Hydrogen Bonding Intermolecular Forces Compounds  Elmhurst College
Other Molecules Boiling Point DNA  Chemistry Department
Density of Ice Proteins    Virtual ChemBook


HYDROGEN BONDING

in ammonia, methanol, ethanol

Introduction:

The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge.

Ammonia with hydrogen bonding:

To recognize the possibility of hydrogen bonding, examine the Lewis structure of the molecule. The electronegative atom must have one or more unshared electron pairs as in the case of oxygen and nitrogen, and has a negative partial charge. The hydrogen, which has a partial positive charge tries to find another atom of oxygen or nitrogen with excess electrons to share and is attracted to the partial negative charge. This forms the basis for the hydrogen bond.

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Ammonia - Chime in new window

In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.

In the graphic on the left, the hydrogen is partially positive and attracted to the partially negative charge on the nitrogen. Nitrogen has only one lone pair, only one hydrogen bond can be made to each nitrogen.

This is a very specific bond as indicated. Some combinations which are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon.

Methanol with hydrogen bonding:

The graphic on the left shows a cluster of methanol molecules in the liquid state. Methanol is a polar molecule, with the oxygen (red) being the negative area and the hydrogen (white) being the more positive area. Opposite charges attract.

Some combinations which are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon. So notice that none of these are shown.

 

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Methanol - Chime in new window

Ethanol - Water with hydrogen bonding:

Various molecules may mix and dissolve in each other if they have approximately the same type of polarity. In the case of water and ethanol, this is the situation. The hydrogen of the -OH group on alcohol is polar as it is in the water molecule.

Therefore, the hydrogen of the -OH group on the ethanol may hydrogen bond to an oxygen of a water molecule (shown) or to an oxygen of an alcohol (not shown).

Again, some combinations which are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon. So notice that none of these are shown.