the introduction of Water Properties

1. Water: The Solvent for Biochemical
Reactions

2. Learning Objectives
Upon completion of this lecture, students
should be able to:
• understand water and polarity
• define hydrogen bonds
• Describe covalent and non-covalent
bonds
• understand acids and bases
• know titration curves and buffers

3. Water is essential to living organisms
• No water = no life on earth
• 71% of earth is covered by water BUT
97.5% of it is salt water i.e. ocean
– 2.5% fresh water: lakes, rivers and ice
• 60% of human body consist of water
• Why is water important?
– Most biochemical processes essential for living
organism takes place in presence of water
– Water is the best solvent known – universal
solvent

4. Why is water the best solvent?
• It is due to the polarity of water molecule.
• What is polarity?
• A polar molecule is one that does not have
the same charge across the molecule.

5. Water and polarity
• H2O is the main component in most cell
• Electronegativity = tendency of an atom to attract
electrons to itself
– Atoms of same element share electrons equally in
a bond – equal electronegativity
• When two atoms with equal electronegativity form a
bond, electrons are shared equally

7. Electronegativity
• BUT if atom with different electronegativity form a
bond, electrons are not always shared equally
– Eg: In H2O, the oxygen is more electronegative
than H
– This give rise to partial positive and negative
charge = polar bonds
– Nonpolar = the sharing of electrons in the bond is
very nearly equal

9. • Water is a bent molecule and the uneven sharing of
the electrons in the two bonds is not cancelled out as
in CO2
– The bonding electrons are more likely to be found
at the oxygen end than the hydrogen end of the
molecule
– Bonds with positive and negative ends are called
dipoles.

10. • Bonds in a molecule may be polar even though the
molecule may be nonpolar because of its geometry.
• The two C=O bonds are polar, but since the molecule
is in linear formation, the attraction of the oxygen for
the electrons on both sides cancel out each other =
zero dipole

11. What does all that info means?
• Because of the polarity of the molecules, water
forms H-bonds.

12. Hydrogen bonds
• Hydrogen bond: the attractive interaction between
dipoles when the;
– positive end of one dipole is a hydrogen atom
bonded to an atom of high electronegativity, most
commonly O or N, and
– the negative end of the other dipole is an atom
with a lone pair of electrons, most commonly O or
N

13. Even though hydrogen bonds are weaker than
covalent bonds, they have a significant effect on
the physical properties of hydrogen-bonded
compounds.

14. Hydrogen Bonding
H
O H O
H
R
Water and a
hydroxyl group
H
O H O C
Water and the carbonyl group
of an aldehyde, ketone, carboxylic
acid, ester, or amide
N H O
H
R
An amino group and
a hydroxyl group
N H O C
An amino group and
a carbonyl group
N H N
An amino group and
another nitrogen-
containing group
Important
in
proteins
and
nucleic
acids
hydrogen
bond donor
hydrogen
bond acceptor
Hydrogen bonding is important in stabilization of 3-D structures
of biological molecules such as: DNA, RNA, proteins.

15. • Because of their polarity, water molecules are
strongly attracted to one another, which gives water
a high surface tension. The molecules at the surface
of the water "stick together" to form a type of "skin"
on the water, strong enough to support very light
objects.

16. But how do water work as solvent?
• Three types of molecules characteristics when mixed
with water.
– Hydrophilic – water loving
– Hydrophobic – fear of water
– Amphipatic – part of the molecule is hydrophilic
while another part is hydrophobic.

17. Water's polarity allows it to dissolve other
polar substances very easily.
• When a polar substance is put in water, the positive
ends of its molecules are attracted to the negative
ends of the water molecules, and vice versa.
• The attractions cause the molecules of the new
substance to be mixed uniformly with the water
molecules.

18. Solvent Properties of Water
 Question: Why do some chemicals dissolve in water while
others don’t?
 The solvent properties of water is largely determined
by its polar nature.
 Compounds with full charges
NaCl, KCl
 Polar compounds with partial charges
Ethyl alcohol, acetone
These compounds are hydrophilic = water-
loving

21. • Hydrocarbons (compounds made up of solely C and
H) are nonpolar.
– Do not readily dissolve in water
– Called hydrophobic = fear of water

22. • Some molecules are called amphipathic – ie have
both polar and nonpolar portions in a single
molecule
• Structure like this tends to form micelles in the
presence of water.

23. • Interaction between nonpolar molecules is very
weak
– depends on the attraction between temporary
induced dipole moments
– called van der Waals interactions
– this allow nonpolar molecules to either condense
to liquid or freeze to solids.

24. Why do oil and water mixed together
separate into layers?

25. Why do oil and water mixed together
separate into layers?
• Oil and water don't mix because oil is made up
of non-polar molecules while water molecules
are polar in nature.
• Because water molecules are electrically
charged, they get attracted to other water
molecules and exclude the oil molecules.
• This eventually causes the oil molecules, or
lipids, to clump together.