Lecture #14
  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Oxoacid strength and molecular structure are discussed a bit in pp. 691-3, but is mostly from lecture material. Dipole moments

Partial ionic character

Molecular Basis of Acid Strength


pK values

Oxoacids (continued)

A purely covalent bond, with perfectly shared valence electrons, has no "polarity" or separation of charges. The degree to which a bond has polarity can be expressed by a dipole moment, defined here (but not in the text.).
Calculating the "percent ionic character" or "partial ionic character" for the diatomic molecule HF using the measured dipole moment and the measured bond length.
The percent ionic character for the hydrogen halides. You should be able to calculate the %'s from the various bondlengths and dipole moments. Note that the percent ionic character decreases as the halogen becomes less electronegative, leading to an almost purely covalent bond (covalent = 0% ionic) for HI.You might want to try one of these. Percent ionic character is not in our text.
As an important application of some of the influence of electronegativity and of resonance, we will now look at the effect of molecular structure on the "strengths" of acids. (This is in Chapter 10 pp. 328-9 to a partial extent only.) What's an acid?  
 A measure of acid strength is its pK value. In this course, we will not work out any numerical calculations, but just use the known values to make qualitative comparisons of acid strength.  
We will look almost entirely on a category of acids known as oxoacids (or oxacids) and how the strength of the acid is understood from the structure of its molecule.
The strengths of some oxoacids are determined by the electronegativity of nearby atoms.  
 Nearby oxygens affect acid strength in a very regular way.  
 Another comparison of related oxoacids. (We will return to the molecular basis of strengths of oxoacids after diverting our attention to molecular geometries for a while.)  
If the nearby oxygens are other OH groups, those oxygens barely participate in determining acid strength. The explanation lies in "resonance stabilization" of the reactant product as we will see.  
Here are shown the Lewis structures for the three phosphorus oxoacids. (You would need to be given "skeletal" information to obtain these structures.) Shown at the bottom is the result of H+ being released; the anion H2PO2- is stabilized by resonance.  
The important feature is the ability of the =O to contribute to stabilizing the product anion as in the previous slide.
Carbonic acid is a carboxylic acid, with its -COOH group. It's not a strong acid because the central atom -- the "X" in X-O-H -- is not electronegative.
The effect of an electronegative neighbor to the central atom can be felt as shown here when chlorine is substituted for one of the hydrogens on the neighboring CH3 group.  
And, in this situation, three fluorines are more effective than one fluorine.