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Hydrogen Bonding

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Introduction to Hydrogen Bonding

In 1920, Latimer and Rosebush introduced the idea of "Hydrogen Bond" to explain the nature of association in liquid state of substance like water, hydrogen fluoride, ammonia, formic acid etc. In a Hydrogen Compound, when hydrogen forms bonds with highly electronegative atom (such as F, O, N) by a covalent bond, the electron pair is attracted towards electromagnetic atoms which is so strong that it results in dipole i.e. one end carries a partial positive charge (H-end) and other end carries a partial negative charge and because of this dipole hydrogen bond is formed The hydrogen bond is most simply regarded as a weak electrostatic attraction between a lone pair of electron on one atom, and a covalently bonded hydrogen atom that carries a fractional charge 𝛿+. 

Hydrogen Bonding in Water MoleculeFig. No. 1 Hydrogen Bonding in Water Molecule

Thus, an attractive force that binds hydrogen atom of one molecule with electronegative atom of other molecule of the same or different substance is known as hydrogen bond.
 

Evidence of Hydrogen Bonding

Many evidence for hydrogen bonding comes from structural studies, various examples include ice, which has been determined both by X-ray and neutron diffraction, the dimeric structure of formic acid, X-ray structures of the solids for Sodium Hydrogen Carbonate and Boric acid and many others.  Another technique for studying hydrogen bonds is Infrared absorption spectra in CCl4 which allows studying of O-H and N-H stretching frequencies.

Hydrogen bonding was first used to explain the weakness of trimethyl ammonium hydroxide as a base compared with tetramethylammonium hydroxide. In the trimethyl compound the OH group is hydrogen bonded to the Me3NH group and this makes it more difficult for the OH group to ionize, and hence it is a weak base. In the tetramethyl compound, hydrogen bonding cannot occur, so the OH group ionizes and the tetramethyl compound is thus a much stronger base. Structure of trimethyl ammonium hydroxide is shown in figure below.

Structure of Trimethyl ammonium hydroxide and Tetramethyl ammonium hydroxide      Fig. No. 2 Structure of Trimethyl ammonium hydroxide and Tetramethyl ammonium hydroxide

What Causes Hydrogen Bonding

In some compounds a hydrogen atom is attracted by strong forces of two atoms. As Hydrogen donor atom is strongly electronegative, it results in pulling of covalent bond towards the electronegative species, giving hydrogen a partial positive charge (As shown in Fig. No. 2). These activities result in creation of dipole-dipole interaction between the hydrogen atom and the lone electron pair on the electronegative compound.

Creation of Dipole Moment in Hydrogen Bond
 Fig. No. 3 Creation of Dipole Moment in Hydrogen Bond

Types of Hydrogen Bonding

Intermolecular Hydrogen Bonding 

Hydrogen bonding in which bonding results between the positive and negative ends of different molecules of the same or different substances. Example - Hydrogen Bonding in water, ammonia, acetic acid. In general the melting and boiling points for a related series of compounds increase as the atoms get larger, owing to the increase in dispersive force.

Hydrogen Bonding in ammonia and Hydrogen Bonding in acetic acitFig. No. 4 Hydrogen Bonding in ammonia and Hydrogen Bonding in acetic acit

This type of hydrogen bonding increases the boiling point and solubility in water due to association of several molecules of the compound.

Intramolecular Hydrogen Bonding

This type of bonding results between hydrogen and electronegative element both present in the same molecule, mostly in organic compounds. Examples are o-nitro-phenol and salicylaldehyde.

Ortho-Nitro phenol and SalicilaldehydeFig. No. 5 Ortho-Nitro phenol and Salicilaldehyde

This type of bonding decreases the boiling point and solubility of the compounds making compounds more volatile.

Hydrogen Bond in Proteins

Hydrogen bonds are of great significance both in biochemical systems and in normal chemistry. They are extremely important because they are responsible for linking polypeptide chains in proteins, and for linking pairs of bases in large nucleic acid-containing molecules. The hydrogen bonds maintain these large molecules in specific molecular configurations, which is important in the operation of genes and enzymes.The yellow dotted line (Fig. No. 5) shows Hydrogen Bond in Protein structure.

Bonding In ProteinsFig. No. 5 Bonding In Proteins

Influence of Hydrogen Bond on Properties

Higher Melting and Boiling Points

Compounds with Hydrogen Bond possess higher melting and boiling point. This is due to the fact that because of hydrogen bonding, the electrostatic force of attraction in the molecules become large and greater energy is required to separate these molecules before they can boil or melt.

Association

Because of intermolecular hydrogen bonding two or more molecules of a compound exist as associated molecules. Example - Even in vapor state Carboxylic acid exists in dimer form, increasing its size and molecular mass.

Influence on Physical State

Hydrogen Bonding has great influence on physical states of substances. For Example -  H2O exist as liquid state at room temperature, while H2S as gas even though both O and S belong to the same group. This happens because of difference in electronegativity values of O and S as O being highly electronegative it forms hydrogen bond.

Solubility

With increase in number of hydrogen bonds, the solubility of substance also increases. For Example, Alcohol is highly soluble in water because of hydrogen bonding.

Volatility

Compounds with hydrogen have higher boiling points, which makes them less volatile compared to other corresponding member of other groups. The enthalpy of vaporization is follows almost similar pattern as shown by their melting and boiling points. For Example, in group 15 à H2S < H2Se < H2Te < H2O

Importance of Hydrogen Bonding

Hydrogen bonds play an important role in formation and properties of various compounds.

Because of Hydrogen Bond Water possess high boiling point about 373 K, but in the absence of hydrogen bond it will boil at very low temperature which is close to CO2 (-1053K).Water has high specific heat capacity i.e. it takes a lot of energy to heat water up thereby making it an important coolant and this all because of hydrogen bonds.

Proteins will not be able to form secondary structure without the help of hydrogen bonds and so could not be functional.

The hydrogen bonding provides many substances with some of most amazing and unexpected properties which are responsible for sustaining life on Earth for example DNA, Water, Proteins etc.

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