Stereoelectronics: Synoptic Tutorial Problems

Question 1 Answer

All three molecules adopt chair conformations for all three rings. The anomeric effect is induced by anti-periplanar alignment of an oxygen lone pair with an adjacent C-O σ^* orbital. This has the effect of contracting the length of the O-C donating bond (forming ⁺O=C) and elongating the C-O σ^* bond (forming ⁺O=C....^-O). In system 2 below, all four C-O σ^* orbitals can align with four of the eight oxygen lone pairs. This mutual effect results in all four C-O bond lengths being more or less equal. In system 3, for one O/C-O pair this reciprocal arrangement is maintained, but for the other it is not. For this latter system, the two C-O bond lengths are no longer equal. For system 4, neither O/C-O alignment is reciprocal, and both pairs of bonds are now unequal in length.

You can inspect this further by identifying whether there is an antiperiplanar alignment of any given C-O bond with either of the two oxygen lone pairs on the other oxygen atom (the lone pairs are shown in purple). You should be able to correlate any such alignments with the assertions made above. The stability of the overall molecule is of course related to how many lone pair....C-O σ^* interactions can be achieved. This results in molecule 2 being the most stable with four alignments, and molecule 4 the least stable with only two.

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