Chapter 14: Delocalized Pi Systems

Question 1

Delocalized Electrons

Answer 1

shared by three or more atomic centers with parallel p orbitals that participate in π-type overlap

Question 2

Conjugated Dienes

Answer 2

alternating double and single bonds that have extended delocalization of π electrons
have a coplanar configuration due to increased barrier to rotation about the single bond due to overlapping delocalized π electrons
more reactive kinetically, more stable thermodynamically

Question 3

Allyl

Answer 3

three carbon intermediates with a carbocation/carbanion/radical adjacent to the π framework of a double bond
has special stability due to contributing resonance forms with three p orbitals (a bonding one, an nonbonding one and an antibonding one)

Question 4

Allylic Carbon

Answer 4

activated carbon in an allyl

Question 5

Radical Allylic Substitution

Answer 5

free radical chain mechanism
common reagents:
- halogenation: X2, ROOR or hv
- bromination: NBS, HBr, hv or NBS in CCl4

Question 6

X2, ROOR or hv

Answer 6

radical allylic halogenation

Question 7

NBS in CCl4

Answer 7

NBS is insoluble in CCl4, produces a steady source of small amounts of bromine for radical allylic bromination

Question 8

Initiation Step

Answer 8

two radicals formed from a stable compound using hv or ROOR

Question 9

Propagation Steps

Answer 9

radical and stable compound form a new stable compound and a new radical

Question 10

Radical Stability

Answer 10

1° > 2° > 3°

Question 11

Mg, THF, 0°C

Answer 11

with an allylic halide, produces a Grignard reagent

Question 12

Nonconjugated Isomers

Answer 12

have two double bonds separated by saturated carbons, has a heat of hydrogenation about 3.5 Kcal/mol

Question 13

Allenes/Cumulated Dienes

Answer 13

have π bonds that share a single sp hybridized carbon and are perpendicular to each other

Question 14

s-cis Conjugated Diene

Answer 14

two π bonds lie on the same side of the molecule, less stable

Question 15

s-trans Conjugated Diene

Answer 15

two π bonds on opposite sides of the molecule, more stable

Question 16

Thermodynamic Control

Answer 16

reaction whose ratio of products reflects their thermodynamic stability

Question 17

Kinetic Control

Answer 17

reaction whose ratio of products reflects their relative rates of formation
less stable product will form with a lower activation barrier

Question 18

Diels-Alder Cycloaddition

Answer 18

occurs when atoms at the ends of a diene add to an alkene double bond, closing a ring
bonds form simultaneously and stereospecifically
4 conjugated atoms containing 4 π e- react with a double bond and 2 π e-

Question 19

Cycloaddition Reactions

Answer 19

reactions between π systems that produce cycloadducts

Question 20

Diene

Answer 20

4 carbon molecule with 2 π bonds, 4 conjugated atoms and 4 π e-

Question 21

Dienophile

Answer 21

alkene with 2 conjugated atoms, 2 π e-

“diene loving”

Question 22

Diels-Alder Substitution on Reactants

Answer 22

dienophile favors electron attracting groups

diene favors electron donating groups

Question 23

Dienophile Reactivity with Substituents

Answer 23

CH3

Question 24

Diene Reactivity with Substituents

Answer 24

none

Question 25

Concerted Reactions

Answer 25

bond breaking and bond making occur simultaneously

Question 26

Diels-Alder Stereochemistry

Answer 26

mechanism is concerted and stereospecific so stereochemistry at original double bond of dienophile is retained in the product

Question 27

Exo Adducts

Answer 27

has two substituents on the same side, cis, of the shorter bridge

Question 28

Endo Adducts

Answer 28

have two trans substituents that point towards the diene
favored so the products with the activating e- withdrawing group of dienophile is located in the endo position
endo transition state is slightly lower in energy

Question 29

Visible Spectroscopy

Answer 29

wavelength range of 400-800 nm

describes the color of organic compounds

Question 30

Ultraviolet Spectroscopy

Answer 30

wavelength range of 200-400 nm
can be used with visible spectrum to investigate electronic structures of unsaturated molecules and to measure the extent of conjugation

Question 31

Electronic Spectra

Answer 31

records changes in electron energies using electromagnetic radiation at UV and visible wavelengths to excite e- from filled bonding or nonbonding orbitals to unfilled antibonding MOs