Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 1 of 16
Electrophilic Addition Reactions of alkenes and alkynes
•
In Chapter 8 we learned about elimination reactions, which produce unsaturated compounds
from suitably-substituted saturated compounds.
•
There were two mechanisms of elimination reaction, E1 and E2 . What was the difference?
•
E1 reactions are known to be equilibria processes. How do we know this?
•
Now consider what happens if we reverse the E1 mechanistic steps:
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 2 of 16
•
The reverse process (usually written left-to-right, of course) is called electrophilic addition.
•
Why? Step 1 of such reactions involves the interaction of π-electron density of alkene or alkyne
(i.e. acting as nucleophile) with an electron-deficient component of the addend molecule (i.e. the
electrophile)
•
Note this is a confusing terminology, because the addition of, for example, H2 O across a double
bond involves the addition of both H+ (electrophile) and – OH (nucleophile) to the alkene. The
name comes only from the first step of the mechanism.
Cyclohexene is used to illustrate 5 key types of electrophilic addition reactions
Chemistry 2600 Lecture Notes
#1
Chapter 9 Electrophilic Addition
Page 3 of 16
Hydration of alkenes is a useful preparation of alcohols
Exercise 9.4
Propose a reasonable mechanism for the following transformation:
H3C
CH3
H2O
H3C
CH3
HC
H 3C
•
•
•
CH3
H3PO 4
H3C HO CH3
Write one transformation per step and include all lone pairs and charges
Watch the charge flow carefully (use the arrow formalism)
H2 O is a better nucleophile than the dihydrogenphosphate ion because of the diffuse charge and
large size of the latter
Chemistry 2600 Lecture Notes
#2
•
Chapter 9 Electrophilic Addition
Page 4 of 16
Addition of HX to alkenes
Addition of HX to alkenes is highly regioselective, and therefore useful.
H
H
1
δ+
H
H
δ-
X
+
+
X-
H
H
H
H
H
2
H
X-
+
H
H
X
•
However, this reaction is extremely slow for terminal and di-substituted alkenes. (The rate is
reasonable for tri- and tetrasubstituted alkenes, which are quite rare). The reaction must be done
in a non-nucleophilic solvent such as CH2 Cl2 or pure (glacial) acetic acid, else the water will act
as the nucleophile.
•
But, on a silica gel suspension, the reaction works well. Effectively the silica (or alumina)
surface acts as a catalyst to speed up this reaction. Typical solvents for such reactions are
CH2 Cl2 .
•
HX-derivatized silica or alumina surfaces can be prepared by reactions such as:
SOCl2
OCBr2
PI 3
+ SiO2 ///{H 2O} →
+
Al2 O3 ///{H 2 O} →
SiO2 ///{HCl} +
SO2
Al2 O3 ///{HBr} + CO2
+ SiO2 ///{H 2 O} → SiO2 ///{HI } +
P(OH )3
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 5 of 16
Addition of Cl2 or Br 2 are reactions where H+ is not the electrophile
#3
•
Cannot operate through a conventional carbocation intermediate:
•
The correct stereochemistry is exclusively trans addition:
•
The mechanism for this reaction recognizes that elemental halogens are very polarizable (no less
than 6 lone pairs for two atoms. Nucleophiles can therefore induce the presence of a positive,
electrophilic, end to the molecule. It is the addition of the unusual electrophiles Br+ or Cl+ that is
unique to this reaction, and that is used in a mechanism that explains the observed
stereochemistry of addition.
•
Long controversial, this mechanism has been substantiated by the crystallization of the following
bromonium ion and its structure determination by X-ray crystallography (Brown, UofA).
Chemistry 2600 Lecture Notes
#4
Chapter 9 Electrophilic Addition
Page 6 of 16
Halohydrin addition occurs when Br 2 and Cl2 react in aqueous solution
•
In aqueous solution, the halogens are in equilibrium with HOX and HX. This is known as a
disproportionation reaction, leading to compounds of halogen in the +1 formal oxidation state.
X2
pHControl
ˆˆˆˆˆˆ
†
+ H 2O ‡ˆˆˆˆˆ
ˆ HOX
+ H+
+
X−
•
This equilibrium is strongly pH dependent, with basic conditions favouring products. Why?
•
In neutral solution, these redox equilibria result in the following:
Cl2
Total solubility
0.091 M
Concentration of X2
0.061 M
Concentration of HOX
0.030 M
Br2
0.21 M
~0.21 M
1.15 × 10–3 M
•
The most electrophilic species present is therefore the halogen in HOX which are in a formal
positive oxidation state. The result is stereospecific trans addition of HOX across the double
bond. For Cl2 :
•
A mechanism for halohydrin addition is the following. It is not important whether the reagent is
written as X2 /H2 O or as HOX.
•
Why is the Br– that is formed in the halogen/water equilibrium not important in this reaction?
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 7 of 16
Iodine can react under HOX conditions
•
It is most commonly used with internal nucleophiles. A classic example is the iodolactonization
reaction:
H
O I
O
#5
I
I
I
O
H
I
H
+
O
O
O
O
O
Oxymercuration has a metal ion as the primary electrophile
•
The primary reaction is usually followed by a reduction using sodium borohydride, which we
learn about in subsequent chapters. In most cases the organomercurial is undesired as a final
product because of the toxicity of mercury.
•
The net reaction is that of the addition of water or alcohol across the double bond. Why would
we bother to do this reaction when #1, the acid catalyzed addition of water, will do the same
thing more cheaply and with far less waste of reagents (mercury is expensive to dispose of)?
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 8 of 16
Addition to acyclic alkenes occurs just as for cyclohexene
•
The products depend on the stereochemistry of the alkene that is reacted:
•
Why are two stereoisomers formed in this reaction?
•
The opposite geometric isomer leads to a meso form.
•
When would this not occur?
•
Because there is no ring for reference, we cannot speaks directly of cis or trans addition, but the
stereochemistry of the products is consistent only with trans addition.
The stereochemistry of the products is described as either threo or erythro
•
Definition of threo and erythro (based on two four-carbon sugars) in Fischer diagram form:
CHO
CHO
H
OH
HO
H
OH
H
CH2OH
OH
CH2 OH
D-Erythrose
erythro has all three groups
of equal type “mirrored”
H
D-Threose
threo has only one equivalent
group “mirrored”
•
“Like” groups here are e.g.
C of a given oxidation level
H
any heteroatom
•
For example:
CH3 and C2 H4
CHO and C(O)CH3
H and H
Br and OH
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 9 of 16
Addition of Bromine to disubstituted alkenes (both symmetric and asymmetric cases)
+
Br
cis alkene
H
H
CH3
CH3CH2
Br2
Step 1: bromonium ion
H
CH3
H
CH3CH2
+
Br
H
H
CH3
CH 3CH2
Br
H
CH 3CH2
H
S S
Br
CH3
Br
+
Br
Br
H
H
CH3
CH 3CH2
H
H
CH 3CH2
Step 2: equal chance of adding
to the left-hand carbon...
CH3
R R
or to the right-hand carbon,
resulting in the creation
of two enantiomers
Br
Br
+
Br
trans alkene
H
CH3
H
CH3CH2
Br2
H
CH3
H
CH3CH2
Step 1: bromonium ion
+
Br
H
CH3
H
CH 3CH2
+
Br
CH3
H
CH3
H
H
CH 3CH2
Step 2: equal chance of adding
to the left-hand carbon...
H
Br
Br
CH 3CH2
CH3
S R
Br
H
Br
H
CH 3CH2
R S
Br
or to the right-hand carbon,
resulting in the creation
of two enantiomers
Br
Stereochemical consequences
The products of addition to cis and trans alkenes are diastereomers of one another (any combination of
the red-boxed and blue-boxed isomers, look back at Figure 5.5, p.183 of Sorrell).
For the trans alkene, if the alkene were symmetric (i.e. two CH3 groups rather than one CH3 and one C2H 5)
then the two "enantiomers" would be identical, i.e. we would form the meso form.
The two enantiomers of addition to the cis isomer are said to be the threo isomers, while...
The two enantiomers (or else single meso form) of addition to the trans form are said to be the erythro isomer(s).
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Addition to unsymmetrical alkenes: regiochemistry
•
Regiochemistry refers to the direction of addition. Regioisomers are constitutional isomers.
•
Consider addition of HX to illustrate the regiochemical selection:
Page 10 of 16
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 11 of 16
An energy level diagram of the two competing pathways for the addition of HBr to methylcyclohexane
•
The regiochemistry of addition to a double bond is given by Markovnikov’s Rule
•
The rule states that whenever a polar reagent HY adds to a double bond, the proton H+ becomes
attached to the alkene carbon atom that bore more hydrogen atoms before addition.
•
The rule is better understood via the mechanistic picture developed above. The electrophilic part of
a polar reagent adds to a π-bond in such a way that the more stable carbocation intermediate is
generated. This last formulation also applies in a slightly modified form when cyclic πbond/electrophile intermediates form (#3, #4 and #5 type reactions).
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 12 of 16
Electrophilic addition to alkynes: #1 Halogen Halides
Electrophilic addition to alkynes: #2 Acid-catalyzed addition of water
•
Keto-enol tautomerism occurs for vinyl alcohols (adjacent double bond and OH group)
H O
H
O
H
O
H
H2O
H
H 3C
H
H3C
H
H3C
H
H OH
•
Why does this occur? The answer can be traced to the bond energies of the initial and the final
product. The gas-phase BDE of a C=C is +602 kJ/mol, while that of a C=O is +732 kJ/mol.
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 13 of 16
Electrophilic addition to alkynes: #3 Addition of halogens
Addition to conjugated dienes: 1,2 and 1,4 addition under kinetic vs. thermodynamic control
•
A conjugated diene is any hydrocarbon in which two double bonds are separated by one single bond.
The simplest example, which we use here for illustration, is 1,3-butadiene in the addition of HBr.
•
Two different regioisomers are formed, in quantities that differ with reaction temperature
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Page 14 of 16
•
The difference between thermodynamic and kinetic control is illustrated using a reaction profile
diagram:
•
Another example of kinetic vs. thermodynamic control occurs with halogen addition to this diene
Br
Br
Br
Br Br
CH2Cl2
Br
Br
Br
Br
Br
1,2-addition (kinetic)
Br
1,4-addition (thermodynamic)
•
Note that the cyclic bromonium ion collapses because the resonance-stabilized allylic carbocation is
formed instead
•
Note also that once this occurs, free rotation about the C–C single bonds leads to loss of
stereochemistry. Neither product is stereospecific.
Chemistry 2600 Lecture Notes
Chapter 9 Electrophilic Addition
Addition Reactions of Carbocation Electrophiles with Alkenes
•
Dimerization of isobutylene (2-methylpropene)
•
Polymerization of isobutylene
Page 15 of 16
Chemistry 2600 Lecture Notes
•
Chapter 9 Electrophilic Addition
Intramolecular addition of a carbocation to a π-bond produces a ring – two examples:
Page 16 of 16