How Structure Affects Acid Strength

In this section we ll introduce some generalizations that will permit us to connect molecular structure with acidity in related compounds. The main ways in which structure affects acidity in solution depend on  

The strength of the bond to the atom from which the proton is lost 

Bond Strength. The effect of bond strength is easy to see by comparing the acidities of the hydrogen halides. 

With HE a second factor concerns the high charge-to-size ratio of F . Other things 

Because of the conjugate relationship between acidity and basicity, the strongest acid (HI) has the weakest conjugate base (I ), and the weakest acid (HF) has the strongest conjugate base (F ). 

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So far, we have focused on how differences in molecular structure affect the solubilities and activity coefficients of organic compounds in pure water at 25°C. The next step is to evaluate the influence of some important environmental factors on these properties. In the following we consider three such factors temperature, ionic strength (i.e., dissolved salts), and organic cosolutes. The influence of pH of the aqueous solution, which is most important for acids and bases, will be discussed in Chapter 8. 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

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