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Hydrogen bonding in ethanol

by Psychae
Tags: bonding, ethanol, hydrogen
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Psychae
#1
Nov13-12, 10:37 AM
P: 6
Hey,

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 :)
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AGNuke
#2
Nov13-12, 12:51 PM
PF Gold
AGNuke's Avatar
P: 456
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.
Psychae
#3
Nov13-12, 03:30 PM
P: 6
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

AGNuke
#4
Nov13-12, 10:59 PM
PF Gold
AGNuke's Avatar
P: 456
Hydrogen bonding in ethanol

You may check this figure. It says something else.
Psychae
#5
Nov14-12, 09:05 AM
P: 6
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

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 :)


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