We often characterize chemistry as “the molecular science.” This description allows us to understand the characteristics of chemical bonding that link the polyatomic molecules together to form the matter we observe around us. One of the most prominent topics that belong to this domain is hydrogen bonding. It underpins the behavior of many substances that we come across every day, such as water. Though hydrogen bonds are present everywhere in almost every aspect of life, the exact nature of the interactions is still somewhat a mystery. Hydrogen bond (H-bond) is fundamentally defined as an electrostatic force of attraction between a hydrogen (H) atom, which is covalently bound to a more electronegative atom or group of atoms, such as nitrogen (N), oxygen (O), and fluorine (F) in particular. However, a more general definition given by IUPAC states that a hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment D–H in which D is more electronegative than H, and an atom or a group of atoms in the same or a different molecule in which there is evidence of bond formation. Such an interaction may be depicted as D–H···A–B, where the three dots denote the hydrogen bond. In this system, D–H represents the hydrogen bond donor; whereas the acceptor may be an atom or an anion A, or a fragment of a molecule A–B, where A is bonded to B. It is not necessary for D and A to be different; however, depending on the type of D and A, hydrogen bonds can be characterized as weak or strong. Moreover, in any event, the acceptor is an electron-rich region such as, but not limited to, a lone pair of A or π-bonded pair of A–B.
To put it in simple words, when hydrogen is bonded to a strongly electronegative element ‘D,’ the electron pair shared between the two atoms moves far away from the hydrogen atom. As a result, the hydrogen atom ‘H’ becomes highly electropositive with respect to the other atom ‘D’. Since there is a displacement of electrons towards D, the hydrogen acquires fractional positive charge ({δ}^{+}), while ‘D’ attain fractional negative charge ({δ}^{-}). This results in the electrostatic dipole-dipole interactions among the molecules that form a hydrogen bond. However, the hydrogen bond also has some similarities to the covalent bond. For instance, it is directional and strong, produces interatomic distances shorter than the sum of the van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a type of valence. These covalent features are more substantial when acceptors bind hydrogens from more electronegative donors. Hydrogen bonds are primarily categorized as intermolecular (occurring between separate molecules) or intramolecular (occurring among parts of the same molecule). In terms of strength, the hydrogen bond is stronger than a van der Waals interaction and weaker than fully covalent or ionic bonds. The hydrogen bonds have a strong influence on the structure and properties of the compounds. Let’s study a few of its examples in real life.
1. Water
Water is a necessary molecule for life to exist. It is an omnipresent substance that covers around 71% of the earth’s surface. Some of its oddly remarkable properties make it a substance of great importance in the study of various phenomena around us. For instance, at room temperature, water exists as a liquid, while most of the molecules similar to it are gasses. Hydrogen bonds are formed between neighboring water molecules when the hydrogen of one atom comes between the oxygen atoms of its own molecule and that of its neighbor. This happens because the hydrogen atom is attracted to both its own oxygen and other oxygen atoms that come close enough. The oxygen nucleus has 8 “plus” charges, so it attracts electrons better than the hydrogen nucleus, with its single positive charge. So, neighbor oxygen molecules are capable of attracting hydrogen atoms from other molecules, forming the basis of hydrogen bond formation.
Though it is quite weak compared to other bonds, the hydrogen bond is strong enough to make a significant difference. For example, due to the presence of hydrogen bonding, the boiling point of water is such that the molecules remain together, and water remains in the liquid state at room temperature, whereas other similar molecules, that don’t form hydrogen bonds, attain the gaseous state. These hydrogen bonds also affect the solid structure of water. In the liquid state, the hydrogen bonds of water can break and reform as the molecules flow from one place to another. When water is cooled, the molecules begin to slow down. Eventually, when water is frozen to ice, the hydrogen bonds become permanent and form a very specific network. The bent shape of the molecules leads to gaps in the hydrogen bonding network of ice. Therefore, the density of ice is less than the density of water at the same temperature; thus, the solid phase of water floats on the liquid, unlike most other substances.
2. DNA
During our childhood, we all have wondered how babies are born? When we grow up, we learn that a 5.1μm sperm cell from a male reproductive organ fuses with an ova or egg in the ovaries inside the female reproductive organ and take almost nine months to turn into a full-grown healthy baby. But have you ever wondered, what tells the sperm cell how to grow inside an ova? Deoxyribonucleic acid (DNA), otherwise known as a storehouse of information, is a complex molecule that contains all of the information necessary to build and maintain an organism. Apart from that, it also serves as the primary unit of heredity in organisms of all types. In other words, whenever organisms reproduce, a portion of their DNA is passed along to their offspring. DNA is found inside all living cells mainly inside the nucleus; however, it can also be found in mitochondria, the storehouse of energy inside a cell.
DNA is composed of four bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). These bases function as the fundamental units of the genetic code. In DNA, each type of base on one strand bonds with just one type of base on the other strand, via hydrogen bonding. Adenine (A) bond only to Thymine(T) via two hydrogen bonds, and Cytosine (C) bond only to Guanine (G) via three hydrogen bonds. These bonds ensure the stability of the double helix structure of the DNA.
3. Proteins
Protein is a macronutrient mainly found in animal products, nuts, and legumes. Most people include a necessary proportion of proteins in their daily diet intake for muscle growth. But proteins are not just muscle growth supplements, instead, they are highly complex biomolecules that are essential for life and have a great nutritious value. Proteins perform various functions within an organism, such as catalyzing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another.
Most proteins consist of linear polymers built from series of up to 20 different amino acids. These amino acids arrange themselves in several patterns to give four levels of protein structures: primary, secondary, tertiary, and quaternary. The secondary structure of the protein is formed as a result of hydrogen bonding between amino acids. The solubility of proteins in water is dependent on the ability to form hydrogen bonds with the protein surface. Proteins that have a greater hydrophilic surface content are generally more capable of forming hydrogen bonds with the surrounding water.
4. Nylon 66
In a modern-day world, nylon 66 is one of the most encountered polymers in day-to-day life. It is a name given to a synthetic polymer composed of polyamides (repeating units linked by amide links). It appears to be a silky thermoplastic material that can be melted and processed into films, fibers, and many other shapes. Along with these properties, nylon 66 is one of the strongest polymers, which makes it vividly employable in the material industry. The credit for the strength of nylon 66 goes primarily to the presence of hydrogen bonding among the adjacent strands of polyamides. Nylon 66 is made of two monomers each containing 6 carbon atoms, hexamethylenediamine ({NH}_{2}({CH}_{2})_{6}{NH}_{2}), and adipic acid (({CH}_{2})_{4}({COOH})_{2}), which reacts to form nylon 66.
The hydrogen bonds that occur between carbonyl and amine groups effectively link adjacent chains, which help reinforce the material. The chain axes are aligned along the fiber axis, making the fibers extremely stiff and strong. However, the presence of hydrogen bonding in many polymers makes them sensitive to the humidity levels in the atmosphere because water molecules can diffuse into the surface and disrupt the network of polymers.
5. Cellulose
Celulose ({C}_{6}{H}_{10}{O}_{5}) is a polysaccharide consisting of a linear chain of several hundred to many thousands of beta-glucose carbohydrate. Cellulose is credited as a building material for plants as it provides them with the required rigidity. Plant cells are primarily composed of cellulose, which makes them the most abundant macromolecule or biomolecule present on earth. Cellulose chains are inter-connected by OH–O-type hydrogen bonds to form flat sheets with CH–O hydrogen bonds. These chains form a structure called microfibrils, which, in turn, are bundled together to form macro fibrils that provide a stiff and firm structure to the plants. Though cellulose cannot be digested by humans due to the lack of required enzymes, it is still introduced into the human diet to act as fiber that helps the food move through our gut; however, animals, such as cows, sheep, and horses, can digest cellulose, which is why they can get the energy and nutrients they need from the grass. The principal commercial use for cellulose includes paper manufacturing, rayon, cotton, and linen fabrication.
6. Alcohol
Alcohol is a generic designation given to a recreational beverage around the world. However, in a chemistry lab, it is an organic compound that includes at least one hydroxyl functional group attached to a saturated carbon atom. More specifically, the beverage we find in the market is a much-diluted version of ethanol or ethyl alcohol present in the lab. It is created when grains, fruits, or vegetables are fermented. In general, the hydroxyl group makes alcohols polar. It can form hydrogen bonds to one another and most other compounds. Owing to the presence of the polar OH group, alcohols are more water-soluble than simple hydrocarbons. Methanol, ethanol, and propanol are miscible in water, whereas Butanol, with a four-carbon chain, is moderately soluble.
7. Ammonia
Ammonia ({NH}_{3}) is the most common substance used by agricultural chemists to make fertilizers. Though it is present in trace quantities in nature, it has vital importance for our ecosystem. It is the preferred nitrogen source for many prokaryotes, fungi, and plants and the end product of nitrogen metabolism in most living cells. As per the valence shell electron pair repulsion theory (VSEPR theory), a molecule of ammonia is composed of one nitrogen atom linked with three hydrogen atoms via covalent bonding to form a trigonal bipyramidal structure.
The central nitrogen atom has five outer electrons with an additional electron from each hydrogen atom. It gives a total of eight electrons or four electron pairs that are arranged tetrahedrally. The ability of ammonia to form hydrogen bonds makes it highly soluble in water and other polar solvents. Moreover, ammonia naturally exists as a colorless gas with a characteristically pungent odor, which can easily be liquified due to its ability to form strong hydrogen bonds. Although in comparison to water, the strength of hydrogen bonds in {NH}_{3} is lower. It results in lower melting point, boiling point, density, viscosity, dielectric constant, and electrical conductivity of ammonia than water.
8. Formic Acid
Formic acid is the chemical responsible for the horrific pain that occurs due to an ant-bite. In fact, the name “formic” comes from the Latin word for ant, Formica, in reference to its early isolation by the distillation of ant bodies. It is the simplest carboxylic acid, which can be found in a chemistry lab by the name methanoic acid. Formic acid (HCOOH) is a colorless liquid with a pungent odor and has the strongest acidity among unsaturated fatty acids. Due to its ability to form hydrogen bonding, it is miscible with water and most polar organic solvents and is somewhat soluble in hydrocarbons. In hydrocarbons and the vapor phase, it consists of hydrogen-bonded dimers rather than individual molecules.
9. Smart Rubber
A material that can mend itself back if torn, sounds like tech from the future. Smart rubber is a polymeric material that has the potential to transform this wishful thinking into reality; it can recover its original mechanical strength within several hours of being split and then subsequently recombined. This is accomplished by hydrogen bonding that can occur simply by pressing two faces of the substance together, allowing the recovery of a continuous hydrogen bonding network. Residual hydrogen bond donors and acceptors responsible for the self-healing properties of the elastomer remain unpaired until the newly exposed surface comes in contact with another complementary surface, allowing the formation of new intermolecular hydrogen bonds.