Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section A(iii) / 5 / ii / b

The expected order from most to least acidic isHI is stronger than HCl because the H–I bond is weaker and the larger iodide ion accommodates negative charge well. In water both strong acids are substantially leveled to hydronium; their intrinsic ordering should not be mistaken for a large difference in the acidity of equal-concentration fully dissociated aqueous solutions.

Removing the marked proton alpha to the nitro group in (3) gives a resonance-stabilized nitronate, with charge distributed onto oxygen. It is far more favorable than forming the unactivated carbon anions in (1) or (2).

The marked aromatic C–H in (1) is more acidic than the alkane C–H in (2). The resulting aryl carbon lone pair has greater s character than an alkyl carbanion's, and chlorine also exerts an inductive effect. The aryl anion's lone pair is not simply delocalized through the aromatic pi system in the same way as a phenoxide ion. The specified proton positions are essential: this is not an ordering of arbitrary hydrogens on the molecules.