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Hydrogen Bonding

The electrostatic force of attraction between the hydrogen atom of one molecule and more electronegative atom of the same or another molecule, is called hydrogen bond. When hydrogen atom gets bonded to atoms of highly electronegative elements such as fluorine, oxygen or nitrogen,the formed molecule is polar.Hence the hydrogen atom forms a weak bond with the electronegative atom of the other molecule. This weak bond is called "hydrogen bond".

Hydrogen bond definition:
A hydrogen bond is an interaction that directs the association of the covalently bounded hydrogen atom with one or more other atoms, group of atoms or molecules into an aggregate structure that is sufficiently stable to make it convenient for the chemist to consider it as an independent chemical species.
The electron pair shared between the two atoms lies far away from the hydrogen atom. As a result, hydrogen atom (H) becomes highly electropositive with respect to the other atom (A). Since the electrons are displaced towards A, it acquires partial negative charge (d-) while hydrogen atom gets partial positive charge (d+). In other words, the bond H-A becomes polar and may be represented as Hd+- Ad-. The electrostatic force of attraction between positively charged hydrogen atom of one molecule and negatively charged atom of the neighboring molecule results in the formation of hydrogen bond. This may be represented as:

Electrostatic Force of Attraction in the Formation of Hydrogen Bond

The hydrogen atom acts as a bridge between two atoms, holding one atom by a covalent bond and the other atom by a hydrogen bond. The hydrogen bond is represented by dotted line (....) while the covalent bond is represented by solid line (—).

For example, in hydrogen fluoride HF, the hydrogen atom, while remaining bonded to its fluorine atom, forms another weak bond with fluorine atom of the neighboring molecule. As a result of hydrogen bonding, HF exists as a cluster of hydrogen fluoride molecules and is represented as (HF)n.

The hydrogen bond may be shown as:

HF ...............HF ...............HF .......HFn..

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Strength of Hydrogen Bond

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Hydrogen bond is much weaker than a covalent bond. The strength of hydrogen bond ranges from 10-40 kJ mol-1 while that of a normal covalent bond is 400 kJ mol-1. Thus, a hydrogen bond is about one-tenth of the strength of a covalent bond. The bond length of a hydrogen bond is larger than that of a covalent bond. For example, in case of HF molecule, the covalent bond between H and F is 109 pm, while the bond length of hydrogen bond between F and H is 155 pm. The molecule should contain an atom of high electronegativity such as F, O or N bonded to hydrogen atom. The common examples are H2O and NH3.

Examples of Hydrogen Bond

Small Size of the Electronegative Atom

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The size of the electronegative atom should be small. The smaller the size of electronegative atom, the greater is the attraction for the bonded electron pair. This causes greater polarity in the bond between H and the electro- negative atom, resulting in stronger hydrogen bond.

For example, in NH3 and HCl, N and Cl have the same electronegativity (3.0) and both N and Cl have Hydrogen atoms, only NH3 shows hydrogen bonding as N is smaller in size than Cl.

The main points of difference between hydrogen bond and covalent bond are:   

Hydrogen Bond Covalent Bond
 This bond involves dipole to dipole movement  This bond involves sharing of electrons.
 It is formed between hydrogen atom and higher
electronegative atom(F,O or N).
It is formed between any two electronegative atom, which may be the same or different elements.
 The bond strength of hydrogen atom is very small,e.g..H..F bond is 41.83 Kjmol-1  Strength of covalent bond is sufficiently high,e.g. H---H bond is 433 kJmol-1

Hydrogen Bonding in Water

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Water readily forms hydrogen bonds with dissolved substances and this is an important factor in evaluating the tendency of groups to hydrogen bond. An intermolecular hydrogen bond between two groups within a protein forms at the expense of two hydrogen bonds with water. Likewise the formation of an intermolecular hydrogen bond between a protein and ligand also involves breaking hydrogen bond with water. This diminishes the energy that hydrogen bonds can contribute to structural stability.

Water readily forms hydrogen bonds. An electrostatic potential surface for two water molecules shows the hydrogen bond involving the negatively charged oxygen atom of one molecule and the positively charged hydrogen atom of a neighboring molecule.

Hydrogen Bonding in Water

Hydrogen bond in water is cooperative. That is an hydrogen bonded water molecule serving as an acceptor is a better H-bond donor than an unbonded molecule. Thus participation in H bonding by H2O molecules is a phenomenon of mutual reinforcement. The H-bonds between neighboring molecules are weak relative to the H-O covalent bonds. As a consequence the hydrogen atoms are situated asymmetrically between the two oxygen atoms along the O-O axis. There is never any ambiguity about which O atom the H atom is chemically bound to nor to which O it is H bonded.

Hydrogen Bonding Examples

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Hydrogen bond is much weaker than a covalent bond. Its strength vary between 10-40kJ per mole and that of covalent bond is of the order 400kJ per mole. H-bond is usually longer than the covalent bond present in the molecule. hydrogen bonds are of two types.

Intermolecular H-bonding (Association)

This type of H-bond is formed between two molecules of the same or different substances.

Example - NH3, H2O, HF, ROH etc. In water and alcohol the H-bonding may extend to several molecules but in carboxylic acids, the H-bonding is limited to the association of two molecules only. The anion HF2- present in KHF2 has the strongest known hydrogen bond. In CuSO4.5H2O one H2O molecule is held to SO42- ion and to two of the four coordinated H2O molecule by H-bond.

Intermolecular Hydrogen Bond

Intramolecular H-bonding (Chelation)

This type of H-bond occurs between H-atom and the electronegative atom within the same molecule.

- ortho nitrophenol, salicilaldehyde.

Intermolecular Hydrogen Bond Chelation

The strength of hydrogen bond decreases in the following order.

H---F > H---O > H---N

Hydrogen bonds is even stronger intermolecular attractive force, as evidenced by the large increase in the melting point and boiling points of the alcohol methane relative to those of the hydrocarbon ethane and both are comparable molecular weight.

More topics in Hydrogen Bonding
Intermolecular Hydrogen Bonding Intramolecular Hydrogen Bonding
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