Hydrogen Bonding (H-Bonding)
Hydrogen Bonding - (H-Bonding)
** When
hydrogen (H) is covalently bonded to a highly electronegative atom X (O, N, F),
the shared electron pair is pulled so close to X that a strong dipole results
** Since the
shared pair is removed farthest from H atom, its nucleus (the proton) is
practically exposed. The H atom at the positive end of a polar bond nearly
stripped of its surrounding electrons, exerts a strong electrostatic attraction
on the lone pair of electrons around X in a nearby molecule. Thus :
** Hydrogen Bonding:
is The electrostatic attraction between an H atom covalently bonded to a highly
electronegative atom X and a lone pair of electrons of X in another molecule
** Hydrogen
bond is represented by a dashed or dotted line.
Points to remember
(1)
Only O, N and F which have very high electronegativity and small atomic size,
are capable of forming hydrogen bonds.
(2)
Hydrogen bond is longer and much weaker than a normal covalent bond. Hydrogen
bond energy is less than 10 kcal/mole, while that of covalent bond is about 120
kcal/mole.
(3)
Hydrogen bonding results in long chains or clusters of a large number of
‘associated’ molecules like many tiny magnets.
(4)
Like a covalent bond, hydrogen bond has a preferred bonding direction. This is
attributed to the fact that hydrogen bonding occurs through p orbitals which
contain the lone pair of electrons on X atom. This implies that the atoms
X–H...X will be in a straight line.
Conditions for Hydrogen bonding
The necessary
conditions for the formation of hydrogen bonding are:
(1) High
electronegativity of atom bonded to hydrogen
** The molecule
must contain an atom of high electronegativity such as F, O or N bonded to hydrogen
atom by a covalent bond.
** The examples
are HF, H2O and NH3.
(2) Small size
of Electronegative atom
** The
electronegative atom attached to H-atom by a covalent bond should be quite
small. Smaller the size of the atom, greater will be the attraction for the
bonded electron pair.
** In other words, the polarity of the bond
between H atom and electronegative atom should be high. This results in the formation
of stronger hydrogen bonding.
** For example,
N and Cl both have 3.0 electronegativity. But hydrogen bonding is effective in
NH3 in comparison to that in HCl. It is due to smaller size of N
atom than Cl atom.
Examples of Hydrogen-bonded compounds
** When
hydrogen bonding occurs between different molecules of the same compound as in HF,
H2O and NH3, it is called Intermolecular hydrogen
bonding.
** If the
hydrogen bonding takes place within single molecule as in 2-nitrophenol, it is
referred to as Intramolecular hydrogen bonding.
** We will
consider examples of both types:
Hydrogen Fluoride, HF
** The molecule
of HF contains the strongest polar bond, the electronegativity of F being the highest
of all elements.
** Therefore,
hydrogen fluoride crystals contain infinitely long chains of H–F molecules in
which H is covalently bonded to one F and hydrogen bonded to another F.
** The chains possess
a zig-zag structure which occurs through p orbitals containing the lone electron
pair on F atom.
Water, H2O
** In H2O
molecule, two hydrogen atoms are covalently bonded to the highly
electronegative O atom.
** Here each H
atom can hydrogen bond to the O atom of another molecule, thus forming large chains
or clusters of water molecules
** Each O atom
still has an unshared electron pair which leads to hydrogen bonding with other water
molecules. Thus liquid water, in fact, is made of clusters of a large number of
molecules.
Ammonia, NH3
** In NH3
molecules, there are three H atoms covalently bonded to the highly
electronegative N atom.
** Each H atom
can hydrogen bond to N atom of other molecules.
2-Nitrophenol
Here hydrogen
bonding takes place within the molecule itself as O–H and N–H bonds are a part of
the same one molecule.
Types of Hydrogen-bonding
Hydrogen
bonding is of two types :
(1)
Intermolecular Hydrogen bonding
** This type of
hydrogen bonding is formed between two different molecules of the same or different
substances e.g. hydrogen bonding in HF, H2O, NH3 etc. It
is shown in the following diagram:
** This type of
hydrogen bonding results in the formation of associated molecules. Generally speaking,
the substances with intermolecular hydrogen bonding have high melting points, boiling
points, viscosity, surface tension etc.
(2)
Intramolecular Hydrogen bonding
** This type of
hydrogen bonding is formed between the hydrogen atom and the electronegative atom
present within the same molecule. It results in the cyclisation of the
molecule.
** Molecules exist
as discrete units and not in associated form. Hence intramolecular hydrogen
bonding has no effect on physical properties like melting point, boiling point,
viscosity, surface tension, solubility etc.
** For example intramolecular
hydrogen bonding exists in o-nitrophenol, 2 nitrobenzoic acid etc. as shown
below:
Characteristics of Hydrogen-bonded compounds
(1) Abnormally
high boiling and melting points
** The
compounds in which molecules are joined to one another by hydrogen bonds, have
unusually high boiling and melting points. This is because here relatively more
energy is required to separate the molecules as they enter the gaseous state or
the liquid state.
** Thus the hydrides
of fluorine (HF), oxygen (H2O) and nitrogen (NH3) have
abnormally high boiling and melting points compared to other hydrides of the
same group which form no hydrogen bonds.
** In the following
Figure are shown the boiling points and melting points of the hydrides of VIA
group elements plotted against molecular weights.
** It will be
noticed that there is a trend of decrease of boiling and melting points with
decrease of molecular weight from H2Te to H2S. But there
is a sharp increase in case of water (H2O), although it has the
smallest molecular weight. The reason is that the molecules of water are ‘associated’
by hydrogen bonds between them, while H2Te, H2Se and H2S
exist as single molecules since they are incapable of forming hydrogen bonds.
(2) High
solubilities of some covalent compounds
** The
unexpectedly high solubilities of some compounds containing O, N and F, such as
NH3 and CH3OH in certain hydrogen containing solvents are
due to hydrogen bonding.
** For example,
ammonia (NH3) and methanol (CH3OH) are highly soluble in
water as they form hydrogen bonds.
(3) Three
dimensional crystal lattice
** As already
stated, hydrogen bonds are directional and pretty strong to form three
dimensional crystal lattice.
** For example,
in an ice crystal the water molecules (H2O) are held together in a tetrahedral
network and have the same crystal lattice as of diamond. This is so because the
O atom in water has two covalent bonds and can form two hydrogen bonds. These
are distributed in space like the four covalent bonds of carbon. The
tetrahedral structural units are linked to other units through hydrogen bonds.
** Since there
is enough empty space in its open lattice structure ice is lighter than water,
while most other solids are heavier than the liquid form.
Water as an Interesting Liquid
** Water is very
interesting solvent with unusual properties:
(1) It
dissolves many ionic compounds and polar organic compounds.
(2) It has high
heat of vaporisation
(3) high heat
of fusion
(4) high specific
heat with melting point 273 K and boiling point 373 K.
** Its structure
as shown is very interesting as it explains many properties:
(1) Ice (solid)
is lighter than water (Liquid)
** The
structure of water is tetrahedral in nature. Each oxygen atom is linked to two
H-atoms by covalent bonds and other two H-atoms by hydrogen bonding.
** In this
solid state (Ice), this tetrahedral structure is packed resulting in open cage
like structure with a number of vacant space. Hence in this structure the
volume increases for a given mass of liquid water resulting in lesser density.
Due to this reason ice floats on water.
(2) Maximum
density of water at 277 K (4ºC)
** On melting
ice, the hydrogen bonds break and water molecules occupy the vacant spaces.
This results in decrease in volume and increase in density (d = m/v).
** Hence
density of water keeps on increasing when water is heated. This continues upto
277 K (4ºC).
** Above 4ºC water
molecules start moving away from one another due to increase in kinetic energy.
Due to this volume increases again and density starts decreasing. This
behaviour of water is shown in the fig:
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
No comments