|Lecture Notes: 26 January
© R. Paselk 2006
Chapter 2: Water, cont.
The high mp, bp, and heat capacity of water all predict relatively strong bonding between water molecules, so let's first review the types of bonding which occur between atoms and molecules. The most stable bonds are of course covalent bonds (with bond energies of 50 [S-S] to 80 [C-C] to 110 [O-H] kcal/mol), occurring when we have significant overlap of atomic orbitals.
Water of course is a covalent structure: H-O-H. But what gives it its special properties is the polarity of its O-H bonds and the resultant dipole moments of the bonds and the molecule itself.
The water molecule itself is bent, with an angle of 104.5° between the hydrogens (compare to 109.5° for sp3 tetrahedron) as seen in Figure 2.1, p 27of your text. [overhead 3.1, NP]
Because of the very strong dipole moments of these bonds and the very small size of the hydrogen substituents on water, a slight degree of orbital overlap occurs between adjacent water oxygens and hydrogens to give partial covalent bonds known as H-bonds (effectively, can only form with O, N, & F).
- Note that the partial covalent character means that they are directional! Figures 2.3 & 2.4 (p 28-9) show representations of H-bonds in water.
- Compare the bond length of water H bonds (0.18 nm) to the covalent bond-length between O and H of 0.096 nm - notice that the bond distance is nearly twice the true covalent bond distance, but significantly less than the van der Waals radius of 0.26 nm. [overhead 3.3, NP]
In addition to covalent bonds and H-bonds there are a variety of non-covalent
bonds/interactions as seen in the table below:
||Average Strength, kcal/mol (kJ/mol)
|| 5 (20) [in water solution]
van der Waals repulsion
*van der Waals interactions, **from Zubay Biochemistry
3rd. Table 4.3, pg. 89.
Within solid bulk water (ice):
- every water molecule is bonded to 4 others, as in the ice
structure seen in Figure 2.5 on pg. 29 [overhead 2.3, VV]
- In liquid water the molecules are still bonded to a large
degree (the heat of fusion for ice is only 13% of the heat of
vaporization for ice, thus most of the H-bonds must survive melting).
- Of course in liquid water the bonds are very unstable (average
lifetime about 10 psec = 10-11 sec), exchanging constantly
to give a "flickering cluster" structure.
- The various properties of water arise from this structure.
(Note hi bp & mp, heat cap., viscosity, and, less obviously,
that ice floats.
- Ice floats because the molecules are in an open lattice rather
than close-packed. Garrett & Grisham (in their text, Biochemistry,
2nd ed) note that close-packed water molecules would only
occupy about 57% of the volume of ice. This would lead one to
expect that ice would float "high." It doesn't because
most of the structure remains in the liquid phase at 0° C.)
Water is an excellent solvent for polar substances since
its dipolar structure enables it to insulate them from each other
and it can make good dipole-dipole and dipole-charge bonds. Figure
2.6 on p 30 shows the hexavalent liganding of water to sodium
and chloride ions to form hydration shells (For sodium ions, the
waters in the inner hydration-shell exchange every 2-4 nsec.).
Anything which can H-bond will also of course be quite soluble.
How does water interact with non-polar molecules?
- The problem here is that in order to dissolve in water a
non-polar molecule must disrupt a series of H-bonds and no new
bonds of equal strength are substituted.
- Thus water tends to exclude non-polar substances.
When we forcefully disperse a non-polar substance into water
then the water must form a cage around the molecule to maximize
H-bonds for each water molecule. [overhead 7-51, VV]
- Now an additional problem arises-the waters hydrating the
non-polar group are "locked" in place - they can't
easily flip about because there is no interior bond for substitution!
- Thus the entropy of these water molecules is greatly
reduced. So the insolubility of non-polar groups in water
has both enthalpic and entropic factors!
- Note that many non-polar molecules may dissolve in water
by the formation of micelles as in Fig. 2.9, p 33 of Text. [overhead 2-7 VV]
Finally, recall that water is a good nucleophile and so will
participate in many chemical reactions-readily hydrolyzes esters,
amides, anhydrides etc.
Last modified 26 January 2007