Chapter 1:- Structure and Reactivity

B.sc 1st year Book
Organic Chemistry
(Page 10)

# Hydrogen bond

Latimer and Radebush 1920 introduced the concept of hydrogen bonding. According to them, it is formed when a slightly acidic hydrogen atom already bonded to a strongly electronegative atom such as F, N, or O is linked with weak electrostatic force by the n-bonded pair of electrons of another atom.

## Hydrogen bond Definition:

A linkage formed between a hydrogen atom that is already covalently bonded to a highly electronegative atom (e.g. F, O, N, and sometimes Cl ) and the same electronegative atom of some other molecule is called a “hydrogen bond“. The hydrogen bond is usually denoted by a dotted line (….).Hydrogen bonding is also referred to as “proton bonding”.

## Characteristic features of H-bond:

1. Hydrogen bonding is a type of polar bonding that occurs only with highly electronegative atoms of small size viz. F, O, and N.
2. Hydrogen bond distance is longer than normal covalent bond distance.
3. The hydrogen bond energy lies between 3 to 10 Kcal/mole while that of a normal covalent bond is in the range of 50−100 Kcal/mole.
4. It is weaker than a single bond and stronger than van der Waals forces(1 Kcal/mole).
5. In a compound having the general formula, H−Z (where Z=N, O, or F ) would involve a hydrogen bond.

6. In general, the strength of hydrogen bonds increases if
(a) greater is the polarity of the H−Z bond.
(b) greater is the electron donor character of Y−M
(c) greater is the stability of polar form Zδ-−Hδ+….Yδ-−Mδ+. and
(d) greater is the linearity of the Z−H…….Y−M bond (where Z and Y are high electronegative bonded atoms of the molecules Z−H and Y−M respectively and M stands for the remainder of the molecule to which Y belongs). Hydrogen bonds are classified into two groups viz. Intermolecular and Intramolecular.

## Type of Hydrogen Bonding

There are two types of hydrogen bonding.

(a) Intermolecular hydrogen bonding:
(b) Intramolecular hydrogen bonding:

### (a) Intermolecular hydrogen bonding:

Such a type of hydrogen bond takes place when two or more similar or different molecules combine together to give a polymeric aggregate. The following examples illustrate this type of hydrogen bonding. Examples:

#### (i) H2O molecules :

In H2O each oxygen atom is bonded with four H-atoms by two normal covalent and two hydrogen bonds.

#### (ii) NH3 molecules :

In NH3 each nitrogen atom is bonded with four H-atoms by three normal covalent bonds and a hydrogen bond.

#### (iii) HF molecules :

In (HF)n clusture, each F atom is bonded with two H-atoms one by a normal covalent bond and the other by a hydrogen bond.

### (b) Intramolecular hydrogen bonding (chelation) :

Intramolecular hydrogen bonding takes place within two atoms of the same molecule. In this type, a planar 5 or, 6- membered chelate is formed. It is therefore this type of hydrogen bonding commonly known as ‘Chelation’. For such bonding, the interacting atoms should be placed in such a way that there is minimum strain occurs during the closure of the rings.
Intramolecular hydrogen bonds are more effective than intermolecular hydrogen bonds.
Or
When a hydrogen bond is formed within a single molecule between two different functional groups is called intramolecular hydrogen bonding or ‘internal hydrogen bonding: This leads to the formation of a 5 or 6-membered ring structure. Thus, this hydrogen bonding is also known as chelation. For example, ortho-nitrophenol, ortho-chloro-phenol, salicylic acid, salicylaldehyde, etc. form chelates due to intramolecular hydrogen bonding.
The boiling point of O-nitrophenol is 214∘C whereas its para and meta isomers have boiling points of 270∘C and 290∘C respectively. The reason for the low boiling point of ortho-nitro-phenol is due to the intramolecular hydrogen bond whereas the other two isomers show intermolecular hydrogen bonds and therefore possess higher boiling points.
The evidence for hydrogen bonding is determined on the basis of spectroscopic studies like X-ray, neutron diffraction, electron diffraction, and infrared studies. For example, the dimeric structure of formic acid is confirmed by electron diffraction studies.

## Limitation of hydrogen bonding:

In the formation of a hydrogen bond between Z-H….Y (between Z and Y ), two conditions are necessary. (i) Z and Y must be highly electronegative.
(ii) the distance between Z and Y must be −2.7Å.

## Causes of  Hydrogen Bonding.

### I. Intermolecular hydrogen bond causes-

(a)  Rise in the boiling point and melting point of the substance, It is because extra energy is required to break the hydrogen bond.
(b) Increase in the solubility, which is due to the solvation of the solute molecules in polar solvents by hydrogen bonding. Thus alcohols, amines, acids, sugars, etc are water-soluble.

### II. Intramolecular hydrogen bonding causes :

(a) Decrease in the solubility in polar solvents due to restricted hydrogen bonding with polar solvents. Thus, the enolate form of ethyl acetoacetate is less soluble in water and more soluble in an apolar organic solvent like cyclohexane, since intramolecular hydrogen bonding prevents solvation in water.
(b) Decrease in the boiling point and melting point because intramolecular hydrogen bonding prevents association and so less energy is required for b.p. and m.p. Thus, o- hydroxybenzoic acid has lower m.p. than its p – and m – isomers.

## Effects of hydrogen bonding on physical properties :

### (i) Solubility in water:

An organic substance is said to be water soluble if it is capable of forming hydrogen bonding with water molecules. On the other hand, compounds in which such bonding with water molecules is not possible would be insoluble or less soluble in water. Thus, organic compounds like alkanes, alkenes, and ethers which lack the formation of hydrogen bonds are insoluble in water. While alcohols, sugars, carboxylic acids, detergents, soaps, urea, etc. are capable of forming hydrogen bonds and are readily soluble in water.

### (ii) Melting and boiling points :

The greater the intermolecular hydrogen bonding, the greater will be the energy required for their separation, and hence greater will be m.p. and b.p. of the molecule containing such type of bond. That is why NH3, H2O, and HF have higher boiling points in comparison to the hydrides of P, S, and Cl respectively.

### (iii). Unique behavior of H2O :

As already discussed above in the crystal structure of ice that each oxygen atom is linked with four H-atoms; two of these H-atoms are covalently bonded with an oxygen atom and the remaining two with hydrogen bonds. Now in the structure of Ice, each water molecule is associated with four other water molecules in the tetrahedral arrangement by hydrogen bonding. Thus, during the formation of such a structure, there is a large empty space between the four water molecules as depicted in figure 3.13. As soon as the ice melts these hydrogen bonds are broken and the space enclosed between four H2O molecules disappears due to thermal expansion. This is why the density of ice is Jess than that of liquid water and ice floats on liquid water. It is also observed that the density of water increases from 0 to 4. It is because as the temperature rises from 0C melting and thermal occurs which brings water molecules close together. Consequently, the density increases. But above 4∘C the density of liquid further decreases with an increase in the kinetic energy of H2O molecules.

### (iv). Boiling points of hydrides :

It was observed that hydrides of most electronegative elements of VA, VIA, and VIIA. like

N, O, and F show many high values of boiling points. The reason is an association of large Tanber of molecules due to intermolecular hydrogen bonding. As we move downward in any particular of these groups, there is a sharp decrease in boiling point but later on, the increasing trend is observed as shown in table 3.3.

Note: The highest value of boiling points of hydrides of most electronegative elements N. O and F is due to the hydrogen bonding.

### (v). The viscosity of liquids :

In the case of molecules having a greater tendency to form intermolecular hydrogen bonding, there is a decrease in the tendency of liquid to flow smoothly. This is due to the fact that a large number of liquid molecules associate together to form a cluster, This cluster formation hinders their smooth flow and increases the viscosity. Let us consider an example of glycerol and alcohol like propanol, both having three C-atoms in the molecule. Glycerol has three OH groups whereas propanol has only one OH group. Hence, the association of glycerol s higher than propanol showing a very high value of viscosity as compared to propanol. Glycerol has a viscosity of 104 millipores whereas propanol has a value of viscosity below 10 millipores.

### (vi). Formation of HF2- ion :

HF molecule combines with Fion and forms HF2− ion,
HF +  F−⟶HF2−
This reaction is possible due to the high electronegativity of fluorine and its small size. On the other hand, other halides like Cl, Br, and 1 – ions do not form such types of ions. The reason is the lack of hydrogen bonds between these ions and their acids.

### (vii). Determination of molecular crystal structure :

Due to the hydrogen bonding, many molecular crystals have their linear chain structure like HCN, zig-zag chain structure of HFCH3OH formic acids, ethers, sheet structure of oxalic acid, and tetrahedral structure of ice. All these structures are possible because of hydrogen bonding.

### (viii) Heat of Vapourization :

The heat required to convert a liquid into its vapor is known as the heat of vaporization. If a compound has hydrogen bonding then it will require more energy to rupture this bonding and the heat of vaporization will be high.

### (ix) Comparison between o, m– and para isomers of aromatic compounds :

O-nitro phenol forms intramolecular hydrogen bonding whereas meta and para isomers show intermolecular hydrogen bonding that is polymerization. Due to this reason, the m.p.’ s of o-nitrophenol is low whereas the other two have higher m.p. There is also a difference in their solubility and volatility as shown in Table 3.4.
Table 3.4: Melting point and other physical properties of nitrophenols.


 Nitro phenol M.P. Solubility in H2O Volatility o 45∘ less soluble highly volatile m 96∘ more soluble less volatile p 114∘ highly soluble non-volatile

### (x) Solubility in water:

When the number of hydroxyl groups in an organic compound is more than the hydrocarbon groups, the substance will be soluble in water to a greater extent. For example, sugar and glycerol are highly soluble, and substances with very high molecular weight which contain (−CH2−CHOH−) n linkage are also sparingly soluble. Detergents that find extensive use in our daily life owe their cleansing action due to hydrogen bonding.

## 1. Significance :

(I) Hydrogen bonding causes a shift in the IR spectra. The position of O−H stretching spectra is shifted to longer wavelengths since lower energy is required to stretch the O−H bond which is to some extent already stretched by hydrogen bonding.
(II) Hydrogen bonding explains the change in bond lengths in formic acid. The theoretical C−O and O−H bond distances are 1.43 A∘ and 0.96 A∘
respectively. But experimentally the C−O and O−H distances in formic acid are found to be 1.36A∘ and 1.07A0 respectively. This change in bond length is because of the fact that formic acid exists as a dimer due to intermolecular hydrogen bonding. The existence of dimer in this carboxylic acid is proved by the presence of an eight-membered ring by electron diffraction studies.
(III) Hydrogen bonding is responsible for the zig-zag structure of nylons in which the chains are arranged in a group of adjacent chains.
(IV) Hydrogen bonding plays up the water-soluble dye due to intermolecular hydrogen bonding between the -OH  groups of cellulose and the hydrogen bonding groups(that make them water soluble) in dyes.
(V) Hydrogen bonding also plays a very important role in biological systems. Most of the water present in plants and animals is attached to proteins by hydrogen bonds.

## Reasoning problems with solution

Problem 1: Examine the following table and answer by giving a suitable reason why water has greater m.p. hydrides of S, Se, and Te.
 Physical property H2O H2S H2Se H2Te Molecular weight 18 34 81 130 Boiling point 100∘C −59.6∘C −42∘C −1.8∘C Melting point 0∘C −83∘C −64∘C −54∘C
Solution: Although water has the lowest molecular weight as compared to H2S, H2Se, and H2Te it thus requires more energy to separate the molecules for vapourization.
whereas no such type of bonding is possible in H2S, H2Se, and H2Te that is why these hydrides are regarded as low molecular weight substances of the lowest melting and boiling points as compared to water.
Problem 2: Assign reason: boiling point of C2H5OH is greater than that of C2H5SH is less (46) than that of C2H5SH (62).
Solution: The boiling C2H5OH is Higher than that of C2H5SH (mercaptan) and corresponding ether (CH3OCH3). Because due to the association of alcohol molecules) and corresponding ether intermolecular attractive forces in ethyl alcohol as compared to C2H5SH and CH3OCH3.
Problem 3: Maleic acid and fumaric acid are both dibasic acids and also geometrical isomers to each other but maleic acid is strong acid than fumaric acid. Explain with a suitable reason.
Solution: The strong acidic character of maleic acid than the fumaric acid is due to hydrogen bonding. If one proton is removed from each of the acids, the corresponding ions are formed. But the maleate ion can be stabilized by chelation because hydrogen and oxygen responsible for forming hydrogen bonds are very near to each other. On the other hand, fumarate ions can not be stabilized by chelation because hydrogen and oxygen are on opposite sides of each other. Hence, the formation of fumarate ions does not take place as shown below: