Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hydrogen bonding in ethanol

  1. Nov 13, 2012 #1

    How do you know how many hydrogen bonds ethanol molecules will form with each other? I know there are 3 sites where H bonds could form (2 lone pairs on oxygen and the hydrogen) but with something like hydrogen fluoride, each molecule only forms 1 H bond on average so shouldn't something similar happen with ethanol since each has only one hydrogen bond to 'donate'?

    Thanks :)
  2. jcsd
  3. Nov 13, 2012 #2


    User Avatar
    Gold Member

    Check the definition of Hydrogen Bonding at wikipedia. H-Bonding is a special, stronger case of dipole-dipole attraction. The bond is slightly polarized, which may be seen as an electric dipole of some small "charge".

    In HF, F has slight negative "charge" and H has an equivalent positive "charge", so F only forms one H-Bond.

    But in Ethanol, O has more negative charge than H has positive charge (Ethyl group has +I effect) so overall, O-atom can enough polarization to form two H-Bond.
  4. Nov 13, 2012 #3
    Thanks for replying :) I've just read Wikipedia's entry but I think I must still be missing something :/

    I understand the +I effect and how the O-atom can form two H-bonds, but the H-atom also forms a H-bond with an O lone pair on another ethanol doesn't it? So does that mean each molecule forms 3 H-bonds then? :S
  5. Nov 13, 2012 #4


    User Avatar
    Gold Member

    You may check this figure. It says something else.
  6. Nov 14, 2012 #5
    Yeah I mean I know that my idea of how it might bond is wrong but I'm just not sure of exactly what makes it different :confused:

    So in your picture, for one molecule of ethanol (on average): one lone pair on the O-atom 'accepts' a H-bond from another molecule, and one of it's H-atoms 'donates' a H-bond to another molecule's lone pair, giving 2 H-bonds per molecule?

    I just want to check that's right first :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook