Although ethyl bromide and isobutyl bromide are both primary halides, ethyl bromide undergoes sn2 reactions more than 10 times faster than isobutyl bromide does. when each compound is treated with a strong base/nucleophile (eto7), isobutyl bromide gives a greater yield of elimination products than substitution products, whereas with ethyl bromide this behavior is reversed. what factor accounts for these results?