Question 8.4: In each case, which bond is more polar? (a) B—Cl or C—Cl, (b...
In each case, which bond is more polar? (a) B—Cl or C—Cl, (b) P—F or P—Cl. Indicate in each case which atom has the partial negative charge.
Analyze We are asked to determine relative bond polarities, given nothing but the atoms involved in the bonds.
Plan Because we are not asked for quantitative answers, we can use the periodic table and our knowledge of electronegativity trends to answer the question.
Solve
(a) The chlorine atom is common to both bonds. Therefore, we just need to compare the electronegativities of B and C. Because boron is to the left of carbon in the periodic table, we predict that boron has the lower electronegativity. Chlorine, being on the right side of the table, has high electronegativity. The more polar bond will be the one between the atoms with the biggest differences in electronegativity. Consequently, the B—Cl bond is more polar; the chlorine atom carries the partial negative charge because it has a higher electronegativity.
(b) In this example, phosphorus is common to both bonds, and so we just need to compare the electronegativities of F and Cl. Because fluorine is above chlorine in the periodic table, it should be more electronegative and will form the more polar bond with P. The higher electronegativity of fluorine means that it will carry the partial negative charge.
Check
(a) Using Figure 8.8: The difference in the electronegativities of chlorine and boron is 3.0 – 2.0 = 1.0; the difference between the electronegativities of chlorine and carbon is 3.0 – 2.5 = 0.5. Hence, the B—Cl bond is more polar, as we had predicted.
(b) Using Figure 8.8: The difference in the electronegativities of chlorine and phosphorus is 3.0 – 2.1 = 0.9; the difference between the electronegativities of fluorine and phosphorus is 4.0 – 2.1 = 1.9. Hence, the P—F bond is more polar, as we had predicted.
