| Secondary versus Tertiary Halides |
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Narrative
Two test tubes contain acetone and a basic solution of bromcresol green indicator. Isopropyl bromide is added to the first tube and tert-butyl bromide to the second. Production of hydrogen bromide causes the indicator to change color (from green to pink).
Discussion
Two test tubes contain acetone and aqueous sodium hydroxide (2.3 : 1 ratio) and bromcresol green indicator. Isopropyl bromide (2-bromopropane) is added to the first tube and tert-butyl bromide (2-bromo-2-methylpropane) to the second. The tertiary halide 2-bromo-2-methylpropane rapidly undergoes a SN1 reaction releasing hydrogen bromide and causing the indicator to change color from green to pink. The secondary halide 2-bromopropane reacts more slowly. This order of reactivity (tertiary > secondary halide) is characteristic for SN1 reactions in which a carbocation is formed.
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