13 julio, 2024

Hydrogen bonding: characteristics, examples

What is a hydrogen bond bond?

Hydrogen bonding is a special type of dipole-dipole interaction, belonging to the Van der Waals forces, in which a hydrogen atom joins two or more molecules without being covalently bonded. There is no talk of a sharing of electrons, but of a mainly electrostatic phenomenon.

As its name suggests, hydrogen acts as a bridge, so it must be between two atoms. Hydrogen is bonded (HX) to a highly electronegative atom (such as N, O, and F), and approaches another electronegative atom but from a neighboring molecule. This gives rise to the formation of the hydrogen bond X···HX.

Note that in the X···HX representation, the dots symbolize the hydrogen bond, while the dash represents the covalent bond between H and X. That being said, let’s look at the hydrogen bond between two water molecules, where X is the oxygen atom: H2O···H-OH (bottom image).

In the image above we see that seven water molecules are held together by their hydrogen bonds, represented by blue lines. A hydrogen bond by itself is not very strong, but when there are billions of them, they give matter abnormal and unexpected properties.

Hydrogen bonding characteristics

compositional

The hydrogen bond should really be represented as X’···HX, where X is what is known as the donor of the hydrogen bond, because it is covalently bonded to (gives) hydrogen.

Meanwhile, X’ is the acceptor of the hydrogen bond, present in a neighboring molecule (receives it). Thus, we have a donor atom (X), a hydrogen (H) and an acceptor atom (X’) composing the hydrogen bond (X’···HX).

geometric

When we think of a bridge, flat or arched surfaces come to mind. Because it is assumed that the bonds do not bend, we will then have two distances: X···H and HX, which make up the X···HX hydrogen bond.

Between these two distances there is an angle, which is often 180º; that is, that the three atoms of our bridge rest on the same horizontal (or vertical) line.

When the angle is different from 180º, the X···HX bridge is no longer straight or linear, but takes on other geometries.

On the other hand, the distances in the hydrogen bonds are not identical. The distance HX is shorter than X···H, which can be seen in the image of the water molecules. Thus, for example, the distance HX is usually 110 pm (1·10-12m), while the other distance X···H is from 160 pm onwards.

associative

A special feature of the hydrogen bond is that it allows molecules to associate much more with each other. They don’t walk around ignoring each other. Therefore, it establishes a momentary order in the sinuses of liquids; and in the case of solids, they contribute to the definition of their crystals.

Where we see a hydrogen bond we can think of association and therefore of a certain order (albeit dynamic and changing) at molecular scales.

energetic

Breaking a hydrogen bond is not very difficult. The water molecules, for example, break them and create them all the time while they are moving. But breaking many of them at the same time would imply disassociating an endless number of molecules. We are talking about supplying such energy that it breaks moles of said hydrogen bonds (6.02·1023 X···HX).

Thus, the strength of the hydrogen bond varies depending on the identities and nature of the molecules. For example, the strength of the hydrogen bond O···HO between water and alcohol is 5 kcal/mol: it takes 5 kcal of energy to break one mole of that hydrogen bond in question.

Examples of hydrogen bonds

Water

At first they talked about hydrogen bonds between water molecules, but the effect it has on their properties was not mentioned. Thanks to them, water boils at 100 ºC, leaving far behind the boiling points of related molecules such as H2S, which boils at -60 ºC; or H2Se, which boils at -41.25 ºC.

This abysmal difference is due to the hydrogen bonds of water, which also define other of its anomalous properties, such as its enormous specific heat, ice crystals, its dielectric constant, etc.

Ethanol

Now let’s look at another hydrogen bond: the one between ethanol molecules, CH3CH2OH (above). Note how the CH3CH2OH molecules are arranged in such a way that their hydrogen bonds CH3CH2HO···HOCH2CH3 are established (dotted lines).

However, the molecules are too ordered to assume that we are talking about liquid ethanol, but instead make up a crystal (solid ethanol).

The hydrogen bond described for ethanol is similar to that of other alcohols, with the difference that their carbon skeletons can hinder the efficiency of these bridges.

Acetic acid

Acetic acid, CH3COOH, is capable of establishing two hydrogen bonds at the same time that join two molecules at the same time. Because they are two molecules joined by hydrogen bonds, we speak of a dimer.

Note that one of these hydrogen bonds is C=O···HO and the other OH···O=C. Acetic acid presents the particularity that in the vapor phase it exists as this dimer.

Cellulose

Now let’s look at more diverse and multiple hydrogen bonds. Cellulose, a natural polymer, consists of chains made up of various β-glucose units.

Each chain remains fixed to another thanks to many hydrogen bonds (top image), which reinforce the cohesion between the chains.

dna

So far we have seen the associative effect that hydrogen bonds have to impose order between molecules. But what about a macromolecule? In a macromolecule, such as DNA, we find internal or intramolecular hydrogen bonds between its nitrogenous bases thymine, adenine, guanine, and cytosine (above).

The intramolecular hydrogen bonds between these pairs of nitrogenous bases cause the DNA molecule to acquire a double helix structure, which is ideal for its replication. If these hydrogen bonds are broken by heating, the double helix will eventually split open into two individual segments or bands.

References

Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill. Edward E. Avila & Asiloé J. Mora. (2004). The hydrogen bridge bond and its applications. Universidad de Los Andes, Faculty of Sciences, Department of Chemistry. Wikipedia. (2020). Hydrogen bond. Retrieved from: en.wikipedia.org Pietri J. & Clark J. (August 21, 2020). Hydrogen Bonding. Chemistry LibreTexts. Retrieved from: chem.libretexts.org The Editors of Encyclopaedia Britannica. (2020). Hydrogen bonding. Retrieved from: britannica.com Helmenstine, Anne Marie, Ph.D. (October 29, 2020). What Are Examples of Hydrogen Bonding? Retrieved from: thoughtco.com

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