Question 1

Two ethylene molecules can undergo a [2+2] cycloaddition, but only under photochemical conditions. Explain this difference in reactivity.

Accepted Answer

In ethylene, the HOMO and LUMO have different phases. The phases do not overlap, so the reaction is forbidden.
   When ethylene undergoes photochemical conditions, one of the electrons can be promoted to a higher energy level, so the HOMO now has p orbitals with opposite phases. This matches the phases of the LUMO, so the photochemical reaction is allowed.

Question 2

Which conditions should you use to perform the reaction and why?

Accepted Answer

A)  heat because the reaction is conrotatory 
B)  light (hF1F1F1S1F1F1F10) because the reaction is conrotatory 
C)  heat because the reaction is disrotatory 
D)  light (hF1F1F1S1F1F1F10) because the reaction is disrotatory 
A)  heat because the reaction is conrotatory 
B)  light (hF1F1F1S1F1F1F10) because the reaction is conrotatory 
C)  heat because the reaction is disrotatory 
D)  light (hF1F1F1S1F1F1F10) because the reaction is disrotatory B

Question 3

Predict the major organic product and show stereochemistry where relevant.

Accepted Answer

The reaction is a ring-opening electrocyclic reaction. There are six electrons involved in the reaction and it is heated, so the reaction will be disrotatory. The two ethyl substituents can be either cis/cis or trans/trans. The cis/cis would be highly hindered, so the trans/trans isomer predominates.

Question 4

Deduce the starting materials needed to synthesize this compound.

Accepted Answer

The product is a cyclohexene r

Question 5

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Accepted Answer

@#IMG-DLM& The reaction is a [2+2] cyclo

Question 6

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Accepted Answer

@#IMG-DLM& The reaction is a [3,3] sigmatropic rea

Question 7

Classify the sigmatropic reaction with bracketed numbers.

Accepted Answer

A hydrogen has migra

Question 8

Sketch the molecular orbitals for 1,3-pentadiene. Fill in the number of electrons in each molecular orbital, then identify the HOMO and LUMO. How many nodes are in each molecular orbital?

Accepted Answer

Although 1,3-pentadiene has five carbons

Question 9

Outline a synthetic sequence to give this transformation. Hint: A sigmatropic rearrangement is a key step.

Accepted Answer

The starting aldehydes are fir

Question 10

This transformation can occur through two pericyclic reactions. Outline a synthesis for the transformation.

Accepted Answer

The first step is to open the ring using

Question 11

Classify the reaction:

Accepted Answer

A)  electrocyclic 
B)  cycloaddition 
C)  sigmatropic reaction 
A)  electrocyclic 
B)  cycloaddition 
C)  sigmatropic reaction

Question 12

Consider the carbocation:
   a. How many pi molecular orbitals are there?
b. Classify each MO as symmetric or antisymmetric.
c. Which MOs are bonding and antibonding?
d. Which MOs are the HOMO and LUMO?

Accepted Answer

The carbocation shown is an allylic carb

Question 13

Predict the major organic products in the boxes and show stereochemistry where relevant.

Accepted Answer

@#IMG-DLM& The first reaction is an electrocyclic

Question 14

Give the product for a [1,5] deuterium shift for this structure:

Accepted Answer

Label the starting material with deuteri

Question 15

Classify the reaction:

Accepted Answer

A)  electrocyclic 
B)  cycloaddition 
C)  sigmatropic reaction 
A)  electrocyclic 
B)  cycloaddition 
C)  sigmatropic reaction

Question 16

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Accepted Answer

@#IMG-DLM& The starting material is an allylic ary

Question 17

Outline a synthesis for this transformation. A key step should be a [2,2] or [4,2] cycloaddition.

Accepted Answer

Start by working backwards. The four-car

Question 18

Classify the sigmatropic reaction with bracketed numbers.

Accepted Answer

This is a [2,3] sigmatropic re

Question 19

Draw a curved-arrow mechanism showing how the alcohol is transformed to the ketone.

Accepted Answer

The alcohol undergoes a [3,3]

Question 20

Draw curved-arrow mechanism showing how this transformation occurs.

Accepted Answer

This transformation is an example of an

Exam 28: Pericyclic Reactions

Two ethylene molecules can undergo a [2+2] cycloaddition, but only under photochemical conditions. Explain this difference in reactivity.

Which conditions should you use to perform the reaction and why?

Predict the major organic product and show stereochemistry where relevant.

Deduce the starting materials needed to synthesize this compound.

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Classify the sigmatropic reaction with bracketed numbers.

Sketch the molecular orbitals for 1,3-pentadiene. Fill in the number of electrons in each molecular orbital, then identify the HOMO and LUMO. How many nodes are in each molecular orbital?

Outline a synthetic sequence to give this transformation. Hint: A sigmatropic rearrangement is a key step.

This transformation can occur through two pericyclic reactions. Outline a synthesis for the transformation.

Classify the reaction:

Consider the carbocation: a. How many pi molecular orbitals are there? b. Classify each MO as symmetric or antisymmetric. c. Which MOs are bonding and antibonding? d. Which MOs are the HOMO and LUMO?

Predict the major organic products in the boxes and show stereochemistry where relevant.

Give the product for a [1,5] deuterium shift for this structure:

Classify the reaction:

Given the starting material and conditions, identify the type of pericyclic reaction, the number of electrons involved in the reaction, and draw the major organic product. Clearly show stereochemistry, if applicable.

Outline a synthesis for this transformation. A key step should be a [2,2] or [4,2] cycloaddition.

Classify the sigmatropic reaction with bracketed numbers.

Draw a curved-arrow mechanism showing how the alcohol is transformed to the ketone.

Draw curved-arrow mechanism showing how this transformation occurs.

Chemical Bonding and Chemical Structure

Alkanes and Organic Nomenclature

The Curved-Arrow Notation, Resonance, Acids and Bases, and Chemical Equilibrium

Introduction to Alkenes and Alkynes

Addition Reactions of Alkenes and Alkynes

Principles of Stereochemistry

Cyclic Compounds and Reaction Stereochemistry

Nomenclature and Noncovalent Intermolecular Interactions

The Chemistry of Alkyl Halides

Free-Radical Reactions, Main-Group Organometallic Compounds, and Carbenes

The Chemistry of Alcohols and Thiols

The Chemistry of Ethers, Epoxides, Glycols, and Sulfides

Introduction to Spectroscopy

Nuclear Magnetic Resonance Spectroscopy

Dienes and Aromaticity

The Chemistry of Benzene and Its Derivatives

Allylic and Benzylic Reactivity

The Chemistry of Aryl Halides, Vinylic Halides, and Phenols

The Chemistry of Aldehydes and Ketones

The Chemistry of Carboxylic Acids

The Chemistry of Carboxylic Acid Derivatives

The Chemistry of Enolate Ions, Enols, and Α,β-Unsaturated Carbonyl Compounds

The Chemistry of Amines

Carbohydrates

The Chemistry of Thioesters, Phosphate Esters, and Phosphate Anhydrides

The Chemistry of the Aromatic Heterocycles and Nucleic Acids

Amino Acids, Peptides, and Proteins

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