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chapter2-kh - Why this chapter? 2. Polar Covalent Bonds:...

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1 2. Polar Covalent Bonds: Acids and Bases Based on McMurry’s Organic Chemistry , 7 th edition (Modified by KEH, 2008) Why this chapter? ± Description of basic ways chemists account for chemical reactivity. ± Establish foundation for understanding 2 concepts of reactivity discussed in subsequent chapters. 2.1 Polar Covalent Bonds: Electronegativity Chemical bonds ± Ionic bonds ± Ions held together by electrostatic attractions between unlike charges ± Sodium chloride ± Highly electronegative chlorine atom removes loosely held electron from sodium atom to give Na + and Cl - . ± Nonpolar Covalent bonds ± Two electrons are shared equally by the two bonding atoms ± Bond in H 2 or carbon-carbon bond in ethane ± Symmetrical electron distribution in the bond 2.1 Polar Covalent Bonds: Electronegativity Most bonds neither fully ionic or covalent ± Polar covalent bonds ± Bonding electrons attracted more strongly by one atom than by the other ± Electron distribution between atoms is unsymmetrical 4 owing to differences in electronegativities of the two atoms.
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2 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 5 ± 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 EN ± EN of C = 2.5 The Periodic Table and Electronegativity 6 Bond Polarity and Inductive Effect ± Nonpolar Covalent Bonds : atoms with similar EN ± Polar Covalent Bonds : Difference in EN of atoms is between 0.5 and 2.0 ± Ionic Bonds : Difference in EN > 2.0 ¾ C–H bonds are relatively nonpolar 7 C–H bonds are relatively nonpolar ¾ C-O, C-X bonds ( more electronegative elements) are polar ¾ Bonding electrons are more attracted to the electronegative atom ¾ C acquires partial positive charge, δ + ¾ Electronegative atom acquires partial negative charge, δ - ¾ Inductive effect : shifting of electrons in a bond in response to EN of nearby atoms Electrostatic Potential Maps ± Electrostatic potential maps show calculated charge distributions ± Colors indicate electron- rich red ) and electron- 8 rich ( ) and electron poor ( blue ) regions ± Arrows indicate direction of bond polarity (from δ + to δ− ). Work Problems 2.1 – 2.2 here
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3 2.1 Which element is more electronegative? b) B or Br c) C or Mg 9 2.2 Use δ+/δ− convention to show the direction of expected polarity for each of the indicated bonds. b) H
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This note was uploaded on 07/17/2011 for the course CHEM 227 taught by Professor Santander during the Spring '08 term at Texas A&M.

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chapter2-kh - Why this chapter? 2. Polar Covalent Bonds:...

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