C15: Conjugated Systems, Orbital Symmetry, UV Spec

Question 1

conjugated double bonds are _____ stable than isolated double bonds

Answer 1

more

Question 2

which is more stable?
pent-1-ene or pent-2-ene
terminal double bond or internal

Answer 2

pent-2-ene, internal double bonds

Question 3

conjugate diene require ____ heat for hydrogenation therefor are ___ stable than the addition of

Answer 3

less, more

Question 4

other names for successive double bonds

stability??

Answer 4

cumulated double bonds, allenes

less stable than isolated and much less stable than conjugated

Question 5

lowest-energy orbital is the ____ stable because
ex: butane
1
2

Answer 5

most
pi 1
1. three bond interaction
\+ + + +
-  -  -  -
2. electrons are delocalized over four nuclei

Question 6

trans conformation is ____ stable than cis conformation in diene

Answer 6

more

Question 7

stability of carbocations

Answer 7

substituted allyic, tertiary > secondary > primary > +CH3

Question 8

H-Br + conjugated diene —> ?????
low temp
high temp

Answer 8

1,2 addition and 1,4 addition
more 1,2 addition
more 1,4 addition

Question 9

kinetic control (occurs at \_\_\_\_ temperature)
which is more prevalent? why?

Answer 9

low temperature

1,2- product because has lower activation energy

Question 10

thermodynamic control (occurs at _____ temperature)
which is more prevalent?
why?

Answer 10

high temperature

1,4- product is more stable

Question 11

How to form radicals?

Answer 11

Br2 —hv—> 2Br. (radicals)

Question 12

Br. + allylic hydrogens —> [1] ??? + ???? —Br2—> [2] ??? + ??? —-NBS—> [3] ???

Answer 12

[1] allylic radical + HBr
[2] allylic bromide + Br.
[3] Br2 + succinimide

Question 13

stability of radicals

Answer 13

.CH3 < primary. < secondary. < tertiary. < allylic, benzylic

Question 14

pure Br2. vs NBS difference

Answer 14

NBS provides constant low concentration of Br2 preventing HBr reactions, safer

Question 15

allyl cation
# pi electrons
pi electron placement

Answer 15

2 pi electrons, (missing one)

pi 1 filled

Question 16

allyl radical
# pi electrons
pi electron placement

Answer 16

3 pi electrons,

pi 1 filled, pi 2 has one

Question 17

allyl anion
# pi electrons
pi electron placement

Answer 17

4 pi electrons

pi 1 filled, pi 2 filled (extra electron)

Question 18

allylic ______ ( ____ of orbitals) in ______ transition state ___the energy ___ the rate

Answer 18

delocalization (orverlaping) in SN2

lowers, increases

Question 19

alkene and alkyne with _____ + conjugated dienes –> ______

Answer 19

electron-withdrawing groups

six membered rings

Question 20

Diene + dienophile = ????

Answer 20

diels-alder product

Question 21

What is the DIels-Alder reaction also called?

What happens in the reaction?

Answer 21

[4,2] cycloaddition

pi electrons shift forming 2 new bonds from a double or triple bond

Question 22

Controls of Diels-Alder reaction:
1
2
3

Answer 22

1. the diene must be in s-cis conformation
2. dienophile in cis produces cis (meso), in trans produces trans (racemic)
3. secondary overlap favors products to have electron-withdrawing groups in endo position (down)