Hydrogen-bonded hydroxyl absorption

Unassociated alcohols show a fairly sharp absorption near 3600 cm i, whereas hydrogen-bonded alcohols show a broader absorption in the 3300 to 3400 cm range. The hydrogen-bonded hydroxyl absorption appears at 3350 cm in the IR spectrum of cyciohexanol (Figure 17.11). 

The configuration was assigned after oxidation of (43) and (44) to the corresponding 3,20-diketones. The diketone obtained from (44) showed an intramolecular bonded hydroxyl absorption at 3450 cm h Inspection of molecular models indicates that only a 12 -hydroxyl function can form an internal hydrogen bond to the 20-carbonyl group. 

The freeze of equilibrium by the complexation method is also applicable to some other compounds. 2-Mercapto substituted tropone 32 has been reported to exist as an equilibrium mixture of 2-mercaptotropone (32 a) and 2-hydroxytropothione (32 b), and the latter is predominant both in solution 17) and in the solid state 18). The equilibrium is frozen and the former was isolated by inclusion complexation with 1. When a solution of 1 and 32 in petrol ether was kept, a 1 1 complex 33 composed of 1 and 32a was obtained in 90% yield as orange prisms of mp 101 to 103 °C19). The structure of 32a in 33 was elucidated by IR spectroscopy which showed vSH at 2482 cm-1 33 gave also absorptions of a strongly hydrogen-bonded hydroxyl group at 3270 cm 1. 

FIGURE 2.15 (a) The IR spectrum of an alcohol (cyclohexyl-carbinol) in a dilute solution shows the sharp absorption of a free" (non-hydrogen-bonded) hydroxyl group at 3600 cm", (b) The IR spectrum of the same alcohol as a concentrated solution shows a broad hydroxyl group absorption at 3300 cm" due to hydrogen bonding. (Reprinted with permission... 

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