1.Weak forces of attraction 2.Concepts of Hydrogen bonding 3.Types of hydrogen bonding 4.Properties of hydrogen bond. 5.Methods of detection of hydrogen bond. 6.Importance of Hydrogen bonding. 7.Vander walls forces a.Ion-dipole b.Dipole-dipole c.London forces. 8.Origin of hydrogen bonds. 9.Consequences of hydrogen bonding. 10.Ice has less density than water. 11.Intermolecular forces.

1. Dr.V.A.Thakur
Deptt.of Chemistry
K.B.P.College,Vashi
(Empowered
Autonomous)
Navi Mumbai
Hydrogen Bonding

2. Points to be covered
➢ Weak forces of attraction
➢ Hydrogen bonding
➢ Concepts
➢ Types
➢ Properties
➢ Methods of detection and importance
➢ Vander walls forces
➢ Ion-dipole
➢ Dipole-dipole
➢ London forces

3. Weak forces of Attractions
 Chemical forces and Interactions :
Types of Interaction Strength Energy – distance function
Covalent Bond Very strong Complex, but comparatively
long range
Ionic Bond Very strong 1/r, comparatively long range
Ion-dipole Strong 1/r2, short range
Dipole-dipole Moderately strong 1/r3, short range
Ion- induced dipole weak 1/r4, very short range
Dipole-induced dipole Very weak 1/r6, extremely short range
London dispersion
forces
Very weak 1/r6, extremely short range

4. Hydrogen Bond
 In compounds of hydrogen with strongly electronegative elements,
such as fluorine, oxygen and nitrogen, the electron pair shared between
two atoms lies far away from hydrogen atom.
 This results into hydrogen atom becomes electropositive with respect to
other atom.
 This phenomenon of charge separation in Hydrogen fluoride is
represented as H + --F-.
 Such a molecule is said to be polar.
 The molecule behaves as a dipole because one end carries a positive
charge and other end a negative charge.
 The electrostatic force of attraction between such molecules should be
strong.
 Because positive end of one molecule attracts and is attracted by the
negative end of other molecule.
 Thus , two or more molecules may associate together to form large
clusters of molecules.

5.  ...H + - F - ...H + – F - ....H + -F - ...H + -F - ....
 The cluster of HF molecules may be described as
(HF)n.
 The attractive force which binds hydrogen atom of
one molecule with electronegative atom(such as
fluorine) of another molecule, generally of the same
substance is known as Hydrogen Bond.
 In this case, the length of the covalent bond between F
and H atoms is found to 1.00A0 and the length of
hydrogen bond between F and H atoms of neighbouring
molecule is found 1.55A0

6. Hydrogen bonds
 Water molecules because of its bent structure, is also a
dipole as oxygen end carrying a negative charge and
hydrogen end carrying a positive charge. Hydrogen
bonding in this molecule can takes place.
 The cluster of water molecules can be represented as
(H2O)n
 Alcohols and carboxylic acids also form associated
molecules.
 In water and alcohols, the hydrogen bonding (association of
molecules) may extend to several molecules.
 But in carboxylic acids, the hydrogen bonding is limited to
association of two molecules only.

7. Nature of Hydrogen bond
 The hydrogen bond is a class in itself.
 It arises from electrostatic forces between the positive end (pole)
of one molecule and the negative end(pole) of another molecule
of the same or some other polar substance.
 E,g.Extensive hydrogen bonding occurs between molecules of
water, molecules of ethanol and also between molecules of water
and molecules of ethanol.
 The strength of hydrogen bond is found to vary between
 10- 40 KJ per mole (i,e.per 6.022 x 1023 bonds.)
 That of covalent bond has found to be of the order of 400KJ
mole.
 Thus, a hydrogen bond is very much weaker than a covalent
bond.

9. The Origin of Hydrogen bonds
 Hydrogen: attached
directly to one of the most
electronegative elements
 Electronegative elements:
not only significantly
negative charged, but also
has at least one "active"
lone pair.
 Electrons: contained in a
relatively small volume of
space which therefore has
a high density of negative
charge.

11. Inter-molecular
Bonding
Intra-molecular
COVALENT
Hydrogen bond
Van der Waals
Etc.
, ,…
Dipole-dipole
Dipole- Induced dipole
Instantaneous dipole-induced dipole London Dispersion
Relative strengths
dispersion forces < dipole-dipole interactions < hydrogen bonds
Ion-dipole
Cation-Pi
Pi-Pi
The term ‘Van der Waals forces’ is sometimes used for a specific type (London Dispersion) rather than the class
We will describe briefly a few of these (only) here

12. Types of Hydrogen bonding
▪ The covalent boding between a hydrogen
atom and a strongly electronegative atom
becomes ‘polar’-covalent
▪ The ‘charged’ hydrogen ‘ion’ can be
attracted to a electronegative atom, such as
nitrogen, oxygen or fluorine
▪ hydrogen bond should not be confused
with a covalent bond to hydrogen.
▪ Types of hydrogen bonds:
➢ Intermolecular (between molecular)
➢ Intramolecular (within a molecule)
▪ E.g. of hydrogen bonding: water
(responsible for the high boiling point of
water compared to say H2S), DNA, partly
responsible for the secondary, tertiary, and
quaternary structures of proteins and
nucleic acids, Polymers
O−
H+ H+
O−
H+ H+
Hydrogen bond

13. Electronegativity is the tendency of an atom in a molecule to attract shared electrons to
itself. An electronegative atom pulls more of the electron density from the bond towards
itself.

16.  A hydrogen bond is the attractive force between one
electronegative atom and a hydrogen covalently
bonded to another electronegative atom.
 It results from a dipole-dipole force with a hydrogen
atom bonded to nitrogen,oxygen or fluorine (thus the
name "hydrogen bond", which must not be confused
with a covalent to hydrogen).
 The energy of a hydrogen bond (typically 5 to 30
kJ/mole) is comparable to that of weak covalent bonds
(155 kJ/mol), and a typical covalent bond is only 20
times stronger than an intermolecular hydrogen bond.

17.  These bonds can occur between molecules (intermolecularly), or
within different parts of a single molecule (intramolecularly).
 The hydrogen bond is a very strong fixed dipole-dipole Vander
Walls-Keesom force , but weaker than covalent, ionic and metallic
bonds.
 The hydrogen bond is somewhere between a covalent bond and an
electrostatic intermolecular attraction.
 This type of bond occurs in both inorganic molecules (such as
water) and organic molecules (such as DNA).
 Intermolecular hydrogen bonding is responsible for the high
boiling point of water (100 °C).
 This is because of the strong hydrogen bond, as opposed to other
group16 hydrides.
 Intramolecular hydrogen bonding is partly responsible for the
secondary, tertiary and quaternary structures of proteins and
nucleic acids.

18. Properties of Hydrogen Bond
❖ 1)It is a bond between two electronegative atoms only. It never involves
more than two atoms(excluding H atoms).
❖ 2)Bond Energy of a H-bond is in the range of 3-10 Kcal/mole. While
that of a normal covalent bond is in the range of 50-100 Kcal/mole.
Thus a H-bond(H---B) is much weaker than a cobalent bond A-H.
❖ The difference in energy between A-H and H...B bond indicates they
have different bond length which in turn shows that H atom in A-H...B
is never midway between two atoms A and B.
❖ It is rather always nearer to atom A which is covalently bonded to H-
atom.
❖ H-Bond has more energy(=3-10 Kcal/mole) than Vander Walls forces(=
1 Kcal/mole).
❖ 3) The formation of H-bond does not involve any sharing of electron
pairs. It is therefore quite different from a covalent bond.

19. Properties of Hydrogen Bond
 4)H-bond in A-H....B is formed easily when both the atoms A &
B are highly electronegative.
 Thus the ease of formation of H-bond in A-H...B increses with
the increases in the electronegativity value of atom A indicating
as A.
 This tendency clearly explains that tendency of A-H bond to
form a H-bond increases from N-H through O-H to F-H as
N  O  F .
 This tendency decreases in passing from O-H to S-H or from F-
H to Cl-H because O  s and  F  Cl
 This shows that F atom with highest electro negativity(F )forms
the stronger H-bond.

20. Formation of hydrogen bonds between H2O molecules.
hydrogen bond
Electrostatic attraction exists between partial positive charge of H
atom and the lone pair electrons of O atom of another H2O.

21. Formation of hydrogen bonds between NH3
molecules
hydrogen bond
Electrostatic attraction exists between partial positive charge of H
atom and the lone pair electrons of N atom of another NH3.

22. Consequences of Hydrogen Bonding
 Properties explained by Hydrogen bonding:
 1. State of H 2O and H 2 S:
 The ease of formation of H-Bond in A-H...B decreases with the
decrease in the electronegativity of atom B.
 Thus as O  s, There is a considerable Hydrogen bonding
in H 2O while in H2S, it is absent.
 In other words, H2O molecule can associate together to
form a polymerized molecule,(H2O)n(called cluster)in
which hydrogen atom acts as a bridge between two oxygen
atoms which are highly electronegative.
 Due to formation of this polymerized (H2O)n molecule
containing Hydrogen bonds water exists as a liquid .
 In H2S there is no Hydrogen bond formation. Hence it does
not form cluster. Hence it exists as a gas.

23. Melting and Boiling points of Hydrides of N,O & F
 If the melting points and boiling points of the Hydrides
of the elements of Group IV A, V A, VI A & VIIA are
plotted against their molecular weights of these
hydrides, we get following plot.
Group 15
Formula Molar mass B.P.(0C)
Group 16
Formula Molar mass B.P. (0C)
Group 17
Formula M.Mass B.P.(.(0C)
SbH3 125 -17.0 H2Te 130 -1.8 HI 127.9 -3.5
AsH3 78 -55.1 H2Se 81 -42.0 HBr 80.9 -67.1
PH3 34 -84.6 H2S 93 -59.6 HCl 36.5 -85.0
NH3 17 -33.0 H2O 18 100 HF 200 +19.4

24. Where does H bond come from?

25.  Although the boiling points in each group decreases with
decrease in molar mass, there is sudden reversal in case of
Ammonia, Water and Hydrogen fluoride in Group 15, 16 & 17
respectively.
 The unusual high boiling points of each compound is a
consequence of strong intermolecular forces due to H-bonding.
 The sudden increase in M.P. and B.P. In these hydrides is due to
inter-molecular H-bonding in between H and N in NH3.
 The existence of H-bonding in these molecules gives
polymerized molecule(NH3)n, (H2O)n and (HF)n having H-
bond.
 To break the H-bond existing in these polymerized more energy
is required.
 Thus M.P. & B.P. of these molecules is suddenly raised.

26. H – bonding and boiling point
Predicted and actual boiling points
-200
-150
-100
-50
0
50
100
Period
Boiling
point
Group 4
Group 5
Group 6
Group 7
2 3 4 5

27. Ice has less Density than water
 In the crystal structure of ice, the oxygen atom is surrounded by four H-
atoms.
 Two H-atoms are linked to O-atom by covalent bonds and the
remaining two H-atoms are linked to O-atom by two H-bonds shown
by dotted line.
 Thus, in ice, every water molecule is associated with four other water
molecules by H-bonding in a tetrahedral fashion.
 Ice has open structure with a large empty space due to existence of H-
bonds.
 As ice melts at 00C , number of H-bonds are broken down and the space
between water molecules decreases so that water molecules move close
together.
 The density of water therefore increases from 00C to 40C at which it is
maximum.

28. a water
molecule
hydrogen
bond
hydrogen atom
oxygen atom

29.  Above 40C , the increase is kinetic energy of the molecule is sufficient
to cause the molecules to begin to disperse.
 This result into steady decrease in density as the temp.increases.
 4. There is a contraction in water when it is warmed up to 40C.
 As water is warmed from 00C onwards more and more Hydrogen bonds
break down so that water molecules come closer and closer to one
another.
 The moving of water molecules closer together results in contraction.
 There is also an expansion in the volume of water.
 It appears that upto 40C the effect of expansion predominates.
 Hence there is contraction in the volume of water when it is warmed
upto 40C
 Above 40C the effect of expansion predominates and hence there is an
expansion in the volume of water when it is heated at a temperature
more than 40C.

30. Methods of Detection of Hydrogen Bond
 The methods have been used for the detection of Hydrogen
Bond in different compounds.
 1.Infra Red Spectroscopy :
 When a complex of the type A-H..B containing Hydrogen
bond is formed, several following changes are observed in
the infra-red region.
 A) The absorption bands due to the A-H stretching
vibrations(fundamental and overtones) are shifted to lower
frequencies.
 These shifts range from about 30cm-1 to several hundreds
cm-1 or more.
 This shifting is due to the weakening of the force constant
for A-H.

31. Methods of Detection of Hydrogen Bond
❑Infrared and Raman Spectroscopy
❑Gas-Phase Microwave Rotational Spectroscopy
❑ Neutron Inelastic Scattering
❑ NMR Spectroscopy
❑Deuteron Quadruple Coupling
❑Diffraction Methods: Neutron and X-Ray
❑ Computational Chemistry
❑Thermochemical Methods

32. Importance of Hydrogen Bonding
➢ Without Hydrogen bonding, water would have existed as a gas
like Hydrogen Sulphide(H2S). Hence no life would have been
possible on this globe.
➢ Hydrogen bonding also exists in all living organism of animal or
of vertebrate kingdom. Thus it exists in various tissues, organs,
blood, skin and bones in animal life.
➢ It plays an important role in determining the structure of proteins
which are so essential for life.
➢ Hydrogen bonding plays an important role in making wood
fibres more rigid. Thus it makes an article of great utility to meet
requirements of housing, furniture etc.
➢ The cotton, silk or synthetic fibres owe their rigidity and tensile
strength to Hydrogen bonding. Thus H-bonding is of vital
importance for our clothing also.

33. Importance of Hydrogen Bonding
 Most of our food materials also consist of
hydrogen bonded molecules. e,g. Sugars and
Carbohydrates have many -OH groups. The
Oxygen of one such group in one molecule is
bonded with OH group of another molecule
through H-bonding.
 The thickness of glue( a protein) or honey(which
consists mainly of water and sugar) is also due to
Hydrogen bonding between -OH or other such
groups of different molecules with one another.

36. What hydrogen bonds help to do?
Multiple hydrogen bonds .
➢hold the two strands of the DNA double helix
together .
➢hold polypeptides together in such secondary
structures as the alpha helix and the beta
conformation
➢helps enzymes in bind to their substrates
➢helps antibodies bind to their antigen
➢helps transcription factors bind to each other and
DNA ……

37. H- bond in complex molecules

38. A Special Molecule: H2O

40. Intermolecular forces
 The forces of attraction exists between polar as well as non-polar
molecules are known as Intermolecular forces or Cohesive forces or
Vander Waals forces.
 These forces originate from two types of interactions.
 1. Dipole-dipole interactions.
 2.Induced Dipole-induced dipole interactions,
 1.Dipole-Dipole Interactions:
 In case of polar molecules which have permanent dipoles, the Vander
Waals forces are mainly due to electrical interaction between the
dipoles known as dipole-dipole interaction.
 Gases such as ammonia, Sulphur dioxide, hydrogen fluoride, hydrogen
chloride etc. have a permanent dipoles as a result of appreciable dipole-
dipole interactions between the molecules of these gases.
 The magnitude of this type of interaction depends on the dipole
moment of the molecule concerned.

41.  The grater is the dipole moment, greater is the dipole-
dipole interactions.
 Because of the attractive interactions, these gases can be
easily liquefied.
 The average interaction energy of the two molecules with
permanent dipole moment 1 and 2 is given by
 Interaction Energy((r) =
 2[1 2/40] 2 (1/r6) (1/3KT)
 Where r = distance between the molecules
 K = Boltzman constant
 40 = Permitivity factor for the medium

42. Induced dipole-Induced dipole interaction, London forces or
Dispersive forces
 We understand that Vander Waals forces exist even in non-polar
diatomic molecules such as O2 and N2.
 It is also present in Non-polar monoatomic molecules such as
He, Ne and Ar etc.
 This attraction is evident from the condensation of these gases
into liquids of sufficiently high pressure and low temperature.
 The existence of Vander Waals forces in these molecules could
not understood for several years.
 In 1930, F. London provides a satisfactory explanation for the
existence of forces of attraction between non-polar molecules.
 According to which electrons of a non-polar molecules keep on
oscillating with respect to nuclei of atoms.
 As a result of this, at given instant, positive charge may be
concentrated in one region and negative charge in another region
of the same molecule.

43.  Thus a non-polar molecule has become momentarily self-
polarised.
 This polarised molecule may induce dipole moment in a
neighbouring as shown in figure.
 The electrostatic forces of attraction between induced
dipoles and original dipoles(due to oscillation of electrons)
are known as London Forces.
 These forces are also called as dispersive forces because
the well known phenomenon of dispersion of light is also
connected with these dipoles.
 For a pair of adjacent molecules, London forces vary
inversely as the sixth power of the distance between them.
 F  1/r6

44.  The approximate interaction energy in this case is given by
 (r) = ---([3 E1 E2/2(E1 + E2)] [ 12/ (40] 2 (r6)
 Where
 E1,E2 = Ionization energies of two molecules.
 Other parameters have same significance.
 TheVanderWaals attraction in non-polar molecules is thus
exclusively due to London forces.

45. Van der Waals
Dipole- Dipole interactions
▪ In the covalent bonding between two atoms
of very different electronegativity the bond
becomes highly polar (introducing partial
charges on the species)
▪ This dipole can interact with other
permanent dipoles
▪ This interaction is stronger than dispersion
forces
Br+ F− Br+
F−

46. Instantaneous dipole-
induced dipole London Dispersion
▪ Instantaneously generated dipole (due to asymmetry in electron charge
distribution around the nucleus) on one atom leads to slight polarization of
the atom
(→ quantum induced instantaneous polarization)
➢ This induces a dipole on the neighbouring atom (temporarily)
▪ The force between these two dipoles is called the London dispersion
forces
▪ The force is very weak and is temporally varying
▪ Can operate between non-polar molecules (H2, Cl2, CO2 etc.)
▪ The strength of the dispersion forces will increase with number of
electrons in the molecule
Ar
Ar

47. London forces
 Induced dipole
A dipole forms in one atom or
molecule, inducing a dipole in the
other
Eventually electrons are situated so that
tiny dipoles form

48. Ion-Dipole
▪ Permanent dipole
interacts with an ion.
▪ This explains for
example the solubility
of NaCl in water.
▪ The figure below shows
the interaction of Na+
and Cl− ions interacting
with the permanent
dipoles in a water
molecule.