CS5, page 1
13Mar2008
Polar Addition to carbon-carbon multiple bonds
I. electrophilic addition
A. AdE2: AE + AN bimolecular stepwise addition
1.
I1
I2
2. addition maybe syn or anti depending on conditions or reagent
a. bridging electrophiles will force anti addition
b. ion pair formation favors syn addition, show on board
c. free ions give mixture (stablized carbocation or polar solvents)
for example, halogenation:
anti for all alkyl (I2 favored) and less stereospecific for aromatic substitutents
(I1 favored)
less stereospecific in polar solvents (I1 favored)
d. for proton = E+, addition is rate determining and H does not bridge
3. favored by strong electrophiles (CF3SO3H in HOAc)
4. rearrangement competes if cations are unbridged (I1)
B. AdE3: AEAN, termolecular concerted addition, reverse of E2
E+
E
-
Nu
Nu
break  bond, make 2  bonds
2. addition is anti
3. mechanism favored when solvent participates (HBr in HOAc)
CS5, page 2
13Mar2008
C. Markovnikov's rule: what is it?
D. Bromine kinetics: three different mechanisms (three terms!)
rate = k1[alkene][Br2] + k3[alkene][Br2][Br-] + k2[alkene][Br2]2
Br
Br
Br
Br
Br
Br2 +
Br
Br
Br
Br
Br2 +
Br
Br
Br
Br3
Br Br2
Br
Br
Br
Br2 +
Br2
Br
E. oxymercuration: Nu = water
X
X
Hg
HgX+ +
Nu-
Hg
Nu
1. less electronegative than bromine cation, less sensitive to alkyl inductive
effect
2. steric effects more important, attack of terminal is faster than internal alkenes
3. anti and Markovnikov addition is favored
II. alkyne addition
A. AdE3 can occur, addition is anti
B. AdE2 generates vinyl cation and non-stereospecific, favored by strong accid
H
H+
R C C H
R C C
H
C. Disubstituted alkynes are bridged by Cl, monosubstituted form vinyl cation
CS5, page 3
Cl
13Mar2008
Cl
C C
R
R
C C
R
H
vinyl is resonance stabilized by Cl needed with H
D. but not 2 R
E. Bromination is anti for alkyl substituted and non-stereospecific for aromatic, why?
F. addition of bromide increases anti addition suggesting bromine molecular
complex is formed initially (probably in all alkyne brominations)
Br
R
Br
Br
C C
C C
R
R
Br
Ph
Br
C C
R
Br
Ph
R
Br
C C
H
Ph
Br
C C
III. allene addition
A. HX addition, proton addition to center carbon is preferred: conjugated allylic
cation (900 out-of-plane) is less stable than vinyl cation
H+
H2C C
CH2
H2C C
CH3
H2C C
CH2
H
B. Nucleophiles attacks at 1 position in bromination and oxymercuration
Br
H2C C
Br
CH2
H2C C
CH2Nu
-
Nu
Elimination
I. loss of two groups
A.  - on same atom
B.  - on adjacent atoms
II. extrusion reactions are related to  elimination, i.e. ethylene sulfone (see below)
III. bimolecular elimination, E2 mechanism
A. AxeDEDN show figure
R
CS5, page 4
13Mar2008
B. concerted, single step reaction, break two bonds make  bond
C. second-order kinetics, first order in base and in substrate
D. hydrogen isotope effect is 3-8 indicating loss of H in RDS
E. loss of E and N are anti or syn
Br
Br
periplanar
1. syn is an eclipsed conformation
H
H
so anti preferred
Base
Br
Ph
Br
2. periplanar elimination allows
maximum overlap of newly formed
Ph
Ph
Ph
H
Ph
Br
Ph
H
Ph
H
H
Ph
Base
Br
orbitals
3. in cyclic systems such as cyclohexane, only diaxial substituents are periplanar
4. anti-periplanar elimination is also preferred for formation of alkynes
HO2C
H
Cl
50x faster
CO2H
HO2C C C CO2H
HO2C
H
CO2H
Cl
F. steric interactions and ion pairing can favor syn elimination
benzene
syn:anti = 55:1
H
H
O
Br
K
-
CS5, page 5
13Mar2008
G. follow Zaitsev's rule (most substituted double bond formed) if leaving group is
negatively charged, Hofmann's rule if neutral (NR3+ or SR2+) (least substituted double
bond formed) why?
IV. E1, unimolecular mechanism
A. DN + DEAxe
B. Leaving group leaves first, then base, usually the solvent removes proton
C. first-order kinetics
D. carbocation leads to competing reactions, SN1 and rearrangements
E. loss of stereochemistry unless
ion pairing favors formation of olefin before internal rotations can occurs
H
Cl
CH3
sol
CH2
CH3
CH3
CH2
CH3
CH3
CH3
CH3
F. Zaitzev's rule is followed if not sterically hindered
V. E1cb, carbanion unimolecular elimination
A. base removes proton first
B. favored by acidic hydrogens  to poor leaving group loose L
C. AxeDE + DN
Z
Z
H
L
base
Z
L
L
CS5, page 6
13Mar2008
D. mechanism varies depending on relative rates of second step and reverse of first
step
E. carbanion prefers Z to be a stabilitizing groups: RCO, CN, NO2, SO2R
VI. E2C rare
A. nucleophile attacks carbon but leaves with proton
Nu
H
B. occurs with poor bases but good nucleophiles
L
C. nucleophile stabilizes partial charge on carbon
D. rates: tertiary > secondary > primary
VII. timing of E and L loss
base
A. L first: carbocation forms, E1
base

H
H
L
base
B. E and L at nearly same time: E2
C. E first: carbanion forms, E1cb

L
H


L
D. E2 favored by polar aprotic solvents: Zaitsev or
Hofmann products depends on conditions and
reactants
F. strong base favors carbanion formation Elcb
G. positive leaving group or very electronwithdrawing groups favor Elcb
H. weak base favors E1, strong base E2 or E1cb
I. ionizing solvents favor E1 and E1cb
J. eliminations often occur from most stable conformations of reactants and
intermediates allowed by the mechanism
1. kinetic products are usually obtained
2. gauche or eclipsing interactions are minimized
CS5, page 7
13Mar2008
VIII. pyrolytic eliminations
A. high temperature >200 C yields syn elimination
B. unimolecular, appear to be
H
CO2Et
CO2Et
concerted, probably intimate
H
H
ion chemistry
O
H
O
C. concerted elimination of
O
O
HX violates symmetry
conservation rules?
IX. elimination without H
A. organometallics with  hydroxy or alkoxy groups eliminate via a bridged
intermediate (M = Hg, Sn, Si) (electrophile catalysis)
M
M
M
HOR
OR
M
OR
H
B. nucleophile attack on -halosilanes also generates alkenes
-
Nu
Nu = F or OR
(CH3)3Si
anti
(CH3)3SiNu
+
Br
C. alkene substitution occurs via addition-elimination
Si(CH3)3
I2
I
I
Si(CH3)3
I
ISi(CH3)3
Si(CH3)3
I
CS5, page 8
13Mar2008
X. selected reactions
A. pyrolysis of esters, >300 C
B. p-toluene sulfonylhydrozones (show resonance forms of dianion)
-
H2O
2 MeLi
N
N
NH
Ts
N
N-
N
Ts-
N2
Ts
C. cleavage of cyclic thionocarbonates 7-22 (alternate mechanism with S
nucleophile?
OEt
OEt
EtO P OEt
EtO P OEt
OEt
EtO P OEt
S
S
S
O
O C O
O
O
O
O
O
D. epoxide conversion to olefin
+
O-
O
Ph3P
-
Ph3P
O
-
O
PPh3
PPh3
E. 7-25, Ramberg-Bäcklund
H
R
R
S Cl
O
O
OH-
R
R
R
S Cl
O
O
R
R CH CH R
S
O
O
O
S O
G. alkene addition: HI, HBr, HCl, HF, H2SO4, H2O, ROH, RSH, RCOOH by electrophilic
mechanism
H. hydrocarboxylation
+
+
CO
H+,CO
H2O
I. epoxidation
H
CO
COOH2
H2O
H
COOH
+
H
-H
H
CS5, page 9
13Mar2008
HO
O
H
O
O
O
O