Unique Reactions of Glycols

Periodic Acid (HIO4) or Lead Tetracetate (PblOAc) ), Oxidative Cleavage 

A r OH yields H2C=0 a 2° OH an aldehyde, RCHO a 3° OH a ketone, RjC=0. In polyols, if two vicinal OH s are termed an adjacency, the number of moles of HIO4 consumed is the number of such adjacencies. 

Problem 14.36 Give the products and the number of moles of HIO consumed in the reaction with 2,4-dimethyl-2,3,4,5-hexanetetrol. Indicate the adjacencies with zigzag lines. M 

Note that the middle —C—OH s are oxidized to a —COOH because C—C bonds on both sides are cleaved, 2. Pinacol Rearrangement 

Acidification of glycols produces an aldehyde or a ketone by rearrangement. There are four steps (1) protonation of an OH (2) loss of H O to form an R (3) 1,2-shift of H, R, or Ar to form a more stable cation (4) loss of to give product. 

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The violent or explosive reactions exhibited by glycerol in contact with many solid oxidants are due to its unique properties of having three centres of reactivity, of being a liquid which ensures good contact, and of high boiling point and viscosity which prevents dissipation of oxidative heat. The difunctional, less viscous liquid glycols show similar but less extreme behaviour. 

A number of carboxylic acids other than acetic were investigated as solvents or promoters. All of these acids which were stable to reaction conditions were found to be effective in promoting glycol ester production (e.g., propionic, pivalic, benzoic, etc.). However, other Br nsted acids of non-carboxylic nature were not found to be effective promoters. Thus penta-chlorophenol, although it has a pKa value (4.82) very close to that of acetic acid (4.76), is not a comparable promoter (Table I, reaction 13). Likewise, phosphoric acid (pK 2.15) is not an effective solvent or co-solvent with acetic acid (Table I, reaction 8). Experiments with lower concentrations of these acids in sulfolane solvent also showed that carboxylic acids are unique in promoting glycol formation. The promoter function of carboxylic acids thus appears not to be dependent (only) upon their acidity, but on some other chemical or structural property. 

Diaza-18-crown-6 derivatives (24-42) can be prepared by using a unique, single-step reaction of primary amines with triethylene glycol diiodide (42) ... 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

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