Addition Reactions of Alkenes
The majority of the reactions of alkenes that will be described 246 fall into
three basic categories:
1.
an electrophile interacts with the alkene π-cloud, activating the alkene
carbons to nucleophilic addition in a second step,
2.
syn addition reactions which are additions occurring on one side of the
alkene π-cloud, by concerted mechanisms and,
3.
oxidative cleavage reactions in which the carbon-carbon double bond is
cleaved to form di-carbonyl derivatives (next term).
• Many of these reactions have mechanisms in common, involving
Lewis acid-Lewis-base reactions.
• If you can identify the Lewis-acid, Lewis-base and lowest energy
intermediates and transition states, you can make good predictions
for reaction mechanisms and products without memorizing each
reaction.
THIS IS OUR GOAL!!!!!!
Everything You Ever Wanted to Know about Additions to Alkenes and
Then Some
H3C
H3C
E
H3C
H3C
C
CH2
Nu
Nu δ
Nu
H3C
H3C
C
CH2
δ
H3C
H3C
C
H
Nu
H
anti
Nu attacks on opposite face of E giving
anti product
δ E
E
E
CH2
E
Nu
H3C
H3C
Nu
E
H3C
H3C
H
syn
H
H
Nu
H
H3C
H3C
E
Nu
H
H
anti
Nu attacks on either face since
carbocation is planar, giving both syn
and anti products
2
So, an alkene π-cloud, acts as a Lewis base (an electron donor), it donates
electron density to a Lewis acid or electrophile (E) (e.g., a proton, a halogen
cation (halonium ion), or mercuric ion).
When the nucleophile is of low reactivity or if a very stable carbocation
intermediate can be formed, the initial complex may rearrange to form a σbond to the Lewis acid, leaving a full carbocation on the adjacent carbon.
•
In some cases the symmetrically bound complex of the Lewis acid and the
π-system is a transition state along the path to a carbocation intermediate and
in others it is an intermediate, depending upon the details of the Lewis acid
and the nucleophile.
If the complex bearing a positive charge is an intermediate, it will highly
reactive towards nucleophiles in the system (anions or water) and will
undergoes attack, to form the final addition products.
•
In the case where the Lewis acid bridges between the carbons as an
intermediate, attack is anti, i.e. from the opposite face of the molecule.
3
Detailed Look into Mechanism of HBr Addition to Alkenes
δ
Br
δ
H
Br
δ
H
Standard Free Energy
H3C
H3C
H
H
H3C
H3C
H
H
Br
H
H3C
H3C
C
CH2
Reaction Coordinate
• The alkene π-cloud, functioning as a Lewis base (an electron donor),
donates electron density to a Lewis acid (in this example, a proton).
The transition state involves bond formation between the carbon and
hydrogen AND bond breaking between the hydrogen and Br.
• A full positive charge develops on the left-hand carbon forming a planar
carbocation intermediate and the bromine develops a full negative charge.
• The carbon that gets the hydrogen begins to rehybridize from sp2 to sp3 .
• Transition states that are polar are stabilized by polar solvents that orient
their dipoles in a manner so as to stabilize any charge that builds up. This
should be obvious by now.
4
Stability of Carbocations
Alkyl groups are inductively donating, and thus carbocations are stabilized
by substitution with such groups. Thus, in simple unstrained non-conjugated
systems, without adjacent heteroatoms, the order of stability of carbocations
will be tertiary > secondary > primary.
This can be explained by the concepts of hyperconjugation wherein a C-H
σ-bond aligns with a dihedral angle of nearly 0º with the empty p-orbital and
can donate some electron density to the empty orbital thereby stabilizing it.
H
R
R'
H
H
CH3 + CH3 CH2 + (CH3 )2 CH+ (CH3 )3 C+
Gas phase stability relative to
(CH3 )3 C+ (kcal/mol),
+82.7
+44.8
+17.3
0
The difference of stability in solution will be less due to solvent
stabilization!
5
Resonance stabilization of Carbocations
Positive charge is stabilized by resonance. For example allyl cation (left) is
much more stable than propyl cation
Gas phase stability relative to
(CH3 )3 C+ (kcal/mol),
+24.1
+38.1
Likewise benzylic cations are very stable relative to alkyl cations.
R
R
R'
R
R'
R'
...etc
Gas phase stability relative to (CH3 )3 C+ (kcal/mol), where R, R’ = H, +7.1
Since tertiary centers have no attached hydrogens, secondary centers have
one and primary centers have two, there is an apparent inverse relationship
between the "number of attached hydrogens" and the likelihood that the
carbocation will form at that center.
6
Markovnikov's Rule
This is the origin of Markovnikov's Rule, which states that...
...in the addition of HX to an alkene, the proton will attach to the
center having the greatest number of hydrogens...
often restated as "them that has, gets".
• While the rule is a useful guide, you should remember that the selectivity is
actually to place the carbocation on the carbon that can best stabilize the
charge.
Once the carbocation is formed, the most favorable reaction will involve the
addition of a nucleophile to form an sp3 center.
• In the reaction with HX (HCl and HBr), the most nucleophilic species in
the system will be the chloride or the bromide anions.
Attack of these on the planar (sp2 ) carbocation can occur from either above,
or below the plane defined by the sp2 center, and the net addition of HX can
therefore occur either syn (cis; on the same side) or anti (trans; on the
opposite side), relative to the hydrogen atom.
D
Br
D
Br
H3C C CH
2
H3C
δ
Br
H
H3C
H
H3C syn
top face attack
gives syn
Br
H3C
H3C
δ D
δ
H3C
H3C C CH2
Br
7
H3C
H3C
D
H
H
Br
anti
bottom face
attack gives anti
D
H
H
Standard Free Energy
Examples of Hammond Postulate
The acid-catalyzed addition of water to alkenes, as well as the addition of HX
are examples of reactions involving carbocation intermediates
T.S. 1º
T.S. 3º
ΔG ‡
1º carbocation
ΔG ‡
3º carbocation
Br
Br
+ HBr
Reaction Coordinate
• The addition of halogen acids to alkenes is a stepwise process that
generally involves a solvent-equilibrated carbocation intermediate.
• If the alkene is asymmetrical, two carbocation intermediates can be formed
that have different energies, as for 2-methylpropene shown above..
• The carbocation that is best able to stabilize the cationic center will, be
lower in energy. Here the 3o carbocation on the left in red.
• In general, the transition state leading to the more stable intermediate will
be of lower energy and will be the preferred pathway. (the red path)
8
Rearrangement Reactions of Alkenes
Once an alkene is protonated to form the initially more stable cation (through
the lower energy transition state), it is possible that there is a kinetically
accessible cation which is even lower in energy that can be created by a
rearrangement. Such rearrangements occur typically by a 1,2-alkyl or 1,2hydride shift.
Reaction involving a 1,2-methyl shift:
+
+
H Cl
Cl
Cl
minor product
knucleophilic
nucleophilic
Cl attack
H
H3 C
attack
Cl
major product
nucleophilic
attack
k1,2 alkyl shift
CH3
OR rearrangement
secondary cation
tertiary cation
Notice that the reaction yields two products. The distribution of the products
is determined by the relative rates for trapping by the nucleophile (to give the
non-rearranged product) or by the intramolecular 1,2 alkyl shift, followed by
trapping by the nucleophile (to give the rearranged product).
9
Since the rearrangement simply involves a migration of the methyl group with
its electrons this reaction can be surprisingly fast and can compete with
trapping by the nucleophile.
The actual distribution of products will be sensitive to the details of the
reactants and the reaction conditions.
• The nucleophiliticity of the nucleophile will be important in determining
the relative rates for rearrangement and trapping.
Rearrangement will occur when the mechanism involves the formation of
carbocation intermediates and when the rearrangement yields a significantly
more stable carbocation. For example a secondary carbocation rearranges to
a tertiary or resonance stabilized carbocation as shown for the alkyl shift
earlier and the hydride shift below.
Reactions involving a 1,2-hydride shift:
+ H Cl
Cl
H
H
H
1,2 hydride shift
Rearrangement
H
+ Cl
tertiary cation
secondary cation
In general 1,2 hydride shifts can be even faster than 1,2 methyl shifts since the
hydride is much smaller.
10
Hydration of Alkenes
If an alkene is treated with a trace of acid in water as a solvent, then rather
than the conjugate base of the acid acting as a nucleophile, water will act as
the nucleophile because it is present in huge excess. Subsequent loss of a
proton (to an additional molecules water in the system) yields an alcohol and
regenerates a proton that can attack another molecule of an alkene.
In this manner, the proton is not consumed in the reaction but it greatly
enhances the rate of addition of water to the alkene by providing a lower
energy reaction pathway. Thus the proton fulfills the requirements of being a
catalyst for this reaction.
OH
+
H+
H2 O
H
H
O
X
H
H
O
CH3
CH3
H
H
O
more stable intermediate
H
Note above the more stable tertiary carbocation is formed.
11
H
OH
H+
+
H2 O
X
H
H2
C
CH2
H
Note above the more stable carbocation is formed.
OH
H+
+
H2 O
CH3
CH3
H
H
H
1,2 hydride shift
Because of the possibility of rearrangement reaction, hydration of alkenes is
not a preferred procedure to make alcohols in the laboratory, except when it
is clear that only the desired carbocation will be formed.
Later in the course we will explore alternative routes to synthesizing alcohols.
12
Addition of X2 , Hg(CH3 COO)2 (Hg(OAc)2 ) and HOX to Alkenes.
The addition of halogens, mercuric acetate and hypohalous acids to alkenes
are all useful reactions.
H3C
H3C
X2
H3C
H3C
HOX, or X2 in H2O
C
CH2
H
X
H
Addition of Halogen
where X = Cl , Br
H3C
H3C
X
X
H
HO
H
Addition of Hypohalous Acid
1) Hg(OAc)2 in H2O
H3C
H3C
Hg(OAc)
H
HO
H
Addition of Mercuric Acetate
2) NaBH4
H3C
H3C
H
H
HO
H
Reduction
Overall Reaction: Oxymercuration-Reduction
13
In each case the reactions proceed by a common initial mechanism:
1 ) Coordination of the Lewis acid to the π-system to make a cyclic
intermediate, with concomitant expulsion of a leaving group from the
initial electrophile.
2) Attack of a nucleophile (either the leaving group or solvent) from the
opposite face of the alkene at the site that will give the most stabilized
transition state, hence the anti addition product is formed.
H3C
E
E
H3C
δ
H3C
H3C
H
H
H
Nu
H
Anti- addition
Nu
Note that the δ+ by the carbons is to
remind you of the build up of
charge positve in the transition state.
Nu
Nu
E
The reaction will follow the path
that gives the more stable transition
state (i.e. more stable carbocation).
E
C
CH2
H3C
H3C
C
CH2
Hence the nucleophile will be
bound to the more substituted
carbon.
X
H3C
H3C
E
E
δ
H3C
H3C
H
Nu
14
H
H3C
H3C
H
H
Nu
Addition of Halogens to Alkene
An electrophilic halogen is attacked by the alkene π-system, to form the
cyclic "halonium" ion intermediate (i.e., bromonium or chloronium ion), with
concomitant loss of a halogen anion (i.e., bromide or chloride).
This cationic intermediate is highly electrophilic and reacts rapidly with the
halide anion that was formed in the previous step.
Bromonium ion
Br
Br
Br
H3C
H
H
H3C
H3C
Br
Br
H
Anti-addition
Br
The halonium ion effectively blocks one face of the intermediate from attack
by halide. Therefore the halide anion attacks from the opposite face of the
halonium ion to form the trans-1,2-dihalide. Thus in the above reaction
please note the stereochemistry, i.e. that the addition is anti across the double
bond.
This is an example of a nucleophilic substitution reaction in which a
nucleophile attacks at the carbon and displaces the leaving group in a single,
smooth, concerted process without formation of a carbocation intermediate.
Thus, rearrangement reactions are not observed.
15
Notes:
Thus, the addition of Br2 to 1-methylcyclopentene has anti stereochemistry.
Bromonium ion
Br
Br
Br
H3C
H
H
H3C
H3C
Br
Br
H
Anti-addition
Br
It is important to choose a solvent that is unreactive with Br2 or Cl2 when
performing these reactions, typically dichloromethane (CH2 Cl2 (A.K.A.
methylene chloride) or carbontetrachloride (CCl4 ) are used. Sale of CCl4 has
been severely restricted due to environmental reasons, and thus its use will
become less and less common.
Bromine reacts extremely quickly with alkenes, and, as a result, its red color
is completely discharged immediately upon contact with a solution of an
alkene. Discoloration of Br2 is used as a qualitative test for alkenes.
16
Formation of Halohydrins
HOX, or X2 in H2O
H3C
H3C
C
H3C
H3C
OR
CH2
N-bromosuccinimde, in H2O, DMSO
O
N
Br
H3C
O
S
X
H
HO
H
a Halohydrin
CH3
NBS
O
As with the addition of halogens, the addition of hypohalous acid (HOX) to
alkenes is a stepwise process that also involves the "halonium" ion
intermediate.
The mechanism is completely analogous to the addition of Br2 .
OH
Br
Br
H3C
H
H3C
OH
17
H
H3C
Br
HO
H
Notes:
OH
Br
Br
H3C
H
H3C
H
H3C
Br
HO
H
OH
• If the alkene is asymmetrical, hydroxide can potentially attack at either
carbon.
• In general, hydroxide anion will attack the carbon which would form the
most stable carbocation, (i.e. carbon most able to stabilize δ+ charge in
transition state).
• This determines the regiochemical selectivity of the reaction.
Attack of hydroxide is by a nucleophilic substitution mechanism giving the
anti addition product.
• Note the use of NBS. This is used because NBS is a solid that slowly,
effectively liberates Br+ and is easier to use than bromine.
• DMSO is used as solvent because, in general, alkenes are relatively
insoluble in water, yet DMSO is both a good solvent for alkenes and
water-miscible..
18
Oxymercuration-Reduction of Alkenes to Give Alcohols.
The reaction of alkenes with mercuric acetate follows the general mechanism
for Lewis acid activation of alkene addition reactions.
HgOAc
H3C
OAc
HgOAc
H3C
HgOAc
H2O, THF
O
H3C
H
H
H3C
H
H
HO
H
HgOAc
H
OAc
H2O
Note that the large excess of water leads to attack by water at the opposite
face and deprotonation gives an organomercurial that is stable (and toxic).
This species can be reduced with sodium borohydride, which replaces the
mercury atom with a hydrogen atom.
HgOAc
H3C
H
H3C
NaBH4
HO
+ Hg
HO
H
H
This reaction sequence occurs without formation of carbocation intermediate
and therefore, without rearrangements to give Markovnikov addition of
water.
Note the use of the isotopic label to illustrate something about the mechanism
of the reaction (i.e. the addition of hydride (deuteride)). Deutrium is
incorporated to both the same and opposite face of the alkene.
H3C
D
H3C
HgOAc
NaBD4
HO
H3C
H
+ Hg
+
HO
H
H
19
HO
D
Notes:
Tetrahydrofuran, THF (shown below) is used to solubilize the alkene in the
water.
O
THF
If an alcohol is used as the solvent instead of water then an ether will be
formed as the final product as shown below, because ROH will act as the
nucleophile instead of water.
OAc
HgOAc
HgOAc
ROH, THF
-H+
H3C
H
H3C
H
R
O
R
H
20
H3C
H
RO
H
NaBH4
O
H3C
HgOAc
H
Hydroboration
Important reaction! Discovered by H. C. Brown who won a Nobel Prize in
1979 for this reaction.
H3C
H
H3C
OH
1) BH3
3
2) H2O2, HO-
BH3 acts as a strong Lewis acid and adds a B-H bond to an alkene. All three
boron hydride bonds can add across three alkenes to give the trialkyl borane
shown below. (Note that the boron is attached to the less substituted carbon).
H
H3C
CH3
+ BH3
H
BH
H3C
H
H
CH3
H3C
B
CH3
H3C
H
H
B
H3C H
H3C H
Notes:
In practice, typically a THF or diethyl ether complex of BH3 is used because
BH3 is so reactive it does not exist as such (it dimerizes). The Lewis acidLewis base complex with THF or diethyl ether “tames” it a bit.
21
Mechanism of the Hydroboration Reaction
The initial reaction of BH3 with a substituted alkene can occur in two ways,
leading to two different transition states wherein the boron atoms is nearer the
more substituted carbon, or the less, substituted carbon.
The reaction occurs in a concerted manner without the appearance of
carbocation intermediates. Thus, bonds are forming between the hydrogen in
the borane and one of the carbons of the alkene and between the boron atom
and the other carbon of the alkene. At the same time, the B-H bond and the
π-bond are being broken.
Therefore the reaction occurs without rearrangements.
The overall reaction of hydroboration followed by reaction with hydrogen
peroxide is complementary to the oxymercuration reaction in that it gives the
opposite regiochemical alcohol, i.e. anti- Markovnikov addition of water.
22
Mechanism of Hydroboration
Because the boron is Lewis acidic, it develops a partial negative charge and
the carbon to which the hydrogen will be bound gains partial positive charge.
Accordingly, it is electronically more stable to bond the boron to the less
substituted carbon. In addition, steric repulsion between the boron and the
more substituted carbon also favors bonding the boron to the less substituted
carbon atom of the alkene
H
H
BH2
H3C
H
H3C δ
BH2
δ
H
H
H3C
BH2
H
more favored on both steric and electronic grounds
δ
H2B
H3C
.
H2B
H
H
H3C
H
H2B
H
δ H
H3C
H
less favored on both steric and electronic grounds
Subsequent reaction with basic hydrogen peroxides cleaves the boron carbon
bond to give a stereochemistry of the addition reaction that is syn (cis) and a
regiochemistry of the product is generally anti-Markovnikov.
23
Oxidative Decomposition of an Organoborane to an Alcohol
The organoborane that is formed can be oxidized by alkaline peroxide to
form the alcohol by the following mechanism:
H
H
O
O
O
+ OH
+ H2 O
O
H
pKa = 14
First, hydroxide deprotonates the peroxide.
pKa = 15.5
The deprotonated peroxide anion (a Lewis base) attacks the Lewis acidic
organoborane. It forms a tetrahedral intermediate that rearranges by alkyl
migration to oxygen with concomitant loss of hydroxide. (Note that this
reaction is analogous to carbocation rearrangements that we have already
examined.)
H
O
H
O
O
R
B
O
R
R
R
R
R
B
B
OR
+ OH
R
R
This reaction can occur two more times to give B(OR)3 as follows:
H
O
H
O
O
R
B
OR
R
R
RO
H
O
H
O
O
R
B
OR
OR
RO
RO
O
R
B
OR
B
+ OH
OR
R
O
B
RO
OR
B
R
OR
+ OH
The resulting borate ester is rapidly hydrolyzed by the alkaline conditions:
OH
H
O
RO
B
OR
OR
RO
RO
H2O
B
HO
OR
24
B
OH
OH
+ 3 ROH
Comparison of Oxymercuration-reduction and Hydroborationoxidation Reactions
H
OH
H
1) BH3 THF
1) Hg(OAc)2 H2 O
2) HOOH, OH
2) Na BH4
H
OH
H
Regiochemistry:anti-Markovnikov
Stereochemistry: syn addition
Markovnikov
anti addition (first step)
In both reactions the electrophilic species is attacked by the π-cloud.
For hydroboration there is a concerted addition of the B-H bond across the
double bond. In the transition state, positive charge builds up on the carbon
which receives the hydrogen. Thus, the carbon atom that is better able to
stabilize a δ+ charge will get the hydrogen.
This is typically the more sterically congested carbon. Since boron is larger
than hydrogen, this also favors boron bonding to the less substituted carbon.
Thus both electronic and steric arguments favor the same regiochemistry of
addition.
For oxymercuration, the electrophile Hg atom is attacked by the π-system
forming a bridging mercurium ion with concomitant loss of acetate. In the
next step there is a nucleophilic attack of OH- from the opposite face of the
alkene. In the transition state for the nucleophilic displacement reaction, the
carbon that will bond to the OH- develops a δ + charge. If there are two
possible regiochemical addition products, the transition state that is better able
to stabilize a δ+ charge will lead to the major product.
25
Radical Reactions
To date most of the compounds and intermediates that we have considered
have an even number of electrons--eight if they have a complete octet, six if
they are Lewis acids (except for H+ which has 0).
Radical species have an odd number of electrons. We briefly mentioned
them when considering a thermodynamic cycle to predict acidities.
H
Br
heterolytic cleavage
H
+
Br
note "fish hook" arrow formalism
to indicate motion of
one electon
homolytic cleavage
H
Br
H
+
Br
Peroxides are known to cleave by homolytic cleavage.
heat or light
O
homolytic cleavage
O
2
O
The radical species thus created tend to be very reactive and can recombine to
form a bond. The ΔH0 for recombination (i.e. two radicals forming a bond) is
very negative.
Br
Br
Br
26
Br
A radical can abstract an atom generating another radical:
O
H
Br
OH
+
Br
A radical can also add to a double bond to generate an alkyl radical:
O
H 2C
CH2
O
CH2 CH2
A radical can also cleave in an intramolecular (within one molecule) manner
as shown below, by a β scission reaction.
R
R
β
+
H 2C
CH2
α
β refers to the position of the carbon relative to the reactive species (the
radical carbon): the adjacent carbon is the α carbon, and as one moves away
from that site each carbon is assigned a higher letter in the Greek alphabet.
For a radical abstraction or addition reaction to effectively compete with a
recombination reaction, the ΔH0 must not be significantly positive.
• If ΔH 0 is positive then recombination (i.e. two radicals forming a
bond for which ΔH0 is very negative) will preferentially occur.
Other factors disfavoring radical recombination reactions are the conditions of
the reaction.
• In general the reaction conditions are designed such that the radicals
are always present in very low concentration relative to other reactive
species in the solution.
27
Radical Chain Reactions
Radical reactions typically have three steps, which taken together are called
radical chain reactions.
1. Initiation. In this step reactive radicals are created.
• Homolysis, abstraction and addition reactions, can be the first steps
of a cycle in which radicals are generated (initiation).
• Typically, bonds that are very weak are broken in the initial
homolysis reactions.
2. Propagation. In these steps radicals react to create new chemically
equivalent radicals (or nearly equivalent) to the previous radical.
• A radical generated in the initiation step reacts with a closed shell
species in solution by addition or abstraction to make and/or break a
bond, regenerating a reactive radical.
• The reaction can happen many times until the concentration of the
reactant is very low.
3. Termination. In this step two radicals combine to make a covalent bond
destroying both radicals.
• The reaction is very exothermic relative to propagation steps.
• Termination steps occur when the concentration of the radical
becomes significant relative to the concentration of the reactants.
28
Free radical Initiated Polymerization
Many polymerizations occur by radical chain reactions and are of industrial
importance. Consider the free radical polymerization steps for ethylene:
Initiation: Tert-butoxy radical is created by homolysis of di-tertbutylperoxide. This radical initiates a polymerization reaction by reacting
with a molecule of ethylene to create an alkyl radical.
O
H 2C
O
CH2
CH2 CH2
Propagation: This alkyl radical can react with a molecule of ethylene
generating a new alkyl radical.
This reaction can happen many times in what are known as propagation steps
to yield a polymer with a terminal radical. Note that in the propagation step
the radical always reacts with a closed shell species. This is called a radical
chain reaction.
O
CH2 CH2
H 2C
n-1
O
(CH2CH2 )n
H 2C
O
CH2
H 2C
CH2 CH2
CH2
O
CH2
29
(CH2CH2 )n+1
Termination: If the radical reacts with another radical, a bond will be formed
annihilating both radicals and the reaction will stop. This is called a
termination reaction.
O
(CH2CH2 )n+1
O
O
(CH2CH2 )n+1 O
Summary: Radical chain reaction cycles thus involve:
i)
initiation steps that generate the active radicals;
ii) propagation steps, (radical chain), to make new bonds and regenerate
active radicals, and
iii) termination steps that end the cycle and destroy the radicals.
30
Radical Addition of HBr to Alkenes
If HBr is added to an alkene in the presence of traces of peroxides the
regiochemistry of the product is generally anti-Markovnikov.
H
H 3C
H 3C
C
Br
H
HBr, peroxides (trace)
CH2
H 3C
H 3C
H
The addition of HBr to an alkene in the presence of peroxides occurs via a
radical chain reaction.
Initiation: Tert-butoxy radical is created by homolysis of di-tertbutylperoxide. This radical reacts with HBr: the most favorable initiation
reaction is abstraction of a hydrogen atom from HBr to generate the bromine
radical.
O
H
O
Br
31
H
+ Br
The bromine radical is electrophilic and can react with the alkene π-system,
forming a σ-bond to the bromine and leaving an unpaired electron (a radical)
and the remaining carbon from the alkene.
• In the case of unsymmetrical alkenes, two radical intermediates can be
formed.
• In general, the pathway leading to the more stable intermediate will have a
lower energy transition state (Hammond Postulate) and will be the
preferred pathway, which here, is the path to the right. (Patience--I’ll
explain).
Br
H 3C
CH3
H
H
Br
H 3C
H 3C
C
C
H
H
top arrows
Br
H
H 3C
H 3C
H
X
bottom arrows
Br
H 3C
Br
CH3
H 3C
H 3C
H
H
Br
H
H
Like carbocation intermediates, carbon radicals are planar (sp2 ) and are
electrophilic (which should give you some idea about why the addition of Br
radical takes the red pathway!).
32
Radical Addition of HBr to Alkenes (cont.)
Propagation: This radical can then abstract a hydrogen atom from another
molecule of HBr to generate the HBr addition product (a closed shell
species):
H
Br
Br
H 3C
H
H 3C
H 3C
H
H 3C
H 3C
H
Br
H
+
H 3C
H
+ Br
H
H
H
Br
and a new Br radical which can continue the chain reaction (n times) by
reacting with another molecule of alkene:
H
H
Br
H
Br
H 3C
H 3C
H
H
n times
Br
H
H 3C
H 3C
33
C
C
H
H
Br
H 3C
H 3C
+
Br
H 3C
H 3C
H
H
H
Radical Addition of HBr to Alkenes (cont.)
Termination steps:
Br
H
H
Br
Br
H
H
Br
Br
H 3C CH3
Br
Br
Br
H 3C CH3
Br
H 3C
H 3C
H 3C
H
Br
H
H 3C
H
Br
Br
34
H
Stability of Radicals
The resulting radical is formed on the carbon of the alkene that is best able to
stabilize the electrophilic site (the unpaired electron).
• In simple unstrained non-conjugated systems, without adjacent
heteroatoms, the order of stability of carbon radicals parallels that of
carbocations, with tertiary > secondary > primary > methyl.
CH3 
Stability relative to
(CH3 )3 C  (kcal/mol), 11.9
CH3 CH2  (CH3 )2 CH 
+5.0
(CH3 )3 C 
+1.9
+0
This can also be explained hyperconjugation wherein a
C-H σ-bond donates some electron density to the
partially filled orbital thereby stabilizing it.
• Stabilization is not as sensitive to alkyl groups on
adjoining carbons, since there isn’t a positive charge
H
R
R'
H
H
Radicals are stabilized by resonance.
• For example allyl radical (left) is more stable than propyl radical.
Stability relative to
(CH3 )3 C  (kcal/mol),
–6.9
+4.7
• Likewise benzyl radical is stable relative to alkyl radicals.
H
H
H
H
H
H
...etc
Stability relative to (CH3 )3 C  (kcal/mol), –5.2
35
Catalytic Hydrogenation of Alkenes
Catalytic hydrogenation of alkenes produces the corresponding alkane, with
syn (cis) addition of hydrogen.
• The reaction requires a metal catalyst, usually Pt, Pd or Ni, and the
mechanism involves adsorption of hydrogen to the metal surface, followed
by adsorption of the alkene (probably through Lewis acid complex of the
π-system).
• Hydrogen transfer occurs and syn addition
+
syn addition
H2
Pd(catalyst)
H
H
H H
H
uncatalyzed
catalyzed
Reactants
Products
H
H
H
A catalyst can provide an alternative
mechanism for a reaction which lowers the
transition state energy for the rate
determining step of the reaction thereby
enhancing the rate of the reaction
Catalysts only change the barrier of the reaction, not the energy of the
products and reactants. Therefore a catalyst can accelerate the rate, but not
change the equilibrium constant for the reaction.
36
Notes:
One criterion for a catalyst is that at the end of the reaction the catalytic
species is regenerated in an unchanged state.
Therefore the catalyst can be used again to catalyze a reaction for another set
of reactants.
Each time a catalyst goes through a complete reaction path, generates a
product, and regenerates itself is called a catalytic cycle.
The number of times that a catalyst proceeds through a complete cycle is
called the number of turnovers.
Enzymes are naturally occurring catalysts that are typically made of proteins.
RNA has been also shown in some cases to have catalytic activity. This
activity is thought to be of critical importance to the early development of life.
37
Stability
Heats of Hydrogenation give an estimate of relative stabilities.
R2 C=CR2 + H2 → HCR2 -CR2 H;
• The more negative the ΔH0 hydrog the less stable the starting alkene.
0
H
H
H 3C
H
H 3C
H
H
H
H
H
ΔH hydrog (Kcal/mol) –32.8 –30.1
relative stability un < mono <
H 3C
H
H
CH3 H 3C
–27.6
di
<
H 3C
CH3
CH3 H 3C
CH3
–26.9
tri
–26.6
< tetra
• Greater substitution leads to more
Antibonding C-C π-orbital (unfilled)
stability due to hyperconjugation:
H Bonding C-H
The stabilizing interaction between a
orbital (filled)
filled σ-bond with and unfilled ptype orbital.
Butenes
0
H3CH 2C
H
H 3C
H
H 3C
H
H
H 3C
H
H
ΔH hydrog (Kcal/mol)
relative stability
–30.1
mono
–28.4
< 1,1-di
–28.6
≅
cis
CH3
H
H 3C
H
H
CH3
–27.6
< trans
• And in general sp2 -sp3 bonds are stronger than sp3 -sp3 bonds, thus mono <
than 1,1 di- for example.
There are exceptions, for example when dealing with very bulky substituents,
but we will not worry about them here.
38