Ch 13b

Molecules or ions that have a p orbital on an atom adjacent to a multiple bond, which forms an extended pi system; the p orbital may be that of another multiple bond or that of a radical, carbocation, or carbanion

 A reaction which increases the number of atoms or groups that are attached to a pair of atoms joined by a double or triple bond; all parts of the adding reagent appear in the product, and two molecules become one

A reaction in which an atom or group is replaced by a different atom or group.

The common name for the 2-propenyl group, H2C=CHCH2 -

A hydrogen atom on any saturated carbon atom that is adjacent to a C=C double bond.

A reaction in which an allylic hydrogen atom is replaced by a nucleophile

The free-radical ∙CH2- CH=CH2 that is stabilized by delocalization of electrons through resonance.

 A general term for any free-radical that has a carbon atom with anunpaired electron that is adjacent to a C=C double bond, with the general formula ∙CR2- C(R)=CR2 ; allylic radicals are more stable than any saturated radical due to delocalization of electrons by resonance.

The dispersal of electron density or electrical charge by resonance, which always stabilizes a system

The isomerization of a double bond that occurs through the delocalization of an allylic intermediate.

The carbocation +CH2-CH=CH2 that is stabilized by delocalization of the positive charge by resonance.

A general term for any carbocation that has a carbon atom with a positive charge that is singly bound to a C=C double bond, with the general formula+CR2-C(R)=CR2 ; substituted allylic cations are more stable than any saturated carbocation due to delocalization of the positive charge through resonance.

Lewis structures of the same molecule that differ only in the position of their electrons; a single resonance structure will not adequately represent a molecule, and the molecule is best represented as a hybrid of all of the resonance structures. Also called resonance contributors.

The extra stability of a molecule that has resonance contributors, since the energy of the actual molecule is lower than the energy that is estimated for any of its resonance structures.

Diene of the type C=C=C, in which one carbon has double bonds to two others.

Diene whose C=C double bonds are separated by two or more single bonds; isolated double bonds react independently, as they do in a simple alkene

System of the type C=C- C=C, in which two pairs of doubly bonded carbons are joined by a single bond; the pi bonding orbitals overlap with each other, and the pi electrons are delocalized by resonance over the four consecutive sp2 hybridized carbons

The energy of stabilization that represents the difference in energy between the actual molecule or ion and that calculated for any resonance structure; this lower energy state arises from delocalization of electrons in the conjugated system, and it results in resonance stabilization

The energy of stabilization that represents the difference in energy between the actual molecule or ion and that calculated for any resonance structure; this lower energy state arises from delocalization of electrons, and it results in resonance stabilization.

The lowest unoccupied molecular orbital; when a molecule in the ground state is excited, an electron may jump from a HOMO into a LUMO.

The highest occupied molecular orbital; when a molecule in the ground state is excited, an electron may jump from a HOMO into a LUMO

An addition reaction in which a polarizable reagent, E-N, is attacked by the pi electrons of an alkene (or alkyne) on the electrophilic portion (E), which initially forms a carbocation intermediate that is subsequently attacked by the nucleophilic portion (N) of the reagent; the product contains E attached to one carbon and N attached to the adjacent carbon. Also called an ionic addition reaction.

 An electrophilic addition reaction between hydrogen halide, HX, and an unsymmetrical alkene, where the halogen adds to the more substituted carbon, and the hydrogen adds to the less substituted carbon

An addition reaction in which two groups add to adjacent atoms in a conjugated system, which is fast and under kinetic control. Also called simple addition.

An addition reaction in which two groups add to atoms that bear a 1,4-relationship in a conjugated system, which is energetically favored and under thermodynamic control. Also called conjugate addition

The difference in Gibbs free energy between the reactants and the transition state of a reaction

A measure of the difference in potential energy between the reactants and the transition state of a reaction; it is almost the same as the free energy of activation, ΔG‡ , when entropy changes are small. Also called energy of activation.

A principle stating that when the ratio of products of a reaction is determined by relative rates of reaction, as measured by their activation energies, and the most abundant product will be the one that is formed fastest. Also called rate control.

A principle stating that the ratio of products of an equilibrium reaction is determined by the relative stabilities of the products, as measured by their standard free energies, ΔG°, and the most abundant product will be the one that is the most stable. Also called equilibrium control.

A cycloaddition reaction between a conjugated diene and a substituted alkene (dienophile) to form a substituted cyclohexene product (adduct); four pi electrons in the diene interact with two pi electrons in the dienophile to form two new sigma bonds and the loss of two pi bonds, such that up to four new stereogenic centers may be created simultaneously.

(1) An alkene with two carbon-carbon double bonds; also called an alkadiene. (2) The component with four pi electrons, usually two conjugated C=C double bonds, that reacts with a dienophile in a Diels-Alder reaction.

The component with two pi electrons, usually one C=C double bond, that reacts with a diene in a Diels-Alder reaction

(1) The product of a Diels-Alder reaction. (2) The product of an addition reaction that contains all atoms of both reactants, with the formation of two new sigma bonds and the loss of two pi bonds.

In a bridged ring system, the isomer with the substituent located syn, or closest to the longest bridge.

 In a bridged ring system, the isomer with the substituent located anti, or furthest from the longest bridge