3-membered rings are strained moieties in organic chemistry. When a 3-membered ring contains a heteroatom, such as in an epoxide or in a bromonium intermediate, the three atoms in the ring become polarized. It is impossible to assign (+) and (-) polarities to a 3-membered ring without having two adjacent atoms with the same polarity. Therefore, whenever a polarized 3-membered ring is opened by a nucleophile, umpolung inevitably results . For example, the opening of ethylene oxide with hydroxide leads to ethylene glycol.