Chapter 10 Allylic Systems

Question 1

allylic carbon

Answer 1

-carbon attached to another carbon=carbon bond

Question 2

conjugated diene

Answer 2

• carbon attached to an allylic carbon on either side
• all C are sp2 hybridized
• ex. H2C=CH-CH=CH2

Question 3

allylic carbocation

Answer 3

• carbocation in which allylic carbon bears a positive charge
• 2 electrons, 3 p orbitals

Question 4

stability of allylic carbocation

Answer 4

• more stable than regular carbocations

- the = bond and the - bond are the same length because the electrons are shared by all 3 carbons involved

Question 5

SN1 rates of allylic halides

Answer 5

-100 times more reactive than nonallylic halides

Question 6

allylic resonance

Answer 6

-positive charge switches between tertiary carbon and primary carbon

Question 7

allylic rearrangement

Answer 7

-reactions of allylic systems that yield products in which = bond migration has occurred

Question 8

SN1 reactions of allylic halides

Answer 8

-uses H2O and Na2CO3 to form 2 alcohols but the tertiary alcohol is the major product

Question 9

SN2 reactions

Answer 9

• primary allylic halide + strong base

- presence of double bond has nothing to do with the SN2 characteristics

Question 10

allylic halides

Answer 10

• are good for SN1 and SN2 reactions
  
  • allylic halide + strong base –> SN2
  • allylic halide + H20 –> SN1

Question 11

allylic free radicals

Answer 11

• free radical where allylic carbon bears an unpaired electron
• 3 electrons, 3 p orbitals
• C1 and C3 share the electron, C2 does not

Question 12

allylic anions

Answer 12

-4 electrons, 3 p orbitals

Question 13

isolated diene

Answer 13

• tow C-C = bond units are separated from each other by one or more sp3 hybridized carbon
• ex. H2C=CHCH2CH=CH2

Question 14

cumulated diene

Answer 14

• one C atom is common with two C-C = bonds

- ex. H2C=C=CH2

Question 15

for allylic halogenation to be effective

Answer 15

• all allylic hydrogens of starting alkene must be equivalent
• both resonance forms of allylic radical must be equivalent

Question 16

alkatriene and alkatetraenes

Answer 16

-compounds with three or four doubles bonds, respectively

Question 17

stability of dienes

Answer 17

• conjugated dienes are more stable than isolated dienes by 15 kJ/mol
• conjugated>isolated>cumulated

Question 18

1,2 addition (direct addition)

Answer 18

• hydrogen is added to C-1 and halogen is added to C-2 of the conjugated diene
• produces a secondary halide
• major product at extreme cold temps (-80C)
• monosubstituted

Question 19

1,4 addition (conjugate addition)

Answer 19

• hydrogen is added to C-4 and halogen is added to C-1
• produces a primary halide
• major product at room temp
• disubstituted

Question 20

carbocation stability

Answer 20

• determines the rate of the reaction

- more stable carbocation results in a faster reaction (1,2 addition)

Question 21

major product

Answer 21

-results in the more substituted alkene (1,4 addition)

Question 22

Diels-Alder reaction

Answer 22

• the conjugate addition of an alkene to a diene

- super important because it forms 2 C-C bonds at the same time

Question 23

s-trans diene vs s-cis diene

Answer 23

-trans diene is more stable than cis due to the van der waals strain of cis

Question 24

requirements for Diels-Alder reaction

Answer 24

• most reactive dienophiles contain a substituent such as carbonyl (C=O) or cyano (-C~N)
• diene must be able to adopt s-cis comformation

Question 25

Diels-Alder products

Answer 25

• the product always has one more ring than the reactants

- ex. if reactants have two rings, the product has 3

Question 26

Diels-Alder reaction is stereospecific

Answer 26

• substituents that are cis in the dienophile remain cis in the product
• same is true from trans

Question 27

endo

Answer 27

• inside the basket

- in the axial position off of chair conformation

Question 28

exo

Answer 28

• outside the basket

- in equatorial position off of the chair conformation

Question 29

Alder Rule

Answer 29

-groups are endo in respect to the diene

Question 30

HOMO

Answer 30

• highest occupied molecular orbital

- when electrons are transferred from a molecule, they are the electrons in the HOMO

Question 31

LUMO

Answer 31

• lowest unoccupied molecular orbital

- orbital in which electrons go into when they are transferred to a molecule

Question 32

the more nodes an orbital has

Answer 32

the higher its energy

Question 33

node

Answer 33

-a region where there is no electron density

Question 34

Diels-Alder reaction and molecular orbitals

Answer 34

-interaction between HOMO of diene and LUMO of dienophile (ex. ethylene H2C=CH2)

Question 35

Diels-Alder reaction and molecular orbitals 2

Answer 35

-if symmetry (nodes of reactants) match up the reaction will move forward

Question 36

allenes

Answer 36

-stereoisomers that are possible are a pair of enantiomers