SolutionsCarboxylic acids and derivatives
Exercise 1
What product is obtained by treating methyl hexanoate with (a) LiAlH4, (b) one equivalent of DIBAL-H at −78 °C and (c) excess DIBAL-H at room temperature?
(a) Hexan-1-ol. (b) Hexanal: at −78 °C the tetrahedral aluminium intermediate is stable and releases the aldehyde only on hydrolysis. (c) Hexan-1-ol: at room temperature the intermediate breaks down, the aldehyde is released in the presence of reducing agent and is reduced further.
Exercise 2
Propose a synthesis of 1-bromopentane from hexanoic acid, and of octane from hexanoic acid and butanoic acid. What is the drawback of the second?
Hunsdiecker reaction: silver hexanoate with Br2 in refluxing CCl4 gives 1-bromopentane and CO2 (one carbon fewer). Octane would be obtained by Kolbe electrolysis of the mixture of the two carboxylates (pentyl + propyl), but the radicals combine at random and a statistical mixture of decane (pentyl + pentyl), octane and hexane (propyl + propyl) is formed, which must be separated. Kolbe electrolysis is useful mainly for symmetrical dimers.