Hydrogen bonding

2. Hydrogen Bonding
Dr.Alhaiqa Farooqi
Pharm-D
RIPS

3. A hydrogen bond is a dynamic attraction
between a pair of typically electronegative
atoms involving a hydrogen atom located
between them. Hydrogen bonds are responsible
for the life-giving properties of water.

5. Hydrogen Bonds:
Polar molecules, such as water molecules,
have a weak, partial negative charge at
one region of the molecule (the oxygen
atom in water) and a partial positive
charge elsewhere (the hydrogen atoms in
water).
The water molecule (O-H) bond is highly
polar bond.

11. Hydrogen
Bonding:

18. An alcohol is an organic molecule containing
an -OH group. Any molecule which has a
hydrogen atom attached directly to an oxygen
or a nitrogen is capable of hydrogen bonding.
Such molecules will always have higher boiling
points than similarly sized molecules which
don't have an -O-H or an -N-H group. The
hydrogen bonding makes the molecules
"stickier", and more heat is necessary to
separate them.

19. Ethanol, CH3CH2−O−H, and methoxy
methane, CH3−O−CH3, both have the same
molecular formula, C2H6O.

20. Ethanol and Methoxy methane
both have same no. of electrons.
Similar length of molecules.
Vander Waals attraction and
dipole-dipole interactions are same
in both cases.
BOILING POINT:
(Continued…)

21.  It has H atom attached directly
to O and O has exactly the
same 2 lone pairs as in H2O
molecule.
 Hydrogen bonding occurs in
Ethanol but not as effective as
in water.

22. Ethanol(continued…)
Hydrogen bonding in ethanol is
limited by the fact that there is
only one H in each ethanol
molecule with sufficient Partial +
charge.

23. CH3OCH3
 The lone pair on O are still there, but
H are not sufficient Partial + charge
for hydrogen bonds to form.
 For its case Hydrogen must have
attach to some highly
electronegative atom for hydrogen
bonding to occur.

24. Ethanol (with hydrogen bonding) 78.5°C
Methoxy Methane (without
hydrogen bonding)
-24.8°C

25. Propane: Molecular Weight: 44
Boiling point: -45°C
Dimethyl ether: Molecular weight:
46
Boiling point: -25°C
Ethanol: Molecular Weight:
46 Boiling Point: +78°C

26. Why Ethanol has high
boiling point?

27. A hydrogen bonded substance can
usually soluble in another
hydrogen bonded substance.
For-example: Alcohols are soluble
in water, but alkanes are not.

28. WHY?
Because non-polar
substances can not break the
hydrogen bonding present in
the water molecule as in the
case with alkenes.

29. ----O H---------O H-----O H
H H H
R H
R H
NON-POLAR ALKANE MOLECULES
CANNOT SLIP INTO THE HYDROGEN-
BONDED SEQUENCE:

30. AN Alcohol molecule, it is capable
of hydrogen bonding. It can slip
into the hydrogen bonded
sequence in water. It can replace
the hydrogen bonds that must be
broken to let it in.

31. Non-Polar solids cannot be
dissolved in water, otherwise they
would break the hydrogen bonds
between the water molecules.
Therefore, it does not happen.