ORGANIC CHEMISTRY 354

CONJUGATED DIENES

DEFINE

ADDITION

RESONANCE

CONJUGATED DIENES

ALTERNATING DOUBLE AND SINGLE BONDS.

EXAMPLES

BONDING IN CONJUGATED DIENES SUGGESTS DIFFERENT PROPERTIES.

MOST REACTIONS ARE SAME AS NONCONJUGATED BUT....

ELECTROPHILIC ADDITION

PRODUCTS FROM HBr

PRODUCTS FROM Br₂

NOTE THAT THE DOUBLE BOND HAS "MOVED" TO GIVE THE UNEXPECTED 1,4 ADDITION PRODUCT.

WHAT INTERMEDIATE CATIONS COULD BE INVOLVED?

COMPARE THE P ORBITALS OF THE TWO CATIONS WHICH COULD GIVE THE TWO PRODUCTS.

THE ALLYLIC CATION IS AN EXAMPLE OF A "RESONANCE HYBRID".

RESONANCE THEORY EXPLAINS MANY PHENOMENA!

• WHY ALL THE C-C BONDS IN BENZENE ARE THE SAME LENGTH.

• WHY THE TWO S-O BONDS IN SO₂ ARE THE SAME LENGTH. ETC.

RULES FOR FOR DRAWING AND UNDERSTANDING RESONANCE FORMS:

1. ALL RESONANCE FORMS (CONTRIBUTORS) ARE IMAGINARY, NOT REAL. THE REAL MOLECULE IS A SINGLE STRUCTURE, A HYBRID, WHICH COMBINES THE CHARACTERISTICS OF ALL OF THE RESONANCE CONTRIBUTORS. THERE ARE NO CHANGING FORMS!

2. RESONANCE STRUCTURES (CONTRIBUTORS) DIFFER ONLY IN THE POSITIONS OF THE ELECTRONS , THE NUCLEI NEVER MOVE NOR DOES THE HYBRIDIZATION CHANGE.

3. THE LOWER ENERGY CONTRIBUTORS ARE MORE IMPORTANT. THE MORE EQUIVALENT IN ENERGY THE LOW ENERGY CONTRIBUTORS ARE, THE MORE STABLE THE MOLECULE.

4. ALL RESONANCE FORMS MUST OBEY NORMAL RULES OF VALENCY.

5. THE MORE LOW ENERGY CONTRIBUTORS THERE ARE, THE MORE STABLE THE MOLECULE. THE HYBRID IS LOWER IN ENERGY THAN ANY OF THE CONTRIBUTORS.

PRACTICE WITH CONTRIBUTORS

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