Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2007

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).

In addition to covalent bonds and H-bonds there are a variety of non-covalent bonds/interactions as seen in the table below:

 Interaction Type

Average Strength, kcal/mol (kJ/mol) Range**
Charge-charge (ionic)

 5 (20) [in water solution] 1/r


Charge-induced dipole

Dipole-induced dipole*

0.1-0.2 (0.4-4)  1/r6


0.1-0.2 (0.4-4)
Hydrogen bond

3-8 (12-30)  

 van der Waals repulsion

*van der Waals interactions, **from Zubay Biochemistry 3rd. Table 4.3, pg. 89.

Within solid bulk water (ice):

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?

Finally, recall that water is a good nucleophile and so will participate in many chemical reactions-readily hydrolyzes esters, amides, anhydrides etc.

Pathway Diagrams

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Last modified 26 January 2007