Polar Covalent Bonds Acids and Bases

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Rules for Resonance Forms

Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)

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 rule applies

The resonance hybrid is more stable than any individual resonance form would be

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Curved Arrows and Resonance Forms

We can imagine that electrons move in pairs to convert from one resonance form to another

A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow

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Drawing Resonance Forms

Any three-atom grouping with a multiple bond has two resonance forms

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Different Atoms in Resonance Forms

Sometimes resonance forms involve different atom types as well as locations

The resulting resonance hybrid has properties associated with both types of contributors

The types may contribute unequally

The “enolate” derived from acetone is a good illustration, with delocalization between carbon and oxygen

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Pentanedione

The anion derived from 2,4-pentanedione

Lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O bond on the left and on the right

Three resonance structures result

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Acids and Bases: The Brønsted–Lowry Definition

The terms “acid” and “base” can have different meanings in different contexts

For that reason, we specify the usage with more complete terminology

The idea that acids are solutions containing a lot of “H+” and bases are solutions containing a lot of “OH-” is not very useful in organic chemistry

Instead, Brønsted–Lowry theory defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors

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Brønsted Acids and Bases

“Brønsted-Lowry” is usually shortened to “Brønsted”

A Brønsted acid is a substance that donates a hydrogen ion (H+)

A Brønsted base is a substance that accepts the H+

“proton” is a synonym for H+ - loss of an electron from H leaving the bare nucleus—a proton