The third and strongest type of inter molecular force is a hydrogen bond
A hydrogen bond is a type of intermolecular force that involves a hydrogen atom bonded to an O, N or F atom in one molecule becoming attached to an O, N, or F atom in a different molecule. (I will insert the textbook reference after)
O, N and F atoms have very strong electron-attracting powers— they are the three most electronegative elements. This means that the bonding electrons O-H, N-H and F-H bonds are strongly attracted to the H atom becoming positively charge. In addition, the small size of the the H atom means that two adjacent molecules can get closer together: the positive H atom, bonded to the O, N or F atom in one molecule, forms a strong attractive intermolecular force. (I will insert the textbook reference after)
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).
Hydrogen bonding in alcohols
Any molecule which has a hydrogen atom attached directly to
an oxygen or a nitrogen is capable of hydrogen bonding. Such molecules will
always have higher boiling points than similarly sized molecules which don't
have an -O-H or an -N-H group. The hydrogen bonding makes the molecules
"stickier", and more heat is necessary to separate them.
Ethanol, CH3CH2-O-H, and methoxymethane, CH3-O-CH3, both
have the same molecular formula, C2H6O.
They have the same number of electrons, and a similar length
to the molecule. The van der Waals attractions (both dispersion forces and
dipole-dipole attractions) in each will be much the same.
However, ethanol has a hydrogen atom attached directly to an
oxygen - and that oxygen still has exactly the same two lone pairs as in a
water molecule. Hydrogen bonding can occur between ethanol molecules, although
not as effectively as in water. The hydrogen bonding is limited by the fact
that there is only one hydrogen in each ethanol molecule with sufficient +
charge.
In methoxymethane, the lone pairs on the oxygen are still
there, but the hydrogens aren't sufficiently + for hydrogen bonds to form.
Except in some rather unusual cases, the hydrogen atom has to be attached
directly to the very electronegative element for hydrogen bonding to occur. (
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