Organic Chemistry Lecture Notes Chem 343, Lecture notes of Organic Chemistry

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Chapter 2A. Polar Covalent Bonds

Bond Polarity and Electronegativity

-^ Polar Covalent BondsBonding electrons attracted more strongly by one atom than by the other;Electron distribution between atoms is not symmetrical •^ Electronegativity (EN):Intrinsic ability of an atom to attract the shared electrons in a covalentBond; Elements with higher electronegativities have more attractions forthe bonding electrons

Electrostatic Potential Maps

-^ Electrostatic potentialmaps show calculatedcharge distributions •^ Colors ( ONLY inelectrostatic potentialmaps ) indicate electron-rich (red) and electron-poor (blue) regions •^ Crossed arrows indicatedirection of bond polarity

Dipole Moments of Molecules

-^ Molecular Dipole moment (

) - the vector summation of individual bond polarities and lone-pair contributions^ –^ ^ - magnitude of charge

Q^ at either end of molecular dipole times distance r^ between charges:

^ =^ Q^ ^ r,^ in^

debyes^ (D)

Resonance Forms & Resonance Hybrids • Some molecules cannot be represented with a single structure • Resonance forms are two or more possible structures of a molecule • The actual molecule is represented by a resonance hybrid of its resonanceforms • In a resonance hybrid, the resonance forms are connected by a double-headedarrow

Rules for Resonance Forms

-^ Individual resonance forms are imaginary - the real structure is ahybrid (only by knowing the contributors can you visualize the actualstructure) •^ Resonance forms differ only in the placement of their

^ or

nonbonding electrons • Different resonance forms of a substance don’t have to be equivalent • Resonance forms must be valid Lewis structures: the octet ruleapplies • The resonance hybrid is more stable than any individual resonanceform would be (delocalization of electrons)

Drawing Resonance Forms

-^ Sometimes resonance forms involve different atom types as well as locations •^ The resulting resonance hybrid has properties associated with both types ofcontributors •^ The resonance forms may contribute unequally to the resulting resonancehybrid^ 2,4-pentanedione

2,4-pentanedione anion(three resonance forms)

Homework

2.2, 2.3, 2.7, worked example 2.3, 2.