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Introduction to General, Organic and Biochemistry
The amide group, in acetamide as well as in all other amides, is best represented as a resonance hybrid (Section 3-9). Following are two contributing structures for the hybrid.
(a) Show by the use of curved arrows how contributing structure (a) is converted into contributing structure (b).
(b) Notice that structure (b) contains an oxygen atom with one bond and three unshared pairs of electrons and that this oxygen bears a negative charge, Compare the Lewis structure of this oxygen with the oxygen atom in the hydroxide ion.
(c) Notice that the nitrogen atom of structure
(b) has four bonds and bears a positive charge. Compare the Lewis structure and bonding of this nitrogen with the nitrogen in the ammonium ion, $\mathrm{NH}_{4}^{+}$.
(d) If the acetamide hybrid is best represented by contributing structure (a), predict the $\mathrm{H}-\mathrm{N}-\mathrm{H}$ bond angle
(e) If, on the other hand, the acetamide hybrid is best represented by contributing structure $(\mathrm{b})$ predict the $\mathrm{H}-\mathrm{N}-\mathrm{H}$ bond angle.
(f) Proteins are molecules that can be described as polyamides (Chapter 22). Linus Pauling, in his pioneering studies on the structure of proteins, discovered that the actual $\mathrm{H}-\mathrm{N}-\mathrm{H}$ bond angle in each amide bond of a protein is $120^{\circ}$ What does this fact tell you about the relative importance of contributing structures (a) and
(b) in the resonance hybrid?
