Acid-Base Strength Lab

Introduction: In this lab, we measured the acid-base strengths of seven separate organic compounds. The lab was painless to perform, it consisted of adding a small amount of one of the organic compounds to a well, and testing the pH via testing paper. The color of the paper was then analyzed to determine what the pH of the compound was. The structures were benzenesulfonic acid, benzoic acid, benzyl alcohol, benzylamine, p-cresol, and p-toluidine. My initial hypothesis about the acidity/basicity without looking too heavily into the structures would lead me to believe the banzylamine and p-toluidine would be most basic, because the acidic hydrogen is bonded to the nitrogen group instead of oxygen like all the others. Alcohol groups tend to
be the most neutral, therefore benzyl alcohol and the p-cresol would be more towards the middle of the spectrum of the compounds on the pH scale. Benzoic acid would be the next most acidic, and finally I hypothesized that the benzenesulfonic acid would be most acidic due to the sulfur acting as an electron withdrawing group.

Results:
Compound
pH benzenesulfonic acid
2
benzoic acid
3
benzyl alcohol
5
benzylamine
10
p-cresol
6
p-toluidine
7

Disussion: The results of my lab supported my null hypothesis.
In order of increasing acidity, the compounds can be arranged as follows: benzylamine < p-toluidine < benzyl alcohol < p-cresol < benzoic acid < benzenesulfonic acid. Benzylamine was found to be most basic. This occurs for several reasons. Firstly, Nitrogen is less electronegative then oxygen. We know that acidity increases from left to right in the periodic table, which would mean that as we move right to left (Nitrogen is to the left of oxygen), compounds become more basic. This occurs because of the electronegativity of the atoms as they move across periods. Nitrogen is less electronegative then oxygen, which means that the unshared pair of electrons of the conjugate base in an NH2 molecule (NH-) is in a higher energy orbital and therefore less stable than that of an oxygen conjugate base which has a lower-energy orbital. The nitrogen has more inclination to lose is electron pair to form a bond to an H+ so it is a stronger base than the oxygen in the other compounds. Benzylamine is a stronger base than p-toluidine because resonance structures make a structures within a compound more stable. If the compound is more stable than it will in turn be more acidic. Benzylamine has no resonance stabilization. P-toluidine, on the other hand, does have stabilization by resonance. This outcome of stabilization helps fully explain the results of this
experiment.

Conclusion: The results of my lab supported my null hypothesis and gave me a deeper insight to the relationship between acids and bases. Clearly, it is crucial to not only pay attention to the individual molecules in a structure to determine acidity, but also to the structure of the compound itself.