Diels-Alder Post-lab questions F17. 1) Why do the methylene protons (Ha & Hb) exhibit different chemical shifts? Hb Ha Hd 2) Why do Diels-Alder reactinns hninoll Solution Methylene protons adjacent to a chiral center will be non equivalent, despite the fact that there is free rotation about the cabon-carbon bond. Such protons are described as diastereotopic, since replacement of either of the two hydrogen in turn by a group X produces a pair of diastereomers..