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The Result of an Experiment

Optically pure (R)-2-bromooctane was made to react with sodium hydroxide (a nucleophile) in DMF (dimethylformamide) as solvent.

Experiment: (R)-2-bromooctane ([α] = −34.6°) with NaOH in DMF gives 80% alcohol ([α] = +9°) and 20% alkenes ([α] = 0°)

The thorough analysis of the reaction mixture tells us that the result is the major production (80%) of an optically active alcohol,
together with a series of optically inactive alkenes. It is reasonable to think that the nucleophile (HO−) attacks the electron-deficient carbon of 2-bromooctane, the one bonded to the bromine. Consequently, the alcohol should be 2-octanol. Let's check a chemical catalog, where we can find the following data:

Handbook data: (S)-(+)-2-octanol [α] = +9.5° and (R)-(−)-2-octanol [α] = −9.5°

The comparison between the measured and tabulated optical rotations tells us that the alcohol is mainly (not exclusively) the S enantiomer. The olefins have been separated and analyzed. The global result is summarized in the following scheme:

Result of the experiment: (R)-2-bromooctane gives 80% substitution (2-octanol with 94% inversion) and 20% elimination (E alkene 95%, Z 5% and traces of terminal olefin)

These results lead to the following conclusions:

  • The halogen was mostly substituted by an OH group.
  • Alcohol configuration is mostly S, contrary to that of the starting haloalkane.
  • Therefore, the substitution reaction mostly happened with INVERSION of configuration.
  • The production of olefins shows that the halogen was in part ELIMINATED.
  • The elimination process must be complex because a number of different olefins are formed.
  • A small amount of the R alcohol was formed, suggesting that a small amount of RACEMIZATION took place.
  • The reaction rate depends on the two reagents, haloalkane and nucleophile, thus suggesting that the two of them must intervene in the rate-limiting step of the reaction.

These logical deductions should allow us to propose a reaction mechanism, i.e. a reasonable sequence of steps describing precisely how and when the bonds are broken and formed.