Hitherto the polar addition of bromine to cyclopropane has been considered
as a two-step process. Current calculations have established the energetics
for the proposed cation-anion pairs required by these mechanisms. An energ
etically lower pathway is proposed here in the form of a syn-cycloaddition
process. Compared to the two-step process, a significantly lower activation
enthalpy for this process has been found. The stereochemical consequences
of the cyclic mechanism are retention-retention for the two adding moieties
. This result is consistent with published experimental data on the bromina
tion of a deuterated cyclopropane.