Friday 1 April 2016

3.8 describe the addition of alkenes with bromine, including the decolourising of bromine water as a test for alkenes

Halogens can react with alkenes to form haloalkenes (this does not need UV light, unlike the formation of haloalkanes). For example, bromine ad ethene react together, forming dibromoethane (as it is composed of two bromine atoms and an ethene molecule). This is known as an addition reaction as the carbon-carbon couple bond is split and a halogen atom (in the case, bromine) is added to each carbon.
Diagram of the addition reaction between ethene and bromine
ethene + bromine ---> dibromoethane

This reaction can also be used to determine whether a substance is an alkene or not. This is because if you add an ethene to bromine, the solution formed (in this case, dibromoethane) will be colourless. If the unknown solution does not contain an alkene, the solution will stay the colour of bromine (yellow-brown).
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2 comments:

  1. Anonymous21 May 2016 at 18:48

    Hi, thank you so much for the time and effort you put into this blog! Just a little question. Is dibromoethene or dibromoethane formed? Because both terms are present here so i'm kinda confused. xx

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  2. Millie11 June 2016 at 10:43

    hi Issy! I'm so sorry I haven't been able to get back to you sooner, I hope paper 1 went well for you & hope that your question is still relevant incase this topic crops up in paper 2!

    When ethene reacts with bromine, dibromoethane is formed - it's a little confusing as the reactant is an 'ene' but the product is an 'ane' but is easy to remember once you've had a few questions on it :) Thank you for spotting the mistake! don't hesitate if you have anything else you are unsure of xx

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