SolutionsAlcohols and phenols
Exercise 1
State which reagent you would use to convert hexane-1,5-diol into (a) 5-hydroxyhexanal, (b) 6-hydroxyhexan-2-one and (c) 5-oxohexanoic acid.
(a) Catalytic TEMPO with NaOCl and KBr, which selectively oxidizes primary alcohols. (b) Oxidizing only the secondary alcohol is harder: the primary one is protected (for example as a TBS ether, which forms faster on the less hindered alcohol), the secondary is oxidized with Swern or Dess–Martin, and the protecting group is removed with fluoride. (c) A strong oxidant that oxidizes both alcohols, such as excess Jones reagent: the primary goes to the acid and the secondary to the ketone.
Exercise 2
Why is the Swern oxidation carried out at −78 °C? What by-products are formed and which one is responsible for the smell?
Chlorodimethylsulfonium chloride and the alkoxysulfonium salt are unstable: above about −60 °C they decompose (Pummerer rearrangement) and give methylthiomethyl ethers instead of the aldehyde. The by-products are CO, CO2, HCl (as triethylamine hydrochloride) and dimethyl sulfide, Me2S, responsible for the unpleasant smell; it is destroyed by oxidizing it with bleach in the waste.