Structural Factors That Influence Acid Strength

3 STRUCTURAL FACTORS THAT INFLUENCE ACID STRENGTH 

If the H-A bond is stronger, it will be more difficult to break, and the acid will be weaker. The overlap to H decreases as we go down a column in the periodic table, so the H-A bond gets weaker. The acidity increases going down the column from fluorine to iodine HF HCl HBr HI. The overlap with the hydrogen s 1 orbital is worst with the 5/7 orbital of iodine, gets a little better with the 4/7 of bromine, better still with the 3/7 of chlorine, and is best with the 2p of fluorine. 

Cyclopentadiene is unusually acidic for a carbon acid, with a p Ta of 16. The conjugate base has six electrons in an unbroken loop of p orbitals and so is stabilized by aromaticity (Section 1.8). Aromaticity stabilizes this ion by more than 25 pATa units. 

Positively charged acids are more acidic than their corresponding neutrals, as illustrated by the lower pATaof the cations of the following compounds. 

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Section 3.3 Structural Factors That Influence Acid Strength... 

Structural features are not the only factors that affect acidity or basicity. The same compound can have its acidity or basicity changed when the conditions are changed. The effect of temperature (p. 364) has already been mentioned. More important is the effect of the solvent, which can exert considerable influence on acid and base strengths by differential solvation. If a base is more solvated than its conjugate acid, its stability is increased relative to the conjugate acid. For example,... 

It has been mentioned above (see section 4.5) that in the case of hydrogen evolution in acidic solutions, the isotope separation factor depends considerably on the cathode potential. This was explained by the fact that the hydronium ion is attracted towards the electrode by the electric field. Naturally, this effect depends on the strength of the field which acts on the ion. Incidentally, irrespective of the mechanism by which the potential influences the isotope effect, the latter must directly depend not on the total metal solution potential drop, but on the potential difference between the metal and site at which the discharging ion is located. Hence, for different electrolyte solutions with different doublelayer structures, the same isotope effect will be observed for identical values of (p - where ipi is the local potential at the site of the discharging ion. 

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