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Elimination vs substitution & ethanol as a solvent

  1. Mar 16, 2010 #1
    Hello!
    I was looking into haloalkane reactions and the factors which determine the proportion of nucleophilic substitution to elimination reactions. I read that ethanol is more conducive to elimination reactions than substitution reactions, it mentions it being less polar than water, which favours substitution. I also read that SN1 are favoured over SN2 reactions as the ethanol can stabilise the carbocation. However, I can't find a source that explains why ethanol favours elimination, it must be to do with the 'extent' of polarity of the molecule.

    Is it because the polarity of the molecule influences the stability of the carbocation, and as water is more polar, the carbocation is more stable and therefore substitution is favoured as the carbocation can exist for a longer period of time?

    I appreciate that is insufficient an explanation on it's own. So, taking in to consideration the activation energy, and that subsitiution has a lower activation energy, these two factors can be combined to produce an explanation:

    Subsitution is favoured in highly polar solvents as the carbocation is more stable and therefiore exists for longer, and as the ativaiton energy is lower, it is more likely to be achieved than elimination. In less polar solvents, the carbocation is less stable, and therefore elimination is more liekly to occur, even though it has a higher activation energy, the carbocation intermediate is going to be somewhat 'rare'.

    It seems confusing, as the classification (primary, secondary and tertiary) keeps popping up, and begs to be included!

    Any help appreciated, apologies that its quite long, an attempt seemed necessary.
    Many thanks,
    Nobahar.
     
  2. jcsd
  3. Mar 17, 2010 #2

    chemisttree

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    Your confusion is partly due to not understanding the rest of the problem. Under what conditions are you examining these two pathways? Strong acid? How strong an acid can you have in water? Anything stronger than H3O+? How about in ethanol? The strength of the acid is critical as well. Your substrate is important too. Primary alkyl halides almost always undergo substitution unless really strong acid is used. Secondary alkyl halides undergo a mix of elimination and substitution and if there is a beta tertiary carbon... you get the picture.
     
  4. Mar 17, 2010 #3
    Mnay thanks for the response chemisttree.
    I appreciate that the other factors will have an influence, it is the various contirbuting factors that make it all the more difficult to grasp. If possible, evaluating the solvent on the basis of its own contirbution, is the statement correct? Although what reaction is favoured, and thus what products are produced, is a consequence of all the various influencing factors, if I hold all constant at some value, which may favour one mechanism or the other (elimination or substitution), by varying the solvent can I conclude that: using increasingly more polar solvents, the amount of reagent undergoing a subsitution reaction will increase; regardless of whether or not it is the most commonly occuring mechanism in the reaction?

    I apologise, this wasn't exactly my last question. But I would appreciate any further responses.
    Many thanks,
    Nobahar.
     
  5. Mar 18, 2010 #4
    Any further input would be much appreciated; and verification on the above?
    Many thanks,
    Nobahar.
     
  6. Mar 19, 2010 #5

    chemisttree

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    You are likely contrasting the two unimolecular pathways, E1 and Sn1. In both of these pathways the first step is the generation of the carbocation. This occurs independent of the nature of the solvents water and ethanol. The next step, however, is all about the solvent. The carbocation is stabilized in both cases and exists as an ion pair between the newly-formed cation and the halide leaving group. This ion pair is closer together in the case of ethanol than in the case for water since water can form a slightly more stable separated ion pair (with water separating them). Thus water itself can become the nucleophile and lead to the substituted product since it has better access to the carbonium ion and the ion pair is more completely separated. In the case for ethanol, the ion pair is closer together or more tightly bound and recombination can more easily occur or elimination if that's possible. I have references if you want to read it for yourself but the discussion deals more with t-butyl halides but the arguments and logic apply.
     
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