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Representation of Molecules: Structural Formulas

All representations of the same molecule!

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Interpreting Dash Formulas

Are these different or identical compounds?

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Writing Molecular Formulas: Dash and Condensed

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Writing Molecular Formulas: Dash and Condensed

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Writing Molecular Formulas

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Writing Molecular Formulas

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Writing Molecular Formulas

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Three-Dimensional Formulas

ethane ethane rotated by 90

Solid wedges point out of the plane of the paper; dashed wedges point into the plane of the paper

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Representation of Molecules: Electrostatic Potential Maps

dimethyl ether Electrostatic Potential Map of dimethyl ether

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Representation of Molecules: Electrostatic Potential Maps

Electrostatic potential maps are often placed over ball-and-stick models

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Constitutional Isomers

C3H6O C3H6O

Constitutional isomers have different physical properties: MP, BP, and densities and different chemical properties (reactivity)

Acetone (nail polish remover) Propylene oxide (food thickener)

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Double Bond Rotational Barrier and Cis/Trans Isomerism

Single bonds can rotate freely; double bonds cant rotate freely

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Double Bond Rotational Barrier and Cis/Trans Isomerism

C2H2Cl2 C2H2Cl2

These compounds have identical molecular formulas but are different

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Double Bond Rotational Barrier and Cis/Trans Isomerism

Cis Trans

Cis/trans isomers are stereoisomers (isomers that differ in their orientation in space)

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Constitutional Isomers and Cis/Trans Isomer

Constitutional Isomers

Cis/Trans Isomers

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Resonance Structures

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Resonance Structures

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Resonance Structures

Doubled-headed arrows do NOT represent an equilibrium!

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Resonance Structures

How many single bonds and double bonds would we expect to find in the structure of the real molecule?

Experimental studies show that all bonds in the real structure are of equal length!

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Representation of Resonance Hybrid

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True bonding structure can be represented as a resonance hybrid

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Resonance Theory

1) When we can write multiple Lewis structures, none of the structures will be a completely accurate representation of the molecule.

2) The real structure of the molecule is actually a hybrid of all the resonance structures.

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Resonance gives us clues about structure and in chemistry structure influences reactivity!

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Resonance Theory and the Real Structure of Molecules

Electrostatic potential map of the carbonate ion

Resonance structures exists only on paper!

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Rules for Writing and Using Resonance Structures

Only move pi electrons or lone pair electrons; never break sigma bonds

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Rules for Writing and Using Resonance Structures

Never break sigma bonds!

X

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Rules for Writing and Using Resonance Structures

Never have an expanded octet on carbon or other second period heteroatoms!

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Second Period Elements and the Octet Rule

Second Period Elements Cant have Expanded Octets!

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Exceptions to the Octet Rule: Expanded Octets

Third period elements (and beyond) can have expanded octets

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Exceptions to the Octet Rule: Incomplete Octets

Elements with incomplete octets are very reactive!