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Chapter 14
Delocalized Pi Systems
42
Organic Chemistry
Undergraduate 2
01/06/2015

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Term
Is the primary allylic carbon-hydrogen bond relatively strong or relatively weak?
Definition
relatively weak; only 87 kcal/mol
Term
Is the primary allylic carbon-hydrogen bond weaker or stronger than a tertiary C-H bond?
Definition
weaker
Term
Do allylic C-X bonds dissociate relatively fast or relatively slow under SN1 conditions?
Definition
relatively fast
Term
Is an allylic carbon more or less stable than other primary carbocations?
Definition
more stable
Term
What is the pKa of an allylic C-H bond?
Definition
40
Term
Is the formation of an allylic carbon anion favored or not?
Definition
favored
Term
allylic carbon
Definition
the carbon bonded to a carbon involved in an alkene functional group
Term
What is the major stabilizing force for allylic carbon forms?
Definition
resonance
Term
Each carbon in the allylic structure is _____ hybridized
Definition
sp2
Term
nonbonding
Definition
a pi orbital that has the same energy as a noninteracting p orbital
Term
The resonance formations for allylic systems indicate that it is mainly the two _____ carbons that accommodate the charges in the ions or the odd electron in the radical
Definition
terminal
Term
radical allylic bromination takes place when bromine levels are _____
Definition
low
Term
conjugated diene
Definition
two double bonds separated by one single bond; comes from the addition of a fourth p orbital; comes from the Latin word for union (conjugatio)
Term
nonconjugated
Definition
two double bonds are separated by saturated carbons; in contrast with conjugated dienes
Term
allenes (AKA cumulated)
Definition
pi bonds share a single sp hybridized carbon & are perpendicular to each other; in contrast with conjugated dienes
Term
noninteracting double bonds
Definition
separated by one or more saturated carbon atoms
Term
s-cis
Definition
two pi bonds lie on the same side of the C2-C3 axis
Term
s-trans
Definition
pi bonds are on opposite sides
Term
The prefix "s" in s-cis and s-trans refers to the fact that the bridge between C2 & C3 constitutes a _____ bond
Definition
single
Term
Are conjugated dienes more or less reactive kinetically in the presence of electrophiles/other reagents than dienes with isolated double bonds?
Definition
more reactive
Term
thermodynamic control
Definition
when a ratio of products for a reaction reflects their thermodynamic stability
Term
kinetic control
Definition
when a ratio of products for a reaction reflects their relative rates of formation/relative heights of the respective activation barriers
Term
Will an irreversible reaction be under kinetic or thermodynamic control?
Definition
kinetic control
Term
extended pi system
Definition
when more than two double bonds are in conjugation
Term
Is benzene generally reactive?
Definition
no!
Term
Diels-Alder reaction
Definition
a conjugated diene adds to an alkene to yield cyclohexene derivatives
Term
What are the products of cycloaddition reactions called?
Definition
cycloadducts
Term
dienophile
Definition
the alkene in a Diels-Alder reaction
Term
Can rings be formed by the linkage of the termini of a single conjugated di-, tri-, or polyene?
Definition
yes!
Term
electrocyclic reactions
Definition
the formation of a ring by a Diels-Alder reaction occurring on a single molecule
Term
pericyclic
Definition
a class of transformations that exhibit transition states with a cyclic array of nuclei & electrons; ex) cycloadditions & electrocyclic reactions; comes from the Greek word "peri" for "around"
Term
photochemical reactions
Definition
using light to drive a reaction in the thermodynamically unfavorable direction
Term
Are electrocyclic reactions concerted and stereospecific?
Definition
yes!
Term
conrotatory
Definition
a process in which the carbon atoms are found to rotate in the same direction in a ring opening
Term
photocyclization
Definition
the photochemical closure of a diene ring; proceeds with stereochemistry exactly opposite that observed in thermal opening
Term
disrotatory
Definition
a process in which reacting carbons rotate in opposite directions
Term
If there are an even number of pairs of participating electrons in an electrocyclic reaction, will the thermal process be conrotatory or disrotatory?
Definition
conrotatory
Term
If there are an even number of pairs of participating electrons in an electrocyclic reaction, will the photochemical process be conrotatory or disrotatory?
Definition
disrotatory
Term
If there are an odd number of pairs of participating electrons in an electrocyclic reaction, will the thermal process be conrotatory or disrotatory?
Definition
disrotatory
Term
If there are an odd number of pairs of participating electrons in an electrocyclic reaction, will the photochemical process be conrotatory or disrotatory?
Definition
conrotatory
Term
What do the Woodward-Hoffman rules describe?
Definition
the stereochemical outcomes of electrocyclic reactions
Term
vulcanization
Definition
a process in which a rubber is treated with hot elemental sulfur to increase its elasticity
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