Example 7.3: Electronegativity and Bond Polarity

Bond polarities play an important role in determining the structure of proteins. Using the electronegativity values in Figure 6, arrange the following covalent bonds — all commonly found in amino acids — in order of increasing polarity. Then designate the positive and negative atoms using the symbols δ+ and δ–:
C–H, C–N, C–O, N–H, O–H, S–H

Solution

The polarity of these bonds increases as the absolute value of the electronegativity difference increases. The atom with the δ– designation is the more electronegative of the two. Table 1 shows these bonds in order of increasing polarity.

\(\mathrm{ΔEN}_{\mathrm{\ce{C-H}}}\) \(= 0.4\)


\(\ce{ C^{δ-}-H^{δ+}}\)



\(\mathrm{ΔEN}_{\mathrm{\ce{S–H}}}\) \(= 0.4\)


\(\ce{ S^{δ-}-H^{δ+}}\)



\(\mathrm{ΔEN}_{\mathrm{\ce{C–N}}}\) \(= 0.5\)


\(\ce{ C^{δ+}-N^{δ-}}\)



\(\mathrm{ΔEN}_{\mathrm{\ce{N–H}}}\) \(= 0.9\)


\(\ce{ N^{δ-}-H^{δ+}}\)



\(\mathrm{ΔEN}_{\mathrm{\ce{C–O}}}\) \(= 1.0\)


\(\ce{ C^{δ+}-O^{δ-}}\)



\(\mathrm{ΔEN}_{\mathrm{\ce{O–H}}}\) \(= 1.4\)


\(\ce{ O^{δ-}-H^{δ+}}\)