Which of the following is the most stable radical?
Rank the following alkyl radicals in order of increasing stability (least
< < <most).
1<3<2<4
What is the product of the following reaction?
Predict the product of the following reaction.
1-methylcyclohexene + HBr/H2O2 ?
Br
Identify the reagent(s) that will promote the following
transformation.
A) HBr, tetrahydrofuran
 B) HBr, peroxides
C) Br2, CCl4
D) Br2, H2O
•Conjugation occurs whenever p orbitals can overlap on
three or more adjacent atoms.
•The four p orbitals on adjacent atoms make a 1,3-diene a
conjugated system.
•Having three or more p orbitals on adjacent atoms allows p orbitals
to overlap and electrons to delocalize.
•1,4-Pentadiene is an isolated diene.
•The  bonds in 1,4-pentadiene are too far apart to be conjugated.
Which of these compounds have delocalized electron density?


•The allyl carbocation is another example of a conjugated
system.
•Conjugation stabilizes the allyl carbocation.
•Drawing resonance structures for the allyl carbocation is a
way to see how to use Lewis structures to illustrate how
conjugation delocalizes electrons.
•The true allyl cation is a hybrid of the two resonance
forms.
•In the hybrid, the positive charge is delocalized over the
two terminal carbons.
•Delocalizing electron density lowers the energy of the
hybrid, thus stabilizing the allyl carbocation and making it
more stable than a normal 1° carbocation.
What are the resonance structures of these compounds?
+
+
+
+
+
+
•Experimental data show that the stability of the allyl cation
is comparable to a more highly substituted 2° carbocation.
Common Examples of Resonance
1. The Three Atom “Allyl” System, X=Y-Z*
Examples are the allyl cation and the acetate anion. The two
resonance structures differ in the location of the double bond,
and either the charge, the radical, or the lone pair, generalized by
[*].
2. Conjugated Double Bonds
•Cyclic completely conjugated rings like benzene have two
resonance structures, drawn by moving the electrons in a cyclic
manner around the ring.
•Three resonance structures can be drawn for other conjugated
dienes, two of which involve separation of charge.
3. Cations Having a Positive Charge Adjacent to a Lone
Pair
The overall charge is the same in both resonance structures.
Based on formal charge, a neutral X in one structure must bear
a (+) charge in the other.
4. Double Bonds Having One Atom More Electronegative
Than the Other
Draw the resonance structures?
-
-
-
+
+
+
The resonance hybrid more closely resembles the major
contributor
Draw the other resonance structure and predict which is more
stable.
H3C
+
C
NH2
H3C
CH3
+
+
C
NH2
H3C
CH3
CH3
O
-
O
O
H3C
NH
H3C
NH
C
H3C
NH
NH2
Draw the three resonance structures of CH3CO2H and rank their
stability.
-
O
O
H3C
OH
H3C
O
OH
H3C
1
3
-
-
O
O
H3C
+
+
OH
+
H3C
OH
2
OH
Electron Delocalization, Hybridization, and Geometry
Consider the two Lewis structures (A and B) for the resonance
stabilized anion (CH3COCH2)¯.
•Based on structure A, the indicated carbon is sp3
hybridized, with the lone pair of electrons in an sp3
hybrid orbital.
•Based on structure B, however, it is sp2 hybridized with
the unhybridized p orbital forming the  portion of the
double bond.
•The electron pair on the carbon atom adjacent to the C=O
can only be delocalized if it has a p orbital that can overlap
with two other p orbitals on two adjacent atoms. Thus, the
terminal carbon atom is sp2 hybridized with trigonal planar
geometry.
•Three adjacent p orbitals make the anion conjugated.
Determien the hybridization of the indicated atoms.
O

O
-
 O
CH

All three are sp2 hybridized.
Conjugated Dienes
•Conjugated dienes are compounds having two double
bonds joined by one  bond.
•Conjugated dienes are also called 1,3-dienes.
•1,3-Butadiene
(CH2=CH-CH=CH2)
is
the
simplest
conjugated diene.
•Three stereoisomers are possible for 1,3-dienes with alkyl
groups bonded to each end carbon of the diene.
•Two possible conformations result from rotation around the
C—C bond that joins the two double bonds.
•Note that stereoisomers are discrete molecules, whereas
conformations interconvert.
Draw the three possible stereoisomers of 2,4-octadiene. Pick which
one is (2E,4E) 2,4-octadiene.

Draw the s-cis and s-trans conformations of (3Z,5Z)-4,5-dimethyl
-3,5-octadiene
s-trans
s-cis
The Carbon—Carbon  Bond Length in 1,3-Butadiene
Four features distinguish conjugated dienes from isolated
dienes.
1. The C—C single bond joining the two double bonds is unusually
short.
2. Conjugated dienes are more stable than similar isolated dienes.
3. Some reactions of conjugated dienes are different than reactions
of isolated double bonds.
4. Conjugated dienes absorb longer wavelengths of ultraviolet
light.
The Carbon—Carbon  Bond Length in 1,3Butadiene
The observed bond distances can be explained by looking
at hybridization.
A resonance argument can also be used to explain the
shorter C—C  bond length in 1,3-butadiene.
•Based on resonance, the central C—C bond in 1,3butadiene is shorter because it has partial double bond
character.
•Finally, 1,3-butadiene is a conjugated molecule with four
overlapping p orbitals on adjacent atoms.
•Consequently, the  electrons are not localized between the
carbon atoms of the double bonds, but rather delocalized
over four atoms.
•This places more electron density between the central two
carbon atoms of 1,3-butadiene than would normally be
present.
•This shortens the bond.
Using hybridization, compare the C-C bonds of the following three
compounds.
H3C
CH3
sp3
25% s character
H2C
CH2
HC
CH
sp2
sp
33% s character
50% s character
Using resonance, why are the two C—O bonds the same length?
O
-
H3C
-
O
O
O
-
H3C
O
H3C
O
The two resonance structures show how the electron density is
delocalized over 3 atoms.