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SolutionsSubstitution vs. Elimination

1. Predict the major product

  1. (CH3)3CBr + CH3CH2ONa in ethanol
  2. CH3CH2CH2Br + NaCN in DMSO
  3. (CH3)2CHBr + CH3CH2ONa in ethanol
  4. CH3CH2CH2Br + (CH3)3COK
  5. (CH3)3CBr in ethanol, without base
  1. 2-Methylpropene, by E2 (tertiary halide and strong base).
  2. Butanenitrile, CH3CH2CH2CN, by SN2.
  3. Mainly propene, by E2, with some ethyl isopropyl ether by SN2.
  4. Propene, by E2: the base is strong and hindered.
  5. Ethyl tert-butyl ether (SN1) and some 2-methylpropene (E1).

2. Temperature

Why does raising the temperature increase the proportion of elimination over substitution?

Elimination produces more molecules than substitution (three versus two), so its reaction entropy is more favourable; at higher temperature the −TΔS term weighs more. In addition, its activation energy is usually somewhat higher.

3. Choose the conditions

Which conditions would you choose to obtain 2-methylpropene from tert-butyl bromide? And to obtain tert-butanol?

For the alkene, a strong base (sodium ethoxide or potassium tert-butoxide) and heat: E2. For the alcohol, water without base at moderate temperature: SN1, although some alkene will always be formed by E1.

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