydrogenation reaction is particularly useful in the food industry -   Margarine is a butter substitute that is made from vegetable oils -   Vegetable oils = mixture of polyunsaturated fats which are recognised as a healthy substitute for butter -   Hydrogen is added across some of the double bonds in the polyunsaturated fats and the oils are converted to a soft, low melting point solid that is more like butter -   Alkenes can also react with hydrogen halides and with water -   Both these reactions are addition reactions -   Reaction between an alkene and a hydrogen halide provides a more controlled way to produce a particular halogenoalkane than the substitution reaction between an alkane and a halogen -   Double bond controls the position of the halogen atom in that it will be on one of the two carbon atoms that were involved in the double bond -   An important industrial addition reaction is the reaction between ethene and water -   This process is called hydration and it is very important in the manufacture of ethanol -   Ethene and steam are passed over a catalyst (phosphoric acid on silica) at 300 o C and 70 atm pressure -   Purpose of phosphoric acid is to provide a lower activation energy pathway for the reaction -   Hydrogen ions, H + , are supplied by the phosphoric acid -   These ions take part in the reaction but are generated at the end -   Example of an industrial process in which a compromise must be reached between rate of reaction and yield -   Reaction to make ethanol is an exothermic reaction and would be favoured by low temperatures at which the rate of reaction would be unacceptably low, so a catalyst is used in conjunction with moderately high temperatures and moderately high pressures  -   Reaction between alkenes and bromine is used to distinguish between an alkene and an alkane -   Both hydrocarbons are colourless and so cannot be distinguished by sight -   Pure bromine is a red liquid that becomes yellow/orange in solution -   When bromine disappears immediately (decolourisation occurs) as the addition reaction occurs -   Reaction between an alkane and bromine will not proceed unless a strong source of UV light is available -   Similarly, purple acidified potassium permanganate, KMnO 4 , is decolourised by reaction with alkenes Addition polymerisation -   A polymer (poly means ‘many’) is made up of very large cha in-like molecules -   These have been generated by the reaction of thousands of monomers (mono means ‘one’) that have joined together as repeating units in the chain -   Production of poly(ethene) is an example of an addition polymerisation reaction -   This refers to reactions in which the monomer contains a double bond -   When the monomers join to each other, the double bond converts to a single bond and the second pair of electrons previously in the double bonds are used to form covalent bonds between the monomers -   There are no by-products in this process  –  only one product forms -   As the product is directly related to just one reactant, the name of the product is always the same as that of the reactant with ‘poly’ added as a prefix  -   Ethene polymerizes to make poly(ethene), propene polymerizes to make poly(propene) -   The equation can be written as follows, where n is a large number and represents the number of repeating units in the polymer chain o   nCH 2 CH 2     (CH 2 CH 2 ) n   o   ethene   poly(ethene)
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