Chapter 10 Allylic Systems Flashcards
allylic carbon
-carbon attached to another carbon=carbon bond
conjugated diene
- carbon attached to an allylic carbon on either side
- all C are sp2 hybridized
- ex. H2C=CH-CH=CH2
allylic carbocation
- carbocation in which allylic carbon bears a positive charge
- 2 electrons, 3 p orbitals
stability of allylic carbocation
- more stable than regular carbocations
- the = bond and the - bond are the same length because the electrons are shared by all 3 carbons involved
SN1 rates of allylic halides
-100 times more reactive than nonallylic halides
allylic resonance
-positive charge switches between tertiary carbon and primary carbon
allylic rearrangement
-reactions of allylic systems that yield products in which = bond migration has occurred
SN1 reactions of allylic halides
-uses H2O and Na2CO3 to form 2 alcohols but the tertiary alcohol is the major product
SN2 reactions
- primary allylic halide + strong base
- presence of double bond has nothing to do with the SN2 characteristics
allylic halides
- are good for SN1 and SN2 reactions
- allylic halide + strong base –> SN2
- allylic halide + H20 –> SN1
allylic free radicals
- free radical where allylic carbon bears an unpaired electron
- 3 electrons, 3 p orbitals
- C1 and C3 share the electron, C2 does not
allylic anions
-4 electrons, 3 p orbitals
isolated diene
- tow C-C = bond units are separated from each other by one or more sp3 hybridized carbon
- ex. H2C=CHCH2CH=CH2
cumulated diene
- one C atom is common with two C-C = bonds
- ex. H2C=C=CH2
for allylic halogenation to be effective
- all allylic hydrogens of starting alkene must be equivalent
- both resonance forms of allylic radical must be equivalent
alkatriene and alkatetraenes
-compounds with three or four doubles bonds, respectively
stability of dienes
- conjugated dienes are more stable than isolated dienes by 15 kJ/mol
- conjugated>isolated>cumulated
1,2 addition (direct addition)
- 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
1,4 addition (conjugate addition)
- hydrogen is added to C-4 and halogen is added to C-1
- produces a primary halide
- major product at room temp
- disubstituted
carbocation stability
- determines the rate of the reaction
- more stable carbocation results in a faster reaction (1,2 addition)
major product
-results in the more substituted alkene (1,4 addition)
Diels-Alder reaction
- the conjugate addition of an alkene to a diene
- super important because it forms 2 C-C bonds at the same time
s-trans diene vs s-cis diene
-trans diene is more stable than cis due to the van der waals strain of cis
requirements for Diels-Alder reaction
- most reactive dienophiles contain a substituent such as carbonyl (C=O) or cyano (-C~N)
- diene must be able to adopt s-cis comformation
Diels-Alder products
- the product always has one more ring than the reactants
- ex. if reactants have two rings, the product has 3
Diels-Alder reaction is stereospecific
- substituents that are cis in the dienophile remain cis in the product
- same is true from trans
endo
- inside the basket
- in the axial position off of chair conformation
exo
- outside the basket
- in equatorial position off of the chair conformation
Alder Rule
-groups are endo in respect to the diene
HOMO
- highest occupied molecular orbital
- when electrons are transferred from a molecule, they are the electrons in the HOMO
LUMO
- lowest unoccupied molecular orbital
- orbital in which electrons go into when they are transferred to a molecule
the more nodes an orbital has
the higher its energy
node
-a region where there is no electron density
Diels-Alder reaction and molecular orbitals
-interaction between HOMO of diene and LUMO of dienophile (ex. ethylene H2C=CH2)
Diels-Alder reaction and molecular orbitals 2
-if symmetry (nodes of reactants) match up the reaction will move forward
allenes
-stereoisomers that are possible are a pair of enantiomers
