|Lecture Notes: 1 February||
Water is a very unusual, even incredible substance whose amazing properties are often unappreciated because of its ubiquitousness. Water's special properties include extremely high mp and bp (0 °C & 100 °C K, compare to methane, -183 °C & -161 °C, with a MW of 16 vs. water's 18); a high heat capacity (18 cal/°C mol vs. 8 cal/°C mol for methane); it has a high viscosity; its solid form is less dense than the liquid form at the same temperature (ice floats on water - very rare), it has a large surface tension, and it has a high dielectric constant (78.5 vs. 1.9 for hexane).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. [slide]
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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:
*van der Waals interactions, **from Zubay Biochemistry 3rd. (1993) 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?
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Finally, recall that water is a good nucleophile and so will participate in many chemical reactions-readily hydrolyzes esters, amides, anhydrides etc.
Last modified 1 February 2010