13: Chapter Conjugated Unsaturated Systems! Reactivity in saturated vs. unsaturated compounds!
+ A-B "-bond
C=C !-bond
C-C AB !-bonds bonds gained
bonds lost
+
+ H-X
H
:
:
C C
C
C
+ X-
carbocation
Focus on molecules that have a p-orbital on the atom adjacent to the double bond.!
H2C
CH CH3
No p-orbital on adjacent carbon!
H2C H2C trigonal planar geometry where carbon is sp2 hybridized
. H C H H electron in p orbital
CH CH2
or!
CH CH2
a planar geometry p +z C sp2
sp2 hybridization
Conjugated Unsaturated Systems have a p-orbital on the atom adjacent to a double bond and give rise to extended, delocalized bonds.!
Conjugation means a !-system extended by p-orbitals on adjacent carbons.
C=C-C
conjugated unsaturated system
C
C
C
an allylic cation, radical or anion
A !-system of three overlapping p-orbitals is called allylic.
Compounds with adjacent multiple bonds are also conjugated unsaturated systems!
H2C
H C
C H
CH2
Conjugated molecules have interesting properties and reactivity! 1. More stable than non-conjugated analogues! 2. Often are highly colored (absorb visible light)! 3. Undergo unusual reactions!
Allylic Substitution and the Allylic Radical
Propene reacts with Br2 and Cl2 by the usual electrophilic addition reaction at room temperature in the absence of ultraviolet radiation.
Electrophilic Addition CH3CH=CH2 + X2
room temp no h!
CH3CHXCH2X
Allylic Substitution
At high temperatures, and under conditions where [X2] is low, allylic substitution occurs:
Allylic Substitution CH3CH=CH2 + X2
high temp low [X2]
XCH2CH=CH2 + HX
Allylic substitution is a general reaction that proceeds by a free radical chain mechanism.
H CH C H H2C H
Allylic Hydrogens!
H CH C Cl H2C H
H CH C H2C H CH C H2C H H
R CH C H2C R CH C H2C R R
Allyl radical and cation!
Allylic radical or cation!
Allylic Chlorination of Propene
Propene and chlorine react at 400oC in the gas phase to produce 3-chloropropene (allyl chloride) and HCl:
CH3CH=CH2 + Cl2
400o C gas phase
ClCH2CH=CH2 + HCl
3-chloropropene (allyl chloride)
A Free Radical Chain Mechanism
initiation
:Cl:Cl:
h! or heat
2 :Cl .
::
:: ::
propagation steps
H H2C=CH-C H + . Cl: H
H H2C=CH-C. + :Cl:Cl: H
:: ::
H H2C=CH-C. + HCl H
allyl radical
In this free radical chain reaction, the chain carriers are the allyl radical and the chlorine radical. The two propagating steps recycle thousands of times before termination reactions scavenge the chain carriers. The energetics of allylic substitution are determined by the bonding changes in the two propagating steps.
::
H H2C=CH-C-Cl + . Cl: H
::
allyl chloride
Energetics
(1) CH2=CHCH2 H BDE -88
(kcal/mol)
+ Cl .
. CH2=CHCH2 + H-Cl
-103
!Ho = -15 kcal/mol
Cl CH2=CHCH2 + Cl .
-60
. (2) CH2=CHCH2 + Cl-Cl
-58
!Ho = -2 kcal/mol
(1) + (2) CH2=CHCH3 + Cl2
-88 -58
CH2=CHCH2Cl + HCl
-60 -103
!Ho = -17 kcal/mol
The above analysis shows that the overall reaction has a favorable enthalpy change, and each propagating step also is favorable. to According the usual relationship between the thermodynamics and kinetics of a reaction, there is not expected to be any large energy barrier that would inhibit this mechanism.
The stability of the allyl radical intermediate, as revealed by the somewhat lower BDE for the C-H bond, is a key feature of this reaction.
A Comparison of BDE Values for C-H Bonds
. CH2=CHCH2 + H.
allyl
BDE (kcal/mol) -88
-92
CH2=CHCH2-H
CH3 CH3-C-H CH3
CH3 CH3-C . CH3
t-butyl
+ H.
H CH3-C-H CH3
H CH3-C . + H. CH3
isopropyl
-95
increasing bond strength
CH3CH2CH2-H
. CH3CH2CH2 + H.
propyl
-98
CH2=CH-H
. CH2=CH + H.
vinyl
-108
Relative Stabilities of the Carbon Radicals
Since the BDE values are the standard heats evolved in forming the C-H bonds, they provide a measure of the relative stabilities of the several different types of carbon radicals. vinyl . CH2=CH increasing + H. stability primary
secondary CH3CHCH3 tertiary . + H. CH3CCH3 allyl CH3 . CH2=CHCH2 + H. + H.
.
CH3CH2CH2 + H.
.
108 98
92
95
!Ho
88
The differences in the standard enthalpies are assigned to the carbon radicals since the hydrogen atom is common to all the dissociated states.
CH2=CH2
CH2=CHCH3
(CH3)CH
CH3CH2CH3 CH3CH2CH3
Allylic Bromination with N-Bromosuccinimide (NBS): A Useful Laboratory Reagent
N-Bromosuccinimide is a derivative of succinimide:
O C OH
O O O
succinic acid anhydride
O N-H O
succinimide
O N-Br O
N-bromosuccinimide
succinic acid
=
NBS reacts with HBr to produce Br2: O O +H + :Br: N N-Br + H-Br Br
O
NBS
=
C OH O
(NBS)
O N-H + Br-Br O
succinimide
::
:
O
Allylic Bromination
The bromination of the allylic position is carried out in the presence of a catalytic amount of peroxide (ROOR), or under irradiation to initiate radical production.
overall
O CH2=CH-CH3 + N-Br
h! or ROOR CCl4
O CH2=CH-CH2Br + N-H O
O
:
A Mechanism for Allylic Bromination with NBS
Initiation of the free radical chain reaction may be by any or all of the reaction sequences below:
RO-OR
h! or heat homolysis
2 RO . . CH2=CH-CH2 + RO-H
chain carrier
CH2=CH-CH3 + RO.
or
O N-Br O CH2=CH-CH3 +
h! or heat homolysis
O . N . + Br
:
chain carrier
O N.
:
O . CH2=CH-CH2 +
chain carrier
O N-H O
O
or
Br-Br
h! or heat homolysis
2 Br .
chain carrier
Propagating Steps
(1) CH2=CH-CH3
. (2) CH2=CH-CH2
chain carrier
+ Br .
chain carrier
. CH2=CH-CH2 + H-Br
chain carrier
+ Br-Br
CH2=CH-CH2Br + Br .
chain carrier
As in all free radical chain reactions, the concentrations of the chain carriers (radical intermediates) are extremely low, so radical-radical recombination reactions (terminations) only occur infrequently. The propagating steps cycle thousands of times, before terminations occur.
The H-Br produced reacts rapidly with NBS to give succinimide and Br2. Thus, while Br2 is not an explicit reagent, it is constantly being produced during the reaction. In the nonpolar solvent (CCl4), and at the low concentration of Br2, little electrophilic bromination of the alkene function occurs.