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SolutionsHaloalkanes

1. Which mechanism is it?

State which mechanism (SN1, SN2, E1 or E2) each description corresponds to:

  1. Rate = k[RX][Nu−] and inversion of configuration.
  2. Carbocation intermediate and racemization of the product.
  3. A single transition state in which H and the leaving group are antiperiplanar.
  4. The carbocation loses a proton from the adjacent carbon.
  1. SN2.
  2. SN1.
  3. E2.
  4. E1.

2. 2-Bromobutane

For 2-bromobutane, CH3CHBrCH2CH3:

  1. Is it a primary, secondary or tertiary halide?
  2. What product does it give with NaCN in DMSO, and by which mechanism?
  3. What is the major product with sodium ethoxide in refluxing ethanol?
  1. Secondary: the carbon bonded to bromine is bonded to two other carbons.
  2. 2-Methylbutanenitrile, CH3CH(CN)CH2CH3, by SN2 (a good, weakly basic nucleophile in a polar aprotic solvent), with inversion of configuration.
  3. Mainly 2-butene (mostly E) by E2: a strong base, a secondary halide and heat favour elimination.

3. Choose the substrate

Which compound would you choose for a fast SN2 reaction, CH3I or (CH3)3CBr? And for an SN1 reaction?

For SN2, CH3I: an unhindered carbon and iodide, an excellent leaving group. For SN1, (CH3)3CBr: it gives a tertiary carbocation, the least unstable one.

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