When a H-atom is bonded to a highly electronegative atom like F, 0 or N (say, Z) by a covalent bond, the bond pair of electrons is displaced towards the electronegative atom. when solitary electron of hydrogen atom lies away from it, its nucleus gets exposed and behaves as a bare proton. Such a bare hydrogen nucleus exerts a strong electrostatic attraction on the electronegative atom of the adjacent molecule. This
interaction between hydrogen atom of one molecule and the electronegative atom of the other molecule is referred to as hydrogen bond. Thus, hydrogen bond is defined as the electrostatic force of attraction which exists between the covalently bonded hydrogen atom of one molecule and the electronegative atom of the other molecule. The hydrogen bond is represented by dotted line( ….. ). For example, in case of hydrogen fluoride the hydrogen bond exists between H atom of one molecule and fluorine atom of another molecule as shown.
It may be noted that the hydrogen atom acts as a bridge between the electronegative atoms of two different molecules, to one atom it is linked through a covalent bond while to the other it is linked through a hydrogen bond. However, the hydrogen atom does not lie in the centres of the two electronegative atoms because hydrogen bond and covalent bond do not have same strength.
STRENGTH OF HYDROGEN BOND
Hydrogen bond is much weaker than covalent bond and its strength ranges between 12.5 kJ mol-1 to 41.5 kJ mol-1The strength of hydrogen bond is governed by the following factors:
(i) Extent of polarisation within the molecule. The molecule must contain highly electronegative atom linked to H-atom. Higher the-electronegativity of the atom greater is the polarisation and larger will be the strength of hydrogen bond.
(ii) Size of the electronegative atom should be small. We know electronegativity increases from N à 0 àF and atomic radius also decreases in the same order. Therefore, H-Bond strength increases as
H-N ….H < H-O. …H < H-F. …H
(iii) Lone pair of electrons must be present on the electronegative atom which can point towards the H-atom of adjacent or near by molecule.
(iv) The angle between the three atoms involved (two electronegative atoms and central H atom) must be 180°. lt may be noted that as the angle decreases, strength ofH-bond also decreases and no, H-bonding occurs when angle falls below 140°.
EXAMPLES OF HYDROGEN BONDING
Let us study some examples of the compounds which exhibit hydrogen bonding.
1. Hydrogen fluoride. In hydrogen fluoride hydrogen atom is bonded to highly electronegative atom fluorine (electronegativity = 4). It has been found that in solid state hydrogen fluoride consists of long zig-zag chains of H-F molecules associated by H-bonds as shown in Fig. 8.l(a).
On heating, progressively, the length of the chain shortens, and associated units become quite small.
Fig. 8.1(a). Zig-zag chains of H-F molecules.
2. Water. In water molecule, oxygen atom is bonded to two hydrogen atoms. Due to large electronegativity, oxygen atom forms the negative centre whereas each of the hydrogen
atom acquires a partial positive charge. Each O atom can form two hydrogen bonds as shown in Fig. 8.1 (b).
Fig. 8.1(b). H-bonds in H2O molecules.
3. Ammonia. In ammonia molecule, nitrogen, an electronegative atom is bonded to three hydrogen atoms. The nitrogen atom forms a negative site of the molecule whereas each of three H-atoms acquires a partial positive charge. The ammonia molecules are associated by H-bonds as shown in Fig. 8.1 (c).
4. Ethanol (C2 H5 OH) and Ethanoic acid (CH3COOH): These molecules contain the highly electronegative oxygen atom linked to H-atom and H atom form associated molecules as shown in Fig. 8.2.
Fig. 8.2. H-bonds in C2H5OH and CH3COOH .
EFFECT OF HYDROGEN BONDING O N THE PROPERTIES
Hydrogen bond has a marked influence on the properties of various substances as discussed as follows:
(i) Association. The hydrogen bonds link up molecules of the same substance to form· large aggregates. This is called association of molecules. For example, H-F molecules are associated with one another by hydrogen bonds. The formula of hydrogen fluoride can be written as (HF)n where, the value of n depends on the physical state of the compound. Similarly enthanoic acid exists as dimer and pertains to formula (CH3COOH)2 The association of molecule through hydrogen bonding results in the unexpected larger values of many physical properties such as melting points, boiling points. enthalpy of vaporisation enthalpy of fusion, etc. For example, if we compare these physical properties among the hydrides of elements of group 16 we find that all these properties are unexpectedly higher for H2O as compared to those of H2S. H2Se and H2 O as shown in Table 8.1. This is because of the fact that H20 molecules are associated through hydrogen bonding whereas hydrides of other elements have van der Waal forces among their molecules.
Table 8.1. Physical Properties of Hydrides of Group 16 Elements
(ii) Melting and boiling points. The compounds whose molecules are associated with one another by hydrogen bonds have abnormally high melting and boiling points. It is due to fact that a large amount of energy is needed to overcome intermolecular hydrogen bonds and to separate the molecules. let us compare the boiling points of hydrides of elements of group 15, 16 and 17 of the periodic table.
The hydrides of first member of each group namely; NH3, and HF have abnormally high boiling points. For example, in case of halogen acids the boiling point decreases with the decrease in the molecular mass, i.e., from Ill to HBr and from HBr to HCl. But there is a sudden rise in the value H- boiling point in case of HF. The relatively high boiling point of HF is due to the presence of intermolecular hydrogen bonds in hydrogen fluoride. In case of other halogen acids no H-bonding occurs due to the large atomic sizes and small electronegativity’s of other halogens.
Similar explanation can be given to explain the abnormal boiling points of H20 among the hydrides of elements of the oxygen group and that of NH3 among the hydrides of the elements of nitrogen group. The relative boiling points of difference hydrides of group 14, 15, 16 and 17 are shown graphically in Fig. 8.3.
It may be noted from the graph that CH4 in group 14 does not have H-bonds as intermolecular forces. Hence, IT does not follow the pattern of hydrides of elements of group 5. 16, 17.
(iii) Solubility. Hydrogen bonding can also provide explanation for the solubility of certain compounds in water. We are familiar with the fact that ionic compounds dissolve in water due to ion-dipole interactions. But the molecular compounds having polar groups in their molecules dissolve in water due to their tendency to form H-bonds with water molecules. For example, lower alcohols are soluble in water because their molecules can form H-bonds with water molecules.
The molecules of glucose, sugar honey, carboxylic acids have polar-OH groups in their molecules. Hence, their solubility, in water is also attributed to the ability of their molecules to form H-bonds with water molecules.
It may be noted that hydrogen bonding also influences other physical properties of the substances such as viscosity, surface tension, etc. For example, glycerol (CH2 H. CHOH.CH20H) has three -OH groups while ethanol (C2HpH) has one -OH group per molecule. The hydrogen bonding is more extensive in glycerol in comparison to ethanol. Consequently, glycerol is more viscous than ethanol.
TYPES OF HYDROGEN BONDS
There are two types of hydrogen bonds intermolecular hydrogen bonds and intramolecular hydrogen bonds
1. Intermolecular Hydrogen Rond. Intermolecular hydrogen bond is formed between two different molecules of the same or different substances. For example:
(i) Hydrogen bond between the molecules of hydrogen fluoride.
(ii) Hydrogen bond between alcohol and water molecules. Intermolecular hydrogen bond results into association of molecules. Hence, it usually increases the melting point, boiling point, viscosity, surface tension, solubility, etc.
2. Intramolecular Hydrogen Bond. Intramolecular hydrogen bond is formed between the hydrogen atom and highly electronegative atom (F, 0 or N) present in different bonds within the same molecule.
Intramolecular hydrogen bond results in the cyclisation of the molecules and prevents their association. Consequently, the effect of intramolecular hydrogen bond on the physical properties is negligible.
A few examples of molecules which form intramolecular hydrogen bonds are given below:
E-Bonding in water and Ice
We have studied in previous unit that water is a polar molecule having dipole moment (µ) = 1.84D as shown in Fig. 8.4.
Fig. 8.4. Polar nature of H2O.
AGGREGATION OF WATER MOLECULES IN LIQUID STATE
In gaseous state, the individual covalent molecules H2O exist as such. However, in liquid state, large aggregates of varying number of H20 units are formed because of their association through intermolecular hydrogen bonds.
In fact, in liquid water, the free H2O molecules an associated H2O molecules exist in a state of dynamic equilibrium.
The extent of association, however, depends upon the conditions of temperature and pressure. Due to intermolecular H-bonding water has abnormally high freezing point, boiling point, enthalpy of fusion, enthalpy of vaporisation. It also has high surface tension and viscosity. Due to high surface tension. water droplets are spherical in shape and surface of water bends to form a curved meniscus in burettes an capillary tubes. Hydrogen Bonding in Ice
In ice, a solid state of water, each H2O molecule is tetrahedrally surrounded by four neighbouring Hp molecules with their oxygen atoms occupying the comers of tetrahedron. There are four H atoms around each 0 atom. Two of the four H atoms are bonded by covalent bonds (bond length 100 pm) whereas the other two are linked through hydrogen bonds (176 pm) as shown in Fig. 8.5. This gives highly ordered three dimensional structure having large vacant spaces Fig. 8.6 which may be compared to open cage.
Fig 8.1. open cage like structure of ice
Due to open cage-like structure, ice has a relatively larger Volume for a given mass of liquid water. Consequently, Sity of ice is less than water and it floats over water.
As the temperature is raised beyond 273 K, open cage e structure starts dismentalling due to cleavage of some H-bonds and ice melts. The breaking of H-bonds causes large aggregates of 0 molecules to move closer resulting in the decrease in volume and thereby increase in density. This continues till 277 K When the density becomes maximum. Beyond 277 K more and H-bonds cleaves and expansion of liquid water starts occurring due to increased K.E. of molecules and the density again starts decreasing, however, it remains higher than ice. Hence, density of water is maximum at 277 K (4°C).
This property of maximum density at 277 K is a boon for the survival of aquatic animals during winter months because when the upper layer of sea water freezes.
The frozen water does not sink to the bottom but keeps ·floating at the surface due to its lesser density. This provides thermal insulation to the water below it.
HYDROGEN BONDS AND STRUCTURE OF MACROMOLECULES
Macromolecular structures such as proteins, carbohydrates, nucleic acids, etc., are joined together by the formation of H-bond through their active groups. Some examples are being discussed as follows:
(i) Proteins are polymers of a-amino acid and they have > c = 0 and – N-H groups in their molecules. These groups form both intermolecular and intramolecular hydrogen bonds.
The a -helix structure (spiral structure) of proteins is due to intramolecular hydrogen bonding (Fig. 8.7.) Further, the protein chains are held together by intennolecular hydrogen bonds to form fibres as in hair, silk and wool, etc. It may be noted that curly nature of hair is because of S-S cross links in addition to H-bonding. This cross linking occurs between one part of a-helix chain over another.
Fig .8.7 .a helix structure of protein.
Proteins are appreciably soluble in water due to their tendency to form H-bonds with water molecules. Proteins are an important part of body tissue. Dissolved protein is absorbed into the blood and other aqueous fluids so that it can be used for growth. Proteins are an essential constituents of food and occur in meat, fish and beans .
Cellulose which forms about 50% of wood structure is a polymer of glucose units. The polymeric chains in cellulose are held by hydrogen bonds and it gives fibrous nature to cellulose which can be observed in wood pulp, cotton, jute etc.