The Naked Scientists

The Naked Scientists Forum

Author Topic: Why does water behave oddly compared to alcohols?  (Read 6243 times)

DiscoverDave

  • Guest
I looked at water and noticed that it has a somewhat similar structure to alcohols except, of course, that it doesn't have any alkyl groups.  In a way, it's an alkyl-less alcohol. 

When I plotted the boiling points of water and alcohols, I noticed that the alcohols behaved rather predictably,; however, they did not predict the behavior of water, which behaved more like 3-carbon propanol than like 2-carbon ethanol or 1-carbon methanol.  Why?



When I looked at their melting points, the alcohols reached their lowest MP at propanol, and the alcohols close to water trended toward water, but water had a MP closer to a 9- or 10-carbon alcohol.  Again, why?



Also, it seems that something inherent to boiling and melting makes the BP's and MP's of the alcohols behave differently: the BP's plot monotonically, and the MP's don't.  Why?
« Last Edit: 20/09/2009 01:39:17 by DiscoverDave »


 

Offline lancenti

  • Full Member
  • ***
  • Posts: 96
    • View Profile
Why does water behave oddly compared to alcohols?
« Reply #1 on: 20/09/2009 05:08:15 »
For melting point, it might be worthwhile to consider that Water arranges itself in a lattice and, because of extensive hydrogen bonding, it should have a higher melting point than methanol which cannot form an equally nice lattice because it's missing the other hydrogen atom.

There's a uniform increase of boiling point probably because of the fact that alcohols can only form one hydrogen bond and the importance of ID-ID interactions of the alkyl chain in keeping the alcohol liquid. Having two hydrogen bonds should clearly give water an advantage over those which can only form one, so it starts out higher. However, alkyl groups contribute to what is known as ID-ID interactions which, if the molcule is large enough, can actually play a greater role than the hydrogen bond, or even overpower two hydrogen bonds. This could also explain why, after hexanol (if I remember right), alcohols are no longer miscible in water.

As for the 'kink' in the melting point curve probably has something to do with the ability of the smaller alcohols to form lattices like water but because they're missing one hydrogen bond they're considerably weaker lattices. After propanol, the lattice is replaced by packing of the side-chains which is largely an ID-ID interaction which, as the chain gets longer, the interactions get stronger.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Why does water behave oddly compared to alcohols?
« Reply #2 on: 20/09/2009 12:18:50 »
With "ID-ID interactions" you mean Van Der Waals interactions, I suppose.
I would only add to your post that hydrogen bond is much stronger in water than in an alcohol.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Why does water behave oddly compared to alcohols?
« Reply #3 on: 20/09/2009 18:57:47 »
I had wondered if the carbon atoms produce a partial positive charge in the hydrogen atoms differently than oxygen does; that is, do carbons make for a less polarized molecule?  And, likewise, do carbon atoms affect the partial negative charge of oxygen atoms differently than hydrogen does?  I was thinking that this would explain why alcohols that are "close to" water actually melt and boil with less thermal energy pumped into them -- because the carbon(s) spoil the polarization.
yes.
 

The Naked Scientists Forum

Why does water behave oddly compared to alcohols?
« Reply #3 on: 20/09/2009 18:57:47 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums