Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS
2.1 CONCEPTS OF POLAR ADDITION AND ELIMINATION REACTIONS
Addition and elimination processes are the reverse of one another. In general, the
two processes follow a similar mechanistic path in opposite directions the final state of the
system depending on the conditions. The principle of microscopic reversibility states that
the mechanism (pathway) traversed in a reversible reaction is the same for the reverse
reaction as for the forward one under the same conditions.
The following discussion will be restricted to reactions that involve polar or ionic
transition states. Several limiting general mechanisms can be described for polar additions:
(A) E—Y E + + Y -
E + + C=C C—C + C—C
(B) E—Y + C=C C—C + C—C
E
E
(C) 2E—Y + C=C C=C —C—C— + E—Y
Y-E
Y-E
Y -
Y -
E
E
Y
E Y
Y
2.2 ADDITION OF HYDROGEN HALIDES TO ALKENES
One aspect of the mechanism to be established was its regioselectivity, that is, the
direction of addition. A reaction is described as regioselective of one of the two possible
addition products; the term regiospecific is used if only one possible product is formed.
The types of studies that have been applied to determining the mechanistic details
concerning addition of hydrogen halides include kinetics, stereochemistry, and the effect of
added nucleophiles.
Addition of HBr or HCl to alkenes usually give third-order process:
Rate = k [alkene][HX] 2
H-X
H
C=C H + + C—C— + X -
H-X
X
H-X
fast
slow
H
or C=C + H—X C=C
HX
C—C + HX
X
H—X
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 1

The stereochemistry of addition of HX to unconjugated alkenes is predominantly
anti. Temperature and solvent can modify the stereochemistry, i.e. syn at lower
temperature.
A significant variation in the stereochemistry takes place when the double bond is
conjugated with a group than can stabilize a carbocation intermediate.
+ X- syn
X
ArCH=CHR + HX ArCH—CHR ARCH-CH 2 R
H
Rate is second order
Ion pair
Show salt effect
+ HCl the added Cl - will increase the rate
+ HBr the added Br - will increase the rate
Skeletal rearrangements have been observed.
HCl
CH 3 NO 2
+
40% Cl
Cl
60%
HCl
CH 3 NO 2
Cl
+
Cl
17 % 83%
D
DBr
D
Br
+
Br
DCl
57%
D
Cl
D
+ +
Cl
Cl
41%
2%
D
2.3 ACID-CATALYZED HYDRATION OF ALKENES
H +
R—CH=CH 2 R—CH—CH 3
H 2 O
OH
Several points have to be considered :
-Is the protonation step reversible?
-Is there a discrete carbocation?
-Does the nucleophile become involved before carbocation formation is complete?
-Is there any rearrangment taking place?
For simple alkene, it is not easy to study mechanistically.There are two reasons for
the difficulty:
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 2

(1) Simple alkenes and water are not mutually soluble, except in very concentrated
acids.
(2) The spectral features of simple alkenes do not allow for very convenient
methods of following reaction rates, absorption in UV below 200 nm.
For this reason, most studies have been done with conjugated alkenes
particularly styrenes.
slow
H + + -H +
PhCH=CD 2 PhCHCD 2 H PhCH=CHD
H 2 O , fast
PhCH-CD 2 H
OH
This mechanism is correspond well with, i.e. rate to reaction increases with
electron-releasing substituents. A substantial solvent isotope effect k H2O /k D2O = 2-4 is
observed.
2.3 ADDITION OF HALOGENS
C=C + X 2 —C—C—
X X
-Is the reaction concerted ?
-Is there a discrete positively charged intermediate, carbocation or a cyclic
halonium ion?
For brominations, anti addition is preferred for simple alkenes, for conjugated
alkenes with aryl groups, the extent of syn addition becomes dominant pathway.
Chlorination is not so stereoselective as bromination, but tends to follow the same pattern
in a lesser extend.
Br - Br + Br - Br
Br
syn
Br
C C C—C C C
+
anti
The existence of halonium ion
CH 3 CHCH 2 Br
SbF 5 /SO 2
CH 3 CH—CH 2
-60 o C
-
SbF 6
F
Br
+
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 3

∆∆H #
from steric repulsions
in transition state
cis
trans
Bromination transition state
resembles a three-membered ring
∆∆H
from steric repulsions in
ground state
In general, the difference in enthalpy of the transition states for bromination was
greater than the enthalpy difference of the isomeric alkenes, This finding is consistent with
a cyclic somewhat closer together than in the alkene.
The kinetics of brominations are often complex, with at least three term making
contributions under given conditions:
Rate = k 1 [alkene][Br 2 ] + k 2 [alkene][Br 2 ] 2 + k 3 [alkene][Br 2 ][Br - ]
Br 2
+Br
Br 2
Br 2
Br
Br -
Br
C=C C=C C — C C—C—
Br 2
Relative reactivity of alkenes toward halogenation
Alkene Chlorination Bromination
1.00 1.00
1.15 0.12
63 27
50 17.5
58 57
11,000 13,700
The Hammett correlation for bromination of styrenes is best with σ + substituent
constants and, gives ρ = -4.8. The rates of reactions increase with solvent polarity and
with additional substitution of electron-releasing alkyl groups at the double bond. All of
these features are in accord with an electrophilic mechanism.
Chlorination generally exhibits second-order kinetics and usually leads to expulsion
of a proton from the chloronium ion intermediate.
R
R 2 C
Cl +
R Cl
R
R
C C C C + H +
R
R 2 C
R
H
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 4

2.4 THE E2, E1, AND E1cb MECHANISMS
An elimination reaction -the expulsion of a small molecule from an organic
substrate- can be classified according to the relative placement of the carbon atoms from
which elimination occurs:
H
..
—C—X C + HX α-elim.
H
X
—C —C C=C + HX β-elim.
H
X
—C—C—C— C C + HX γ-elim.
The β-eliminations can be further subdivided by closer examination of the mechanisms
involved.
E1
B:
—C—C + — C=C + B:H +
slow
H
fast
X
X
E2
—C—C— —C — C— BH + + C=C + X -
B:
H
H
B
E1cb
B:
X -
B:H + + C C C=C + X -
Variable-transition-state theory was proposed to cope with the intermediate
mechanisms.
Increasing C-H breaking in the transition state
C
X
B-H δ+ δ-
B-H δ+ δ-
B—H δ+
B H δ+
H
E1cb
X
X
E1cb-like
X
Synchronous E2
E1-like
δ+
X δ-
E1
δ+
X δ-
The most important structural features to be considered concerning the mechanistic
types are:
(1) The nature of the leaving group
(2) The nature of the base
(3) Steric factors in the substrate
(4) Solvent effects
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 5

Br
KOBu -t
85%
Br
KOBu -t
30-40%
Br
CH 3
Cl
3COK
+
75% 25%
Ph
Br
Br
O
AcONa
Ph
Br
O
64-73%
Br
NaNH 2 /NH 3
Br
PhC≡CH 45-52%
Ph
OTs
aq. OH -
HC≡C
HC≡C-C=CH-CH 3
∆
CH
OH -
3 CH 3
98%
Ionization is favored by
- electron-releasing groups
- good leaving groups
- solvents of high ionizing strength
- stronger bases favor the E1 path over the S N 1 path
The precise nature of the transition state of the E2 mechanism is a function of the
leaving group, and the solvent. E1cb mechanism is not observed with simple alkyl halides
or sulfonates, but with functional groups that capable of stabilizing negative charge on
carbon.
2.5 ORIENTATION EFFECTS IN ELIMINATION REACTIONS
The experimental evidence that has been gathered concerning direction of
elimination from substrates reacting by the E1 mechanism indicates that the relative
stability of the product olefins is a major factor in determining direction of elimination.
H
R 2 C δ+ X δ-
+ N(CH 3 ) 3
R 2 HC + X -
A′
B′
Thermodynamically
Controled product
R 2 CHX
A (less-substituted olefin)
B (more-substituted olefin)
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 6

In the E1cb mechanism, the direction of elimination is governed by the kinetic
acidity of the available protons, which, in tern, is determined by the inductive and
resonance effects of nearby substituents and by the degree of steric hindrance to approach
of base to the proton. Preferential proton abstraction from unhindered positions leads to the
formation of less-substituted alkenes, i.e. “Hofmann rule” is followed.
E2 eliminations that proceed through transition states with high double-bond
character give mainly the more substituted olefin because the stability of the olefin is
reflected in the transition state, i.e. said to follow the “Saytzeff rule.”
Base/solvent
+ +
X
X = I MeO - /MeOH 19 63 18
Cl ″ 33 50 17
F ″ 69 21 9
OTs ″ 33 44 23
I t-BuO - /t-BuOH 78 15 7
Cl ″ 91 5 4
F ″ 97 1 1
OTs ″ 83 4 14
Poorer leaving groups favor Hofmann products. Stronger bases favor formation of
the less-substituted olefins. Highly hindered bases favor Hofmann products.
2.6 STEREOCHEMISTRY OF E2 ELIMINATION REACTIONS
E2 elimination reaction may proceed via syn or anti mode:
B:
B:
H X
H
syn
anti
C C C=C C C
X
In most cases, E2 elimination proceed via anti mode. Syn elimination is possible,
and becomes dominant mode when special structural features retard anti elimination. In
general trend revealed that anti mode is normally preferred for reactions involving good
leaving groups such as bromide and tosylate, With poorer leaving groups, eg. fluoride,
trimethylamine syn mode becomes important.
D anti
D
H
syn
+ NMe 3
H
H
Ion pair promotes syn elimination of anionic leaving group the syn mode will be
diminished.
H
RO -
C C
H +
X
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 7

The proportion of cis-and trans-isomer of internal olefins formed during
elimination reactions depends on the identity of the leaving group. Halides usually give
predominautly the trans-olefins. Bulkier groups, particularly arenesulfonates, give higher
proportions of the cis-olefin. The normal preference for trans-olefin probably reflects the
greater stability of the trans-olefin; i.e. the unfavorable steric repulsions present in the cistrans
are also present in the E2 transition state leading to cis-olefin. High cis:trans ratio are
attributed to a second steric effect that become important only when the leaving group is
large.
H
R
B:
O
H
O
S-Ar
O
R
H
trans
R
O
S-Ar
O
H
This is favored when the leaving group and base are large
because it permits the bulky base and leaving groups to occupy
positions removed from the alkyl substituents
R
B:
O
H
H
cis
2.7 DEHYDRATION OF ALCOHOLS
The dehydration of alcohols take place under acidic conditions involving and E1
mechanism.
slow
H + -H 2 O
-H +
—C—C— —C—C— —C—C— C=C
+
HO H
H 2 O + H H
- Orientation follows the Saytzeff rule
- Relative stability: 3° > 2° > 1° carbocations
- Rearrangement is found
- Exchange of the hydroxyl group with the solvent (S N 1)
- Shows isotope effect at β-deuterated alcohols.
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 8