hydrogen bonding

Hydrogen bonding

Hydrogen bonding is a type of dipole-dipole attraction between molecules containing O-H, N-H and H-F groups. It is also the strongest form of intermolecular bonding with hydrogen bonds in water having around 5% the bond strength of the O-H covalent bonds in water. Hydrogen bonding is found in covalently bonded molecules which contain a hydrogen atom bonded to nitrogen, oxygen and fluorine atoms only. The reason for this is that N, O and F are the three most electronegative elements in the periodic table. This means that the electron density in the polar covalent bond between the hydrogen atom and any N, O or F atoms attached to it will be very much concentrated towards the electronegative element and away from the hydrogen atom. This will leave an exposed hydrogen nucleus or proton with a large δ+ charge which will form a very strong intermolecular bond with the lone pairs of electrons on the small atoms of nitrogen, oxygen and fluorine. This is shown in the diagram opposite:

Drawing hydrogen bonds

For a hydrogen bond to form you need 3 atoms and a lone pair of electrons, it is important that when you are asked to draw a hydrogen bond that the atoms and the lone pair of electrons involved in the hydrogen bond are in a straight line with each other. The diagram below shows how one hydrogen bond can form in molecules of water and ammonia. If you look at the hydrogen bond in the water molecule you will see that the oxygen atom-----hydrogen atom---lone pair ----oxygen atom involved in forming the hydrogen bond are all in a straight line. It is a similar story with the hydrogen bond in the ammonia molecule, the three atoms involved in forming the hydrogen bond MUST be in a straight line.

Hydrogen bonding in molecules containing the -OH group

The hydroxyl group (-OH) is very common in many molecules, such as water, carboxylic acids and alcohols. Any molecule which contains this functional group can undergo hydrogen bonding. Hydrogen bonding is particularly strong in water since each water molecule is able to form 4 hydrogen bonds as shown in the image below (note the number of hydrogen bonds formed by each molecule will vary with temperature and the kinetic energy of the particular water molecules). Although molecules such as hydrogen fluoride and ammonia can in theory form just as many hydrogen bonds as water in reality they rarely do. This extended hydrogen bonding found in water is responsible for many of its characteristic properties including a very high melting and boiling points and viscosity considering its very small molecular size and mass, it is also responsible for the very high surface tension found in water.

The presence of hydrogen bonding can help explain many of the unusual changes in the physical properties of many of the compounds which contain hydrogen bonding. As an example consider the hydrides of the group 6 elements: O, S, Se and Te. We would expect the boiling points of the hydrides to increase down any group, simply due to the increase in the mass of the molecules and the amount of Van der Waals bonding present. So in this case hydrogen oxide (water- H2O) should have the lowest boiling point and tellurium hydride (H2Te) to have the highest boiling point. However this is not what is observed, in fact the smallest molecule has the highest boiling point, the total opposite of what we might have expected! Water has a much higher boiling point than would be predicted; this is shown in the graph below. This hugely inflated boiling point for water tells us that much more energy is needed to separate the water molecules than would be expected. This additional energy is due to the presence of hydrogen bonding in water molecules.

graph to show the boiling points of the group 6 hydrides

The structure of ice

A water molecule has two lone pairs of electrons; this will enable each molecule of water to form 4 hydrogen bonds as discussed above. The image below shows how the water molecules are arranged in ice, hopefully you will notice some similarities between this structure and that of diamond.

ice structure

When water is in the liquid state the water molecules are free to move and slide over each other, this means that the hydrogen bonds must be continually breaking and reforming as each water molecule moves. However as the temperature drops close to the freezing point of water the movement of the water molecules slows and at 40C the density of the water reaches it maximum, then below 40C the water molecules start to move apart as more and more hydrogen bonds start to form. When water reaches its freezing point the water molecules will be unable to move as they are held in place by the hydrogen bonding between adjacent molecules. In order to form the ice structure the water molecules are not as densely packed as they are in water, this explains why ice is less dense than water and floats on it. It is easy to see when you look at the ice structure how open it is. This is a very unusual property since for most substances the solid state is more dense than the liquid state.

Hydrogen bonding in alcohols and carboxylic acids

Carboxylic acids and alcohols both contain extensive hydrogen bonding. Most carboxylic acids exist as dimers where two carboxylic acid molecules are attracted to each other and held by 2 hydrogen bonds, this is shown below. This hydrogen bonding in carboxylic acids causes them to have much higher boiling and melting points than expected. Both alcohols and carboxylic molecules can form hydrogen bonds with water; this explain why they are soluble in water but molecules such as alkanes are not. Many organic molecules such as the alkanes are non-polar which means they cannot form hydrogen bonds with polar solvents such as water which is why they are insoluble in water. However for alcohols and carboxylic acids as the molecules get larger and the chain length of the orgnaic part of the molecule increases their solubility decreases.

Hydrogen bonding in biological molecules

Hydrogen bonding is critical in maintaining the shape of many important biological molecules that are essential for life. These molecule include protein and DNA. Proteins consist of long chains of amino-acids which are arranged in long helical chains or sheets. These large protein polymers are held in place by hydrogen bonds between the N-H and C=0 bonds on different amino acid molecules within the protein chains or sheets. The DNA polymer consists of helical shaped strands which are held together by a series of hydrogen bonds between 4 bases in the strands. These bases are called guanine, cytosine, adenine and thymine. Like the protein polymers these hydrogen bonds occur between N-H groups and C=O groups on the bases in different strands. This is shown in the image below:

Key Points

Practice questions

Check your understanding - Questions on hydrogen bonding