Hydrogen bonding in protic solvents

CN, cf the behaviour of OMe. Nevertheless it seems possible that hydrogen-bonding in protic solvents could be a factor enhancing the electron-attracting effect of CN see structure 8. 

Finally, it should be noted that although it is solvent strength which primarily controls retention, hydrogen bonding in protic solvents may also influence the retention. 

Hydrogen bonding with protic solvents or reagents occurs widely in azines even when they are not appreciably basic and the protic compounds are very poor acids. The latter do not have to be present... 

However, all amines can participate in hydrogen bonding with protic solvents, so amines have similar water solubilities to comparable alcohols. Low molecular weight amines are freely miscible with water. Low molecular weight amines have an offensive fishlike smell. 

This entropy decrease for the formation of the eoloured form (12b) may be due to the planarization of the molecule (formation of the N—H N bridge) and the fixation of the di-ctT-form (12b), while in (12a) there is free rotation about the central C—C bond. In protic solvents, the colourless form (12a) is already stabilized by hydrogen bonding and T AS° is small. Therefore, the AG° values are greater in these solvents and AH° is positive [68],... 

Solvent shifts are useful as a criterion to identify n,7t transitions in absorption spectra because hydrogen bonding of protic solvents with the carbonyl oxygen stabilizes the np lone pair and gives rise to a hypsochromic shift of the n,7t absorption bands see the positions of the n,7t absorption band of acetone in heptane and water (Figure 6.5, top). This contrasts with 7t,7t transitions that tend to be shifted bathochromically in polar solvents. Also, the photophysical and photochemical properties often serve to identify the nature of the lowest excited state. Lone-pair interaction in biacetyl splits the two np-orbitals giving rise to two n,7t transitions at v = 2.23 and 3.54 im In the spectrum of 1,4-naphthoquinone in methanol, the n,7t band is barely detectable as a shoulder on the red edge of the 71,71 absorption. 

Much of this chapter is concerned with hydrogen bond making or breaking. This is because, for polar solutes in protic media, such bonds dominate solvation. Also, anions in protic solvents are solvated by hydrogen bonds. This is not true for cations, but when a cation is solvated it uses H-bond sites (lone-pairs of electrons), which changes hydrogen bonding in the solvent. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

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