Polar Covalent Bonds Acids and Bases

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2. Polar Covalent Bonds: Acids and Bases

Based on McMurry’s Organic Chemistry, 7th edition

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Why this chapter?

Description of basic ways chemists account for chemical reactivity.

Establish foundation for understanding specific reactions discussed in subsequent chapters.

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Polar Covalent Bonds: Electronegativity

Covalent bonds can have ionic character

These are polar covalent bonds

Bonding electrons attracted more strongly by one atom than by the other

Electron distribution between atoms is not symmetrical

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Bond Polarity and Electronegativity

Electronegativity (EN): intrinsic ability of an atom to attract the shared electrons in a covalent bond

Differences in EN produce bond polarity

Arbitrary scale. As shown in Figure 2.2, electronegativities are based on an arbitrary scale

F is most electronegative (EN = 4.0), Cs is least (EN = 0.7)

Metals on left side of periodic table attract electrons weakly, lower EN

Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly, higher electronegativities

EN of C = 2.5

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The Periodic Table and Electronegativity

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Bond Polarity and Inductive Effect

Nonpolar Covalent Bonds: atoms with similar EN

Polar Covalent Bonds: Difference in EN of atoms < 2

Ionic Bonds: Difference in EN > 2

C–H bonds, relatively nonpolar C-O, C-X bonds (more electronegative elements) are polar

Bonding electrons toward electronegative atom

C acquires partial positive charge, +

Electronegative atom acquires partial negative charge, -

Inductive effect: shifting of electrons in a bond in response to EN of nearby atoms

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Electrostatic Potential Maps

Electrostatic potential maps show calculated charge distributions

Colors indicate electron-rich (red) and electron-poor (blue) regions

Arrows indicate direction of bond polarity

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Polar Covalent Bonds: Dipole Moments

Molecules as a whole are often polar from vector summation of individual bond polarities and lone-pair contributions

Strongly polar substances soluble in polar solvents like water; nonpolar substances are insoluble in water.