NUCLEOPHILIC REPLACEMENT REACTIONS OF SULPHONATES .6. A SUMMARY OF STERIC AND POLAR FACTORS

Since replacement reactions of sulphonic esters of carbohydrate derivatives normally proceed by an SN2 mechanism, the formation of the highly polar transition state is subject to polar factors emanating from other electronegative substituents in the molecule. Consideration of these factors offers an explanation for the diminished reactivity of aldopyranoside 2-sulphonates, 1-sulphonates of hexulopyranose derivatives, and 6-sulphonates of galactopyranose derivatives. Furthermore, the presence of an axial, electronegative substituent adjacent to a sulphonate group on a pyranose ring inhibits replacement with charged nucleophiles, because of polar factors in the transition state of the reaction. Steric inhibition of displacement by a β-trans-axial substituent is also discussed. © 1969.