Pericyclic Reactions and Radical Chemistry: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry II Topic 7, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
A concerted reaction proceeding through a cyclic transition state with no intermediate, so stereochemistry is dictated entirely by orbital symmetry.
Cycloaddition
Two pi systems join end to end, forming two new sigma bonds and a ring in one step.
Electrocyclic reaction
A conjugated chain closes to a ring or a ring opens to a chain, converting one pi bond into a sigma bond or the reverse.
Sigmatropic rearrangement
A sigma bond migrates across a conjugated system while the pi bonds shift, moving a group without ever forming an ion.
Frontier orbital analysis
Predicts pericyclic outcomes by matching the donor orbital of one partner with the acceptor orbital of the other and checking that the lobes overlap in phase.
Thermal versus photochemical control
Light promotes an electron to the next orbital, reversing the symmetry of the reacting level, so photochemical and thermal reactions give opposite stereochemistry.
Conrotatory closure
Both ends of a conjugated chain rotate the same way during ring closure, the thermally allowed path for systems with four pi electrons.
Disrotatory closure
The two ends rotate in opposite directions, the thermally allowed path for systems with six pi electrons.
Cope rearrangement
A neutral all-carbon sigmatropic shift through a six-membered chair-like transition state, driving toward the more substituted or conjugated product.
Claisen rearrangement
The oxygen-containing counterpart, converting an allyl vinyl ether into an unsaturated carbonyl compound in one concerted step.
Cheletropic reaction
A cycloaddition in which both new bonds form at a single atom, as when sulfur dioxide adds to and later leaves a diene.
Retro-cycloaddition
The reverse of a cycloaddition, driven at high temperature by the entropy gain of splitting one molecule into two.
Radical initiation
The first step of a chain, in which a weak bond splits homolytically under heat or light to create the first radicals.
Radical propagation
The self-sustaining steps that consume starting material and regenerate a radical, so a few initiator molecules convert many substrate molecules.
Radical termination
Two radicals combine, ending that chain. Because radical concentration is tiny, termination is rare relative to propagation.
Radical stability trend
Radicals follow the same stability order as cations, with allylic and benzylic positions most stabilized, which sets where substitution occurs.
Selectivity of halogen radicals
Bromination is highly selective for the weakest carbon-hydrogen bond, while chlorination is far less selective and gives mixtures.
Anti-Markovnikov addition
Radical addition of hydrogen bromide in the presence of a peroxide places bromine at the less substituted carbon, opposite to the ionic outcome.
Radical polymerization
Chain growth in which a radical adds repeatedly to alkene monomers, with chain length controlled by the ratio of initiator to monomer.
Autoxidation
Slow radical reaction of organic material with atmospheric oxygen, producing peroxides. It is why old ether stocks are hazardous.
Antioxidant
A compound that donates a hydrogen atom to give an unusually stable radical, breaking a chain instead of continuing it.
Radical clock
A substrate that rearranges at a known rate once a radical forms, used to prove that a radical intermediate is involved.
Persistent radical effect
Pairing a reactive radical with a long-lived one suppresses unwanted coupling and makes controlled chain growth possible.
Photochemical excitation
Absorbing a photon promotes an electron to a higher orbital, creating a species whose reactivity and selectivity differ from the ground state.
Norrish-type cleavage
An excited carbonyl fragments next to the carbonyl group, a classic demonstration that photochemical routes reach products thermal ones cannot.