Chapter 1:- Structure and Reactivity
B.sc 1st year Book Organic Chemistry (Page 10)
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:
Characteristic features of H-bond:
- Hydrogen bonding is a type of polar bonding that occurs only with highly electronegative atoms of small size viz. F, O, and N.
- Hydrogen bond distance is longer than normal covalent bond distance.
- 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.
- It is weaker than a single bond and stronger than van der Waals forces(1 Kcal/mole).
- 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.
(iv) Urea-water system :
(b) Intramolecular hydrogen bonding (chelation) :
Limitation of hydrogen bonding:
Causes of Hydrogen Bonding.
I. Intermolecular hydrogen bond causes-
II. Intramolecular hydrogen bonding causes :
Effects of hydrogen bonding on physical properties :
(i) Solubility in water:
(ii) Melting and boiling points :
(iii). Unique behavior of H2O :
(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.
(v). The viscosity of liquids :
(vi). Formation of HF2- ion :
HF + F−⟶HF2−
(vii). Determination of molecular crystal structure :
(viii) Heat of Vapourization :
(ix) Comparison between o, m– and para isomers of aromatic compounds :
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|
(x) Solubility in water:
1. Significance :
Reasoning problems with solution