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high BP = high MP @[email protected]?

Discussion in 'DAT Discussions' started by joonkimdds, Dec 28, 2008.

  1. joonkimdds

    joonkimdds Senior Member
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    I think this sounds stupid but I am confused (and feeling high @[email protected];;) so I am sorry if this question sounds too stupid.

    Does high boiling point mean high melting point? (I don't know why but I have been thinking that high boiling point means low melting point):scared:

    I found a question asking for which one has the highest boiling point and the solution says we look for the one that can form the H bond and COOH was the answer.

    And then I found another question but this time asked for highest melting point and the answer was the same.
     
    #1 joonkimdds, Dec 28, 2008
    Last edited: Dec 28, 2008
  2. doc3232

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    Ya, I am not sure, but I always did think that the two were correlated like that.
    I like to think of a diamonds characteristics when I am confused. Works for me, might work for others.
     
  3. Panther85

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    higher mp is due to branching

    higher bp is due to a straighter chain interaction

    look at destroyer ochem #152
     
  4. joonkimdds

    joonkimdds Senior Member
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    Unfortunately, my destroyer ochem stops at #117 :( (it's from yr 2006)
    According to Kaplan
    H bonding = inc BP and MP
    unsaturation = inc BP
    more branch = dec BP
    longer = inc BP

    That's all the info I could gather.

    Panther85// you said higher MP is due to branching. But branching lowers BP. Does that mean BP and MP are not always correlated?
     
  5. zuma35

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    Higher boiling point usually goes with higher melting point, but not always. One exception, for example, is cis and trans. A cis isomer will have a higher boiling point and a lower melting point than its trans isomer. Another example is a very symmetrical molecule. Although branching usually decreases the melting pt (as it does to the bp), if a molecule is very symmetrical, like 2,2 dimethyl propane, the melting point will increase due to the molecule's ability to form into a perfect crystal.
     
  6. joonkimdds

    joonkimdds Senior Member
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    I have one more question.
    Does higher boiling point also mean higher solubility in H2O?

    I learned from old thread of SDN that
    solubility in H2O is
    COOH>Phenol>Alcohol>Alkyne>Alkene>alkane

    and then I learned from my other materials that
    COOH has high BP, so does alcohol because of OH nature forming H bonding,
    and then alkyne and alkene has higher BP than alkane.

    so when I gather all these info, it seems
    High MP, high BP, and high solubility in water.

    and I think it also has something to do with acidity level.
     
  7. Danny289

    Danny289 Member
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    questions in DAT will be very clear. when you are comparing you must compare apple to apple! for example which one has higer boiling point?
    1) n- pentane
    2)2- methyl butane
    3)Ethane
    4) neo pentane (2,2 dimethyl propane)
    here answer will be "n-pantane" no branch boiling point high, ethane is out because it is small molecule compare to pentane.
    in above question, if ask about melting point answer will be neo pentane because the molecule will be fit better in solid form (draw it you will see)
    maybe you notice we don't have any hydrogyn bond here( lack of , N, O, F )
    now look at to this question?
    which one has higher boiling ( melting point)
    1)CH3-CH2-CH3
    2) CH3-0-CH3
    3)CH2-CH2-CH2-OH
    4)CH2-CH2-COOH
    the answer is "4" because hydrogyn bond (stronger hydogyn bond in acid compare to alcohol).
    give a time to yourself to understand this subject cold.
    good luck :thumbup:
     
  8. Zubnaya Feya

    Zubnaya Feya Combinator
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    I think destroyer had a good explanation about it (or maybe my old textbook?).

    High melting point does not go with high boiling point.

    The more possibility of molecules getting together (Van der Waals attractions) the higher the boiling point. Therefore, the less branching in a chain - the higher the boiling point.

    Melting point increases with increasing branches because (I don't remember why) branching somehow favors crystallization.

    I remember that for sure... and thinking now, it was in Destroyer.
     
  9. Zubnaya Feya

    Zubnaya Feya Combinator
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    I cannot come up with an example... but for some reason I doubt that higher boiling point is proportional to higher solubility.

    Bigger polarity and higher probability of dissociating into ions go together with higher solubility in water.
     
  10. joonkimdds

    joonkimdds Senior Member
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    I guess I should get the 2nd Destroyer ....
     
  11. Danny289

    Danny289 Member
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    boiling point related to the number of molecule(moles) in your solution.
    for example one mole NaCl(58.5 gram NaCl) in one liter water has higher boiling point of one mole suger( 342.3 gram C12H22O11)in one liter water because NaCl dissolves in water and make 2 moles ions(NaCl===> Na+ + Cl-) but sugar makes just one(not ionized).

    anyway this subject is covered in G chem not in organic chemistry.
     
  12. joonkimdds

    joonkimdds Senior Member
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    Danny290, do you think higher boiling point is same as having higher water solubility?
     
  13. doc3232

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    Think diamonds, diamonds have a higher bp than nearly anything, but that doesn't dissolve in water.
     
  14. joonkimdds

    joonkimdds Senior Member
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    ok:)
    But if BP and solubility have different characteristics, what are they?
    For example, characteristics we look for BP is branching, length, H bonding.
    what characterestics do we look for solubility in water?


    Funny thing is I just read from Kaplan that says
    inc chain length = inc BP, MP, density
    inc branching = dec BP, MP, density

    This info is different from what two of you said.

    So I searched SDN and found two similar thread
    it was
    Kaplan vs. Destroyer http://forums.studentdoctor.net/showthread.php?t=542327
    and then
    EK vs. TPR http://forums.studentdoctor.net/showthread.php?t=111078

    in the end, Kaplan and EK lost ... :D
     
    #14 joonkimdds, Dec 28, 2008
    Last edited: Dec 29, 2008
  15. Danny289

    Danny289 Member
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    yes you can think that way kinda. read carefully my previous post. if you have more solubility, you will have more moles in your solution, pay attention in the same timeto the ionization of solute( like NaCl in my example).
     
  16. Danny289

    Danny289 Member
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    open your Textbook! for a moment forger BB and destroyer and EK....!
     
  17. doc3232

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    inc chain length = inc BP, MP, density
    inc branching = dec BP, MP, density

    THIS IS correct.
    But you must assume when you increase branching that you are not changing the number of carbons.
    If you increase branching then the molecules can't stack as well and hence have a lower density. Also, they have less interactions and hence can "leave" each other in the form of gas (BP).
     
  18. Zubnaya Feya

    Zubnaya Feya Combinator
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    I thought we are talking about pure substances. Ochem covers this subject too.
     
  19. Danny289

    Danny289 Member
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    you are right but "joonkimdds" tallked about solublity in water and boiling point. :thumbup:
     
  20. Zubnaya Feya

    Zubnaya Feya Combinator
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    :xf: whatev:D
     
  21. doccure

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    inc BP= inc in chain and dec in branching
    inc MP= inc in chain and dec in branching
    inc solubility in water = short chain
    Correct me if I am wrong
     
  22. doc3232

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    So I came across a tough question:
    Which will have higher MP: cis or trans alkene?
    Same question with BP

    The answer is not what is expected...
     
  23. joonkimdds

    joonkimdds Senior Member
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    diff books are talking about it differently.
    I decided to stick with what people here say which is
    dec MP = dec branching

    tell us what the answer was :)
     
  24. doc3232

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    So answer is: MP is higher if it is trans and BP is higher for cis.
    cis because it is polar.
     

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