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Hydrogen Bonding

The hydrogen bond is an electrostatic force of attraction between a hydrogen atom attached to more electronegative atom and some other electronegative atom of the same or different molecule. For example, in water H2O, hydrogen is attached to electronegative atom oxygen. The O–H is a polar covalent bond. Hydrogen atom has partial positive charge and oxygen has partial negative charge.

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An electrostatic force of attraction is set up between hydrogen atom of one molecule and oxygen atom of other molecule. Hydrogen bond is represented by dotted line. As a result of hydrogen bonding, H–atom links the two electronegative atoms simultaneously. One of them is a covalent bond and the other by a hydrogen bond. Hence, it is said to form a hydrogen bridge.

Hydrogen bonding is not present everywhere in chemistry but it is used everywhere in organic chemistry. For example: If we want to know that which substance is more soluble in water, then we will see its hydrogen bonding. The substance which makes good hydrogen bonds would be more soluble in water. If we want to see the boiling point difference in two substances, we would see the H–bonding in the substance. The substance which makes good hydrogen bond would show higher boiling point.

Conditions for Hydrogen Bonding

The following conditions must be fulfilled for the formation of hydrogen bond.

  1. Molecule must contain a highly electronegative atom linked to H–atom. Higher the electronegativity, greater is the polarization of the molecule.
  2. The size of the electronegative atom should be small. Smaller the size of the electronegative atom, greater is the electrostatic attraction.

A is an electronegative atom which has E.N. equal or greater than 3. Only F4.0, O3.5, N3.0 (C2.55 and Cl3.16 are special cases in organic chemistry) can be the atom which makes bond with hydrogen as covalent bond. Bond length of covalent bond is smaller than hydrogen bond due to very strong bond. B is electronegative atom with small size and lone pair, which is making hydrogen bond with hydrogen can be F, O, N (C and Cl are special cases in organic chemistry).

Examples of Hydrogen Bonding

Hydrogen Bonding in Water 

Water molecule contains highly electronegative oxygen atom linked to hydrogen atom. The negative end of one molecule attracts the positive end of the other and hydrogen bonding takes place. The properties of water as high boiling point, high heat of vaporization and formation of solid ice of low density can be best explained by hydrogen bonding in it.

Hydrogen Bonding in Ammonia 

It contains the highly electronegative atom nitrogen (N) linked to H–atoms. Hence hydrogen bonding takes place as follows:

Hydrogen Bonding in Hydrogen Flouride 

Flourine, having the highest value of electronegativity forms the strongest hydrogen bonds. Orientation of various atoms and bond lengths are indicated below:

It is a zig-zag collection of HF molecules. Very low acidic strength of HF as compared to HCl, HBr and HI can be justified by it.

Hydrogen Bonding in Alcohols and Carboxylic Acids

The molecules contain the highly electronegative oxygen atom linked to H–atom and hence, form associated molecules as follows:

The high M.P, B.P and other thermodynamic properties are due to H–bonding in alcohols and acids. Dimer formation is the best outcome of this concept of H – bonding.

Hydrogen Bonding in Chloroform and Acetone

This is the special case where you can observe that hydrogen bond is taking place between C – H…O

                                         Chloroform (CHCl3) and Acetone (CH3COCH3)

Strength of Hydrogen Bond

The strength of hydrogen bonds is intermediate between the weak van der Waal’s force and the strong covalent bonds. Bond dissociation energy of a covalent bond is in the range 209-418 kJmol-1. The dissociation energy of the H–bond depends upon the attraction of the shared pair of electrons and, hence, on the electronegativity of the atom.

For H………F, the bond dissociation energy is 41.8 kJ mol-1, H……..O is 29.3 kJ mol-1 and H….…N is 12.6 kJ mol-1.

Van der Waal force ˂ H–bonding ˂ Covalent or Ionic Bond

B.E ˂ 8 KJ/mole B.E = 8– 42 KJ/mole B.E ˃ 200 KJ/mole

Bond Energy 

It is a measure of the strength of a chemical bond.

  1. B.Ex ˃ B.Ey Always Possible
  2. B.Ex = B.Ey Also Possible
  3. B.Ex ˂ B.Ey Not possible

Bond energy of the covalent bond is commonly greater than bond energy of hydrogen bond. The bond energy of the covalent bond can be equal to bond energy of hydrogen bond in one case if HF2 where it has bond energy equal to covalent bond.

Comparison between Hydrogen Bond and Covalent Bond

S# Hydrogen Bond S# Covalent Bond
i.
Hydrogen bond involves dipole-dipole attractive interactions
i.
Covalent bond involves sharing of electrons.
ii.
The strength of this bond is very small, e.g., bond strength of H……F is 41.8 kJ mol-1.
ii.
The bond strength of this bond is sufficiently high, e.g., bond strength of H – H bond is 433 kJ mol-1.
iii.
It is formed between hydrogen atom and a highly electronegative atom such as F, O and N.
iii.
It is formed between two electronegative atoms which may be of the same element or of different elements.

Applications of Hydrogen Bonding

High Melting and Boiling Points

The compounds having hydrogen bonding show abnormally high melting and boiling points. High melting and boiling points of the compounds containing hydrogen bonds is due to the fact that some extra energy is needed to break these bonds during the process of melting and boiling.

A stronger H–bond exists in HF than in water, yet water boils at higher temperature than HF. The reason is that each water molecule can form two hydrogen bonds involving their hydrogen atoms plus two further hydrogen bonds utilizing the hydrogen atoms which are attached to neighboring water molecules. Water can form four H–bonds as compared to one in HF.

H2O is liquid whereas H2S, H2Se and H2Te are gases at room temperature. This is because hydrogen bonding causes association of the H2O molecules with the result that the boiling point of water is more than that of the other compounds.

Ethanol has higher boiling point than diethyl ether and diethylamine due to H–bonding in ethanol.

Aldehydes and ketones cannot have strong H–bonds between their molecules, they have low boiling points than corresponding alcohols.

The molecules of amides with one or no substitution on N–atom are able to form strong hydrogen bonds to each other. Such amides have higher M.P and B.P.

Dissociation

In aqueous solution, hydrogen flouride dissociates to give the diflouride ion HF2, but not F. This is due to H–bonding in HF and explains the existence of KHF2. On the other hand, the molecules of HCl, HBr and HI do not have H–bonding. The reason being that Cl, Br and I are not so highly electronegative. There is no existence of compounds like KHCl2, KHBr2 or KHI2.

Association

Carboxylic acids exists as dimers because of the hydrogen bonding. The molecule masses of such compounds are found to be double than those calculated from their simple formula. For example molecular mass of acetic acid is found to be 120.

Solubility

Compounds that form hydrogen bonds with the solvent are soluble in such solvents. For example, lower alcohols are soluble in water because of hydrogen bonding which can take place between water and alcohol molecules are shown below:

Higher alcohols are not much soluble.

Similarly, ammonia (NH3) is soluble in water because of hydrogen bonding as represented below:

Viscosity and Surface Tension

The substances which contain hydrogen bonding exists as associated molecules. So, their flow becomes comparatively difficult. In other words, they have high viscosity and high surface tension. Honey, mostly glucose and fructose (see image below) is a good example of a liquid which owes its viscosity to hydrogen bonding.

Low density of Ice than Water

Hydrogen bonding gives as cage like structure to solid ice. As a matter of fact, each water molecule is linked tetrahedrally to four water molecules. Ice has low density (0.9 g/cm3) at 0 ℃ as compared to water (1 g/cm3) at 4 ℃ because the molecules are not so closely packed as they are in liquid state. When ice melts, this cage like structure collapses and the molecules come close to each other.

                                                           Cage like structure of Ice

We assume that it is filled with water but originally it is hollow inside filled with air.




Types of Hydrogen Bonding

Hydrogen bonding is of two types:

Intermolecular Hydrogen Bonding

When hydrogen bonding takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding. Hydrogen fluoride, water, alcohols and solutions of alcohol or ammonia in water involve intermolecular hydrogen bonding.

Homo molecular

It occurs between two or more same molecules.

Example: Two molecules of H2O

Hetero molecular

It occurs between two or more different molecules.

Example: R–OH and H2O

Intramolecular Hydrogen Bonding

When hydrogen bonding takes place within a molecule, it is called intramolecular hydrogen bonding. It takes place in compounds containing two groups.

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