Knowledge Bank

On the basis of the mixed solvent techniques with carbon tetrachloride as inert solvent, hydrogen bonding was studied for 2-hexanone and $\gamma$-butyrolactone. The solvents used for such investigations are o-cresol (a strong proton-donor), $n$-butanol (a less strong proton-donor), and chloroform (a weak proton-donor). The effect of different solvents on the frequency, intensity, and half width of the carbonyl band was studied and the complexes were determined. 1:1 and 1:2 complexes were formed at fairly low concentrations for both 2-hexanone-o-cresol and $\gamma$-butyrolactone-o-cresol systems. Only a 1:1 complex was formed at high concentrations for both 2-hexanone-$n$-butanol and $\gamma$-butyrolactone-$n$-butanol systems. Only a 1:1 complex was formed at comparatively higher concentrations for both 2-hexanone-chloroform and $\gamma$-butyrolactone-chloroform systems. Formation constants and the spectra of individual species were determined for these complexes. The observed low frequency shift for the 1:2 complex of the 2-hexanone-$o$-cresol system favors a structure in which the second $o$-cresol molecule is not directly bonded to the carbonyl group but to the oxygen atom of the first $o$-cresol molecule, whereas the observed large frequency shift for the 1:2 complex of $\gamma$-butyrolactone-$o$-cresol system favors a structure in which two molecules of $o$-cresol are directly bonded to the carbonyl group. Free energies of formation calculated at $25\circ C$ for these complexes show that the interaction increases in going from the 1:1 complex to the 1:2 complex, and in considering the 1:1 complex the interaction decreases in the following order: ketone-$o$-cresol $>$ ketone-$N$-butanol $>$ ketone-chloroform. The results have been discussed in relation to the intensity changes, frequency shifts, and half-width changes.