Hydrogen-bond





Hydrogen-Bond

[Introduced By Latimer and Rodebush]

It may be defined as, “the peculiar weak bond between the hydrogen atom of one molecule and highly electronegative atom (such as N, O, F) of same or another molecule”. For example several H –F molecules are associated by hydrogen bond as:

\cdots \overset{+\delta}{H} - \overset{-\delta}{F} \cdots \overset{+\delta}{H} - \overset{- \delta}{F} \cdots \overset{+ \delta}{H} - \overset{+ \delta}{F} \cdots

 

The bond energy of hydrogen bond is of the order of 20 k J \text{mol}^{-1} hydrogen bond is represented by dotted line (…….) while the covalent bond is represented by solid line.

The hydrogen bond energies of some typical molecules are shown below:

H- \underset{30}{F} \cdots H \; H H \underset{12.5- 25.0}{-O \cdots} H \; N \underset{12.5- 21}{-H \cdots} N \; O \underset{20 kJ \text{mol}^{-1}}{- H \cdots} N

 

It is clear from the above that bond energy decreases with decrease of electronegativity of the bonded atom with hydrogen. The hydrogen bonds are of two types:

 

(i) Intermolecular hydrogen bond is formed when hydrogen atom of one molecule is connected with the electronegative element of the other molecule e.g.

\cdots H- F \cdots H- F \cdots H- F \\ \cdots \underset{\underset{H}{|}}{O}- H \cdots \underset{\underset{H}{|}}{O}- H \cdots \underset{\underset{H}{|}}{O}-H \cdots \\ \cdots \overset{\overset{H}{|}}{ \underset{\underset{H}{|}}{N}}- H \cdots \overset{\overset{H}{|}}{ \underset{\underset{H}{|}}{N}}- H \cdots \overset{\overset{H}{|}}{ \underset{\underset{H}{|}}{N}}- H \cdots

 

R – OH (alcohols). R – COOH, Nucleic acid, DNA. etc. all have intermolecular H-bonding.

HF^-_2 or KHF_2  is formed due to intermolecular H-bonding.

KHX_2  is not possible as other halogens cannot form H-bonding.

Due to this H-bonding

NH_2 > PH_3

HF > HX

R – OH > R – O – R

R – COOH > R – COX (Acid derivatives)

More:  B.P., Surface Tension, Viscosity Solubility in H_2O, Thermal Stability due to Intermolecular H-bonding.

 

(ii) Intermolecular hydrogen bond: is formed when in the same molecule hydrogen atom is connecting with strong electronegative element e.g.

Hydrogen-bond

  • only H_2O molecule can have H-bonding with four other water molecules.
  • Due to this, acidic nature decreases but volatile nature increases.

E.g. o-nitrophenol is less acidic but more volatile than p-nitrophenol. Both are separated by steam distillation.

Properties of Hydrogen Bond

(i) Strength of hydrogen bond decreases with decrease in the electronegativity of the two bonded atoms.

(ii) Generally the types A – H. . . B hydrogen bonds can be taken lo be approximately linear.

(iii) A hydrogen bond is comparatively weak. its bond energy is of the order of 20 k J \text{mol}^{-1}

(iv)  The hydrogen bonds are simple cases of dipole-dipole or ion-dipole attraction.

(v) In all hydrogen bonds the hydrogen atom is nearer to one of the two adjacent electronegative atoms to the other.

(vi) The size of the electronegative element should be small.

 

Effects of hydrogen bonding

1. Density of ice is less than that of liquid water but more volume.

2. Melting and boiling points of such molecules are high. This is due to the fact that some extra energy is required to break dawn hydrogen bond

E.g. H_2O, NH_3, HF o-chloro-and nitro phenols etc.

3. The lower alcohols are soluble in water because they form hydrogen bond with water.



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