1. CH 14 Delocalized Pi Systems
The Allyl System: H2C=CH—CH3 (propene)
Effects of a neighboring double bond on a CH3 group
Weakens the C—H bond
Allyl C—H DHo = 87 kcal/mol
Primary C—H DHo = 98 kcal/mol
Secondary C—H DHo = 94.5 kcal/mol
Tertiary C—H DHo = 93 kcal/mol
SN1 solvolysis rate is increased
pKa is greatly lowered (more acidic)
pKa = 50
pKa = 40

2. Delocalization explains these observations
The three observations generate primary C., C+, and C- respectively. How can they be stabilized?
Delocalization = resonance structures spread the p-electrons over several C’s. This helps stabilize the radical, carbocation, and carbanion.
Molecular Orbital (MO) Representation of p-orbitals
H2C=CH—CH2 has each C sp2 hybridized
One unhybridized p-orbital for each carbon
C—C bonds are equivalent
Treat s-bonds as in Lewis structures

3. MO theory needed to describe the delocalized p-bond
3 p atomic orbitals must give us 3 MO’s
We get a bonding (O nodes), a nonbonding (1 node), and an antibonding (2 nodes) MO
Fill up these orbitals with the appropriate number of p-electrons
cation has 2 e-, radical has 3 e-, anion has 4 e- (B.O. = 1 for all)

4. 4) Resonance Indicates that the charge or radical is found only on terminal C’s
MO theory shows differences only in pnb e-
That orbital has a node at C2
MO theory agrees with the resonance hybrids
Allylic Reactions
Radical Allylic Halogenation
We would expect normal chlorination of propene (and we get it)
But, at low Cl2 concentrations, a radical reaction occurs for allylic groups

5. NBS Radical Bromination is the most common way to do this reaction
NBS = N-Bromosuccinimide
NBS gives a very low concentration of Br2 in CCl4 + HBr
Light (hv) or a radical initiator (RO.) needed to start the reaction
Allyl Radical resonance forms make two products possible.
Symmetric allyl groups give only one product
Assymetric allyl groups give product mixtures

6. Nucleophilic Substitution of Allylic Halides
SN1 is possible due to resonance stability of carbocation
At room temperature, major product is the less stable alkene
Kinetic Control = less stable isomer is formed faster through a lower energy transition state
Thermodynamic Control = most stable isomer is formed due to overall lower energy of products
Even though the less stable isomer is formed first under room temperature conditions (kinetic control), we can get the most stable product by heating the reaction for long periods (thermodynamic control).
H+ conditions mean the reaction is reversible
Allylic Alcohol reverts back to carbocation
Eventually, we will form the most stable product

7. f) Unstable isomer has lower Ea because its transition state is the most stable carbocation

8. SN2 Reactions of allylic halides are faster than for alkyl halides
Overlap of the p-bond with the p-orbital in the transition state stabilizes the T.S. for allylic reactants
Example:

9. Allylic Organometallic Reagents
Synthesis of Allylic Organometallic Reagents
Reaction with an Alkylithium reagent
Alkyl anion is more basic (less acidic) than allylic group
Alkyl anion deprotonates allylic group
TMEDA (like HMPA) is a good polar aprotic solvent
Normal Grignard Formation
Allylic Organometallic Reagents work as good 3C nucleophiles

10. Conjugated Dienes Naming Dienes (compounds with two double bonds)
Allenes have both double bonds on the same C. H2C=C=CH2
Two p-bonds on the same C must be perpendicular to each other
The central C is sp hybridized
Nonconjugated
Conjugated = two double bonds separated by a C—C single bond
All C’s are sp2 hybridized with a single p-orbital left
The 4 p-orbitals can overlap
H2C=CH—CH=CH2

11. Naming is similar to alkenes, except diene at the end
Stability of Dienes
1) Conjugated dienes are more stable than nonconjugated dienes: DHo
DHo = -30 kcal/mol
DHo = -60 kcal/mol
DHo = -57 kcal/mol

12. 2) Conjugated Dienes are stabilized by resonance
3) Resonance Energy = 3.5 kcal/mol

13. Overlap of p-bonds Creates Conjugation
Four p-orbitals overlap over the entire molecule
Rotational barrier around the single bond is raised. Planarity is needed for conjugation.
The s-cis molecule has its p-bonds on the same side of C—C
The s-trans molecule has its p-bonds on the opposite sides of C—C
The “s” tells us there is a single bond between the C=C’s
The s-trans molecule is 3 kcal/mol more stable due to steric interactions in the s-cis molecule

14. The M.O. Picture for butadiene
4 p-orbitals give 4 MO’s
Number of nodes increases up
4 p electrons fill p1 and p2
p-Bond Order = 2

15. Electrophilic Attack on Conjugated Dienes
Conjugated dienes are more reactive that alkenes
3-chloro-1-butene is the normal Markovnikov product (1,2 addition)
1-chloro-2-butene is the result of 1,4 Addition to a butadiene
2) It is common for dienes to give both 1,2 and 1,4 addition products