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Boiling point of Cyclohexene and 2-methylbut-1-ene

  1. Dec 19, 2012 #1
    So i just took a final and the question was

    Arrange the following solvents in order of boiling point.
    water 2-methylbut-1-ene cyclohexane

    So the answer was Water>cyclohexene> 2-methylbut-1-ene.

    I understand by the hydrogen bonding that water is definitely #1. But I can't seem to understand why cyclohexene is greater in BP. Unless I specifically know the property, I dont see how I could figure it out?
     
  2. jcsd
  3. Dec 19, 2012 #2
    Boiling point is directly proportional to the molecular weight.
    So,higher the molecular weight,higher the boiling point.
    Mol wt of cyclohexene=82
    Mol wt of 2-methylbut-1-ene=70
    Hence boiling point of cyclohexene is greater than that of 2-methylbut-1-ene.
     
  4. Dec 20, 2012 #3

    Borek

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    Staff: Mentor

    In general you are right, but there are also other effects and I would never use this approach to compare quite different molecules. Despite having identical molar mass isopropyl and butene don't have identical boiling points.
     
  5. Dec 20, 2012 #4
    Hi!Borek.

    In the case of iso-propane and butane, butane has higher boiling point than iso-propane.
    This is because branching decreases surface area.
    More the surface area more the Vander Waals' force of attraction(hence more the boiling point).

    We have different approaches in comparing the boiling points of two compounds.
    We can also compare boiling points with their relative tendency to form hydrogen bonds.
    So for a given set of compounds we can compare using any of the above methods,but we should see which method suits the best.

    In the case of cyclohexane and 2-methylbut-1-ene we cannot compare their boiling points using their relative H-bonding or by the surface area method.So the best method to compare is using their molecular weights.
     
  6. Dec 20, 2012 #5

    Borek

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    Staff: Mentor

    It is the simplest method that can be applied, but for the reasons you have listed it is not guaranteed to work, so calling it "the best" is an exaggeration.
     
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