5.5 : Molecular Structure and Acidity

An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the anion is unstable, then the acid is weaker. Hence, to determine the acidity of the compound, the stability of its conjugate base is studied using various factors.

The size effect explains the change in atomic size on acidity. When comparing the acids formed from elements that belong to the same column in the periodic table, their atomic sizes are compared. Compounds, which contain an element with a larger atomic size can stabilize a negative charge better by spreading it over a larger space volume. Hence, these compounds correspond to the stronger acid.

Further, consider compounds formed with elements that belong to the same row in a periodic table. In such cases, the electronegativity of the element dictates the acidity. The more electronegative element forms the stable anion that corresponds to the stronger acid. This is called the charge effect.

If the compounds having the negative charge on a similar atom are compared, another factor, called resonance, determines the acidity. The compound with more stabilizing resonance structures is more acidic. To determine the acidity of compounds having an equal amount of resonance, the induction effect is analyzed. In this effect, the presence of electronegative elements at varying distances is used to identify the strength of the acid. The compound with an electronegative element closer to the acidic hydrogen is determined more acidic.

Hybridization is used to help analyze the acidity in compounds with no resonance, no electronegative elements, and with the negative charge on a similar atom, as, for example, an alkane, alkene, and alkyne. In an alkyne, the carbon atom has an sp orbital with a 50 % s character. In an alkene, the carbon atom has a 33.3 % s character in its sp² orbital, and the carbon with a single bond has a 25 % s character in its sp³ orbital. A higher s character means that electrons are closer to the nucleus. Therefore the negative charge of the conjugate base can be more stabilized, making alkynes the most acidic compare to an alkene or alkane.