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Some doubts I have while trying to compare boiling points/ solubility

  1. Sep 10, 2012 #1
    I was trying to compare some solubilities (in water) and some boiling points, and I could not explain it for some molucules. Also, I have some doubts in the theory itself.

    Why is 2-pentanone more soluble than pentanoic acid?
    Why is a ketone more soluble than the respective aldeyde?

    When I compare boiling points, I was tauch to compare the interactions between the molecules, and the size of the molecule. How can I justify the size of a molecule interfering in solubility? I have 2 theories but I realy don't know if any of them are right. First I would say a big molecule would do more interactions than sa smaller one, so the boiling point should be greater. Also I would say that at the same temperature, all molecules has the same kinetic energy, and how kinetic energy is something like mv²/2, big molecules should have less velocity, so it would be harder for them to escape the liquids. Is it right?

    Another doubt I had was when I had to compare solubility.

    In this case I know the number of hyfrogen bond is important. Is the size important in this case too?

    In an exercise of my chemistry school book I was asked to compare the solubility of acetylsalicylic acid and salicylic acid.

    http://upload.wikimedia.org/wikipedia/commons/6/67/Aspirin-skeletal.svg
    http://upload.wikimedia.org/wikipedia/commons/8/8e/Salicylic-acid-skeletal.svg

    The first one makes 9 hydrogen bonds with water (is it right?) and has a bigger size (more interaction) also, the second one makes only 8 hydrogen bonds and has a smaller size. The answer, though, says that salicylic acid has a greater solubility. Why is this true?
     
  2. jcsd
  3. Sep 10, 2012 #2
    the first one has only 1 hydrogen bond donor (OH groups). the second has 2 hydrogen bond donors (OH groups).

    Always remember: OH groups trump all in terms of hydrogen bonding.
     
  4. Sep 10, 2012 #3
    But shouldn't the elektron pairs in =O be considerated too? In water these molecules would be pretty much diluted so there should be a lot of H in the H2O molecules making hidrogen bond with the pairs of electrons of the oxygens of the carbonyl. Am I wrong?
     
  5. Sep 10, 2012 #4
    think about it this way:

    a single O can accept hydrogen bonds. However, it cannot donate hydrogen bonds. The degree of solubility of something is determined by how much it can interact with the solvent, or put it simpler, how much it is similar to the solvent. In a polar solvent such as water, something with more things that are "like water" in terms of being able to both accept and donate hydrogen bonds will probably be soluble.

    acetylsalicylic acid is less soluble in water because it is "less water-like" than salicylic acid. another easy example is diethyl ether is less soluble than ethanol, propanol (which has more carbons) or butanol.

    now what i've said is generally true, not totally true, but it is a good rule of thumb. it also only applies to small molecules (molecular mass < 1000 daltons). for polymers, things get tricky because they can fold back on themselves or get tangled together and all sorts of messy stuff.
     
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