Unit 14: Conjugation, Dienes, and Pericyclic Reactions
Electrons spread across more than one bond at once, and two pi systems swinging together to build two new bonds in a single concerted step.
7 topics · 25 questions
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Conjugation and Overlapping p Orbitals
- How it worksHow do adjacent p orbitals overlap across a formally single bond to let electron density spread over several carbons?
- Worked exampleFind whether 1,3-pentadiene or 1,4-pentadiene has its two double bonds conjugated.
- Going furtherWhat happens to a diene's stability, measured by its heat of hydrogenation, once its double bonds become conjugated?
Your turn1 of 2
Which molecule has its two double bonds conjugated, 1,3-pentadiene or 1,4-pentadiene?Allylic Cations and Extended Resonance
- How it worksHow does the empty p orbital of an allylic cation overlap with the adjacent π bond to delocalize the positive charge?
- Worked exampleFind both resonance structures and both possible sites of nucleophilic attack on the allylic cation from ionizing 3-bromo-1-butene.
- Common mistakeWhy isn't an allylic cation just an ordinary carbocation that happens to sit near a double bond?
- Going furtherWhat happens to a nucleophile's options when it can attack an allylic cation at either end of its resonance structure?
Your turn1 of 2
3-bromo-1-butene, CH2=CH-CHBr-CH3, ionizes to form an allylic cation. What are the two resonance structures of this cation, and where can a nucleophile attack?1,2- vs 1,4-Addition to a Diene
- How it worksHow does the allylic cation formed from a diene get attacked by bromide at either the near carbon or the far one?
- Worked exampleFind the 1,2- and 1,4-addition products when HBr adds to 1,3-butadiene.
- Going furtherWhat happens to the ratio of 1,2- versus 1,4-product once the reaction temperature is raised significantly?
Your turn1 of 2
1,3-butadiene reacts with HBr. What are the 1,2- and 1,4-addition products?Kinetic vs Thermodynamic Control in Diene Addition
- How it worksHow does the 1,2-product form faster at low temperature while the more stable 1,4-product builds up once the reaction can reverse?
- Worked exampleFind which product, 1,2- or 1,4-, dominates when 1,3-butadiene reacts with HBr at negative 80 degrees Celsius.
- Common mistakeWhy doesn't the faster-forming kinetic product stay the major product once the reaction gets enough time and heat to equilibrate?
- Ties togetherHow does this same kinetic-versus-thermodynamic logic from the mechanism unit resurface here in diene chemistry?
Your turn1 of 2
1,3-butadiene reacts with HBr at negative 80 degrees Celsius, a low temperature where the reaction cannot reverse. Which product dominates, 1,2- or 1,4-addition?The Diels-Alder Reaction
- How it worksHow do the diene's HOMO and the dienophile's LUMO overlap on both faces at once to form two new bonds simultaneously?
- How it worksHow does the diene need to twist into its s-cis conformation before its two ends can reach the dienophile together?
- Worked exampleFind the product formed when 1,3-butadiene reacts with maleic anhydride in a Diels-Alder reaction.
- Common mistakeWhy doesn't a Diels-Alder reaction proceed through separate carbocation or radical intermediates the way ionic additions do?
- Going furtherWhat happens to a Diels-Alder reaction's rate when the dienophile carries an electron-withdrawing group like a carbonyl?
Your turn1 of 2
1,3-butadiene reacts with maleic anhydride in a Diels-Alder reaction. What is the product?Diene and Dienophile Reactivity in the Diels-Alder
- How it worksHow does an electron-donating group on the diene paired with an electron-withdrawing group on the dienophile speed the reaction up?
- Worked exampleFind whether cyclopentadiene or 2,3-dimethyl-1,3-butadiene reacts faster with a given dienophile, and why.
- Common mistakeWhy doesn't every alkene make a good Diels-Alder dienophile just because it happens to have a double bond?
Your turn1 of 2
Which diene reacts faster with a given dienophile in a Diels-Alder reaction: cyclopentadiene, or 2,3-dimethyl-1,3-butadiene?Electrocyclic Ring Closing and Orbital Symmetry
- How it worksHow does heat rotate the two ends of a diene's π system in a disrotatory motion to form a new sigma bond?
- Worked exampleFind whether a thermal electrocyclic ring closure of a diene proceeds disrotatory or conrotatory.
- Ties togetherHow does the same orbital-overlap thinking from the Diels-Alder reaction extend to explain an electrocyclic ring closing?
Your turn1 of 2
A diene undergoes a thermal electrocyclic ring closure to form a cyclobutene. Does this proceed disrotatory or conrotatory?