Chapter 16: Conjugated Dienes and Pericyclic Reactions

Question 1

Electrophilic addition to a conjugated diene

Answer 1

1. Identify all vinylic locations where protonation can occur that will produce a allylic carbocation
2. Draw resonant structures for each location
3. Draw nucleophile attacking at these locations and consider stereochemistry

Question 2

Predicting major product of diene addition

Answer 2

1. Identify all allylic locations where protonation can occur
2. Draw resonant structures for each location
3. Draw nucleophile attacking at these locations and consider stereochemistry
4. Identify kinetic and thermodynamic products

Question 3

Types of pericyclic reactions

Answer 3

Pericyclic reactions have 4 characteristic features:

1. The reaction mechanism is concerted; it proceeds without any intermediates
2. The mechanism involves a ring of electrons moving around a closed loop
3. The transition state is cyclic
4. The polarity of the solvent generally has no effect on the reaction rate (transition state has little, if any, charge)

Question 4

Diels-Alder Reactions

Answer 4

The reactants are the diene and dienophile

Dienophile needs to possess an electron withdrawing group or the reaction will require high temp, which does not favor product formation

Only work with cis diens

Question 5

Bicyclic Diels-Alder Reactions

Answer 5

When bicyclic systems form the terms ENDO and EXO are used to describe functional group positioning

Question 6

Electrocyclic Reactions

Answer 6

Pericyclic process where a conjugated polyene undergoes cyclization

Overall, a π bond is converted to a σ bond

Conrotary- both sets of lobes rotate in the same direction

Disrotary- both sets of lobes rotate in different directions

Question 7

Sigmatropic Rearrangements

Answer 7

A pericyclic reaction in which one sigma bond is replaced with another

The location of the π bonds changes as well

Can be a [1,5] rearrangement or a [3,3]

Question 8

Cope Rearrangement

Answer 8

A [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are carbon atoms

Remember, equilibrium favors more stable (substituted) alkene

π bonds must be separated by exactly three σ bonds

Question 9

Claisen Rearrangement

Answer 9

A [3,3] sigmatropic reaction that is the oxygen analogue of a Cope rearrangement

Commonly occurs with allylic-vinylic ethers

Also with allylic aryl ethers

Equilibrium favors more stable carbonyl bond (C=O)

Question 10

UV-Vis Spectroscopy

Answer 10

Compounds with conjugated π systems absorb UV or visible light

More conjugation means lower energy gap

• Wavelength of light needed can shift closer to visible range because smaller energy gap, the greater the λ_max

The group of atoms responsible for absorbing UV-Vis light is known as the chromophore