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    When testing for a halogen alkane, you mix silver nitrate (AgNO3) and the halogenalkane with ethanol. But do you have to use ethanol, or can you use water, and why do people often use ethanol instead of water?

    Thanks
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    (Original post by Hi, How are you ?)
    When testing for a halogen alkane, you mix silver nitrate (AgNO3) and the halogenalkane with ethanol. But do you have to use ethanol, or can you use water, and why do people often use ethanol instead of water?

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    Water is very polar. Haloalkanes are pretty much nonpolar. Can you see where this might be going?
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    (Original post by bananarama2)
    The tests works because you react the haloalkane with sodium hydroxide to give the anion of the halogen which reacts with the Ag+. To ensure you get the anion you need the reaction of the haloalkane with sodium hydroxide to be a substitution and not elimination. The way you ensure this is my reacting it in ethanol.
    I believe A level syllabi teach that hydroxide in ethanol leads to elimination. I also do not think hydroxide is necessary for this test.
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    (Original post by illusionz)
    I believe A level syllabi teach that hydroxide in ethanol leads to elimination. I also do not think hydroxide is necessary for this test.
    http://www.chemguide.co.uk/organicpr...nes/agno3.html
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    To elaborate, silver salts can somrtimes abstract a halide anion from an organic species, but there are other factors which determine how easy it is.

    My point about A level syllabi teaching hydroxide in ethanol with heat leads to elimination still stands.

    The reason the ethanol is used is a solubility one, not a chemical reactivity one.
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    (Original post by illusionz)
    Water is very polar. Haloalkanes are pretty much nonpolar. Can you see where this might be going?
    So i presume that ethanol can react with the halogenalkane (as a nuclophile as the oxygen has 2 lone pairs of e-'s). Right?
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    (Original post by illusionz)
    To elaborate, silver salts can somrtimes abstract a halide anion from an organic species, but there are other factors which determine how easy it is.

    My point about A level syllabi teaching hydroxide in ethanol with heat leads to elimination still stands.

    The reason the ethanol is used is a solubility one, not a chemical reactivity one.
    Yeh you're right, my bad. I hate organic
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    (Original post by Hi, How are you ?)
    So i presume that ethanol can react with the halogenalkane (as a nuclophile as the oxygen has 2 lone pairs of e-'s). Right?
    Nope. Haloalkanes won't dissolve in water, which means you'd have a bit of a tough time trying to do the reaction. Ethanol is a very poor nucleophile.
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    (Original post by illusionz)
    ...
    How does it react then?
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    (Original post by bananarama2)
    How does it react then?
    Well everything is soluble in the water/ethanol mixture - it is nonpolar enough for the haloalkane and polar enough for the silver salt.

    The silver coordinates to the alkyl halide, and depending upon the nature of the alkyl group (basically how stably the alkyl cation would be), you may get heterolytic fission of the C-X bond, leaving an alkyl cation.

    Water or ethanol (probably water) can then react with the alkyl cation.

    If the alkyl cation would not be stable, then instead of getting the cation, you simply have an activated alkyl halide, with the silver coordinated to the halogen, giving the halogen a positive charge. This makes the alkyl halide a better electrophile, and now it will be reactive enough to react with water/ethanol (again, probably water).

    You do not need to know the details of the reaction though.
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    (Original post by illusionz)
    Well everything is soluble in the water/ethanol mixture - it is nonpolar enough for the haloalkane and polar enough for the silver salt.

    The silver coordinates to the alkyl halide, and depending upon the nature of the alkyl group (basically how stably the alkyl cation would be), you may get heterolytic fission of the C-X bond, leaving an alkyl cation.

    Water or ethanol (probably water) can then react with the alkyl cation.

    If the alkyl cation would not be stable, then instead of getting the cation, you simply have an activated alkyl halide, with the silver coordinated to the halogen, giving the halogen a positive charge. This makes the alkyl halide a better electrophile, and now it will be reactive enough to react with water/ethanol (again, probably water).

    You do not need to know the details of the reaction though.
    Thanks, that's quite interesting.

    Well no now. But I might at uni

    I also noted you said that a-level syllabuses teach that elimination happens in alcohol. Are you implying it's more complicated than that?
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    (Original post by bananarama2)
    I also noted you said that a-level syllabuses teach that elimination happens in alcohol. Are you implying it's more complicated than that?
    Yeah it is a bit. Hydroxide can act as a nucleophile or a base. You can tweak the reaction conditions to favour one over the other. It so happens that hydroxide in ethanol, at high temperatures favours elimination, and hydroxide in water at room temperature favours substitution.

    I'll be honest, I don't actually know what difference water vs ethanol makes - none of the courses I've done covered solvent effects in any sort of detail. The biggest thing I can think of is the bigger increase in entropy of elimination, which explains why it is favoured at higher temperatures.
 
 
 
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