Some charged nucleophiles are actually poor bases. ... So if you see a nucleophile like NaCl, NaBr, KCN, and so on, it will favor SN2 over E2. In contrast, the bulky base below (tert-butoxide ion) is a strong base but a poor nucleophile due to its great steric hindrance, so an E2 reaction is much more likely than SN2.Nov 30, 2012read more >>