Chemistry - STEREO ISOMERISM Question with Solution | TestHub

(a) In a conformational array A-B-C-D, where A and B are second-row electronegative atoms such as N, O or F, or unshared electron pairs, the often observed preference for the gauche (skew) conformation of A and B is called the gauche effect.

preferred conformation of ${\mathrm{H}}_2{\mathrm{O}}_2$ (gauche effect) preferred conformation of ${\mathrm{H}}_2\mathrm{NN}{\mathrm{H}}_2$ (gauche effect)

(b) Viewing ethanal along the C-C bond, with the front $\mathrm{s}{\mathrm{p}}^2$ carbon carrying the =O group and H :

eclipsed conformation bisecting conformation

In the eclipsed conformation the $\mathrm{C}=\mathrm{O}$ bond eclipses a C-H bond of the methyl group; in the bisecting conformation the $\mathrm{C}=\mathrm{O}$ bond bisects an H-C-H angle of the methyl group.