Transcript Document

Reactions of Alkenes: Addition Reactions
Hydrogenation of Alkenes – addition of H-H (H2) must be
catalyzed by metals such as Pd, Pt, Rh, and Ni.
H
H
H
C
+
C
Pd/C
H
H
EtOH
H
H
C-C π-bond
= 243 KJ/mol
H-H
= 435 KJ/mol
H
C
C
H
H
H
H°hydrogenation = -136 KJ/mol
H
C-H
= 2 x -410 KJ/mol
= -142 KJ/mol
• The catalysts is not soluble in the reaction media, thus this
process is referred to as a heterogenous catalysis.
• The reaction takes places on the surface of the catalyst. Thus,
the rate of the reaction is proportional to the surface area
of the catalyst.
Carbon-carbon -bond of alkenes and alkynes can be reduced
to the corresponding saturated C-C bond. Other -bond bond
such as C=O (carbonyl) and CN are not easily reduced by
catalytic hydrogenation. The C=C bonds of aryl rings are not
easily reduced.
O
O
H2, PtO2
ethanol
O
C5H11
OH
H2, Pd/C
CH3(CH2)16CO2H
Linoleic Acid (unsaturated fatty acid)
Steric Acid (saturated fatty acid)
O
O
OCH3
H2, Pd/C
OCH3
ethanol
C
H2, Pd/C
N
ethanol
C
N
Table 6.1 (pg 228): Heats of Hydrogenation of Some Alkenes
Alkene
H2C=CH2
H
H
H3 C
H
monosubstituted
H
H° (KJ/mol)
136
125 - 126
H
117 - 119
H3C
CH3
H
CH3
disubstituted
H3C
H3C
H
114 - 115
H
116 - 117
H3C
H
H3C
H
H3C
CH3
H3 C
CH3
H3 C
CH3
trisubstituted
tetrasubstituted
112
110
Stereochemistry of Alkene Hydrogenation
Mechanism:
H H
H2C CH2
H H H2C CH2
H2C CH2
H2
H
H
H
H
C C
H
H
H
H H
C
H
H
C H
The addition of H2 across the -bond is syn, i.e., from the
same face of the double bond
CH3
CH3
H
H2, Pd/C
EtOH
H
CH3
CH3
H
CH3
H
syn addition
of H2
CH3
Not observed
Electrophilic Addition of Hydrogen Halides to Alkenes
C-C -bond: H°= 368 KJ/mol
C-C -bond: H°= 243 KJ/mol
-bond of an alkene can
act as a nucleophile!!
Electrophilic addition reaction
H
H
Br
C C
H
+
H-Br
H
nucleophile
H
H
C C
H
H
H
electrophile
Bonds broken
C=C -bond 243 KJ/mol
H–Br
366 KJ/mol
Bonds formed
H3C-H2C–H -410 KJ/mol
H3C-H2C–Br -283 KJ/mol
calc. H° = -84 KJ/mol
expt. H°= -84 KJ/mol
Reactivity of HX correlates with acidity:
slowest
HF << HCl < HBr < HI
fastest
Regioselectivity of Hydrogen Halide Addition:
Markovnikov's Rule
R
R
R
H
C
R
C
R
C
C
H
C
H
C
R
H
H-Br
Br H
R C C H
H H
+
H
H-Br
Br H
R C C H
R H
+
H
H-Br
Br H
R C C R
R H
+
H Br
R C C H
H H
none of this
H Br
R C C H
R H
none of this
H Br
R C C R
R H
none of this
H
R
C
C
R'
H
H-Br
Br H
R C C R
H H
+
H Br
R C C R'
H H
Both products observed
Mechanism of electrophilic addition of HX to alkenes
Mechanistic Basis for Markovnikov's Rule:
Markovnikov’s rule can be explained by comparing the
stability of the intermediate carbocations
For the electrophilic addition of HX to an unsymmetrically
substituted alkene:
• The more highly substituted carbocation intermediate is
favored (hyperconjugation).
• The more highly substituted carbocation is formed faster
than the less substituted carbocation and also
thermodynamically more stable.
Carbocation Rearrangements in Hydrogen Halide
Addition to Alkenes - In reactions involving carbocation
intermediates, the carbocation may sometimes rearrange if a
more stable carbocation can be formed by the rearrangement.
These involve hydride and methyl shifts.
H
C
H3C
C
H3C
Cl
H
H-Cl
C
H
C
H3C
H3C
H
H
H
H
C
C
H
H
+
H
~ 50%
expected product
H
C
H3C
C
H3C
Cl
CH3
H
C
H
H-Cl
H3C
C
C
Cl
H
H3C
H
C
C
CH3
H
H
H
H
~ 50%
H
C
H3C
C
H3C
H3C
H
+
C
H3C
H3C
H
H
C
C
Cl
H
H
Note that the shifting atom or group moves with its electron pair.
A MORE STABLE CARBOCATION IS FAVORED.
Free-radical Addition of HBr to Alkenes
H3CH2C
H3CH2C
R
R
R
H
H
C
H
C
H
C
R
C
R
C
R
C
C
H
C
H
H
H-Br
Br H
H3CH2C C C H
H H
H
H-Br
Br H
H3CH2C C C H
H H
peroxides
(RO-OR)
H-Br
C
H
C
H
C
R
C
R'
H
ROOR
(peroxides)
H
H-Br
ROOR
H
H-Br
ROOR
H
H-Br
ROOR
+
+
H Br
H3CH2C C C H
H H
none of this
H Br
H3CH2C C C H
H H
Polar mechanism
(Markovnikov addition)
Radical mechanism
(Anti-Markovnikov addition)
none of this
Br H
R C C H
H H
none of this
Br H
R C C H
R H
none of this
Br H
R C C R
R H
none of this
Br H
R C C R
H H
+
+
H Br
R C C H
H H
H Br
R C C H
R H
+
H Br
R C C R
R H
+
H Br
R C C R'
H H
Both products observed
The regiochemistry of
HBr addition is reversed
in the presence of
peroxides.
Peroxides are radical
initiators - change in
mechanism
136
The regiochemistry of free radical addition of H-Br to alkenes
reflects the stability of the radical intermediate.
H
H
R C•
R C•
H
Primary (1°)
R
R C•
R
<
Secondary (2°)
R
<
Tertiary (3°)
137
Acid-Catalyzed Hydration of Alkenes - addition of water
(H-OH) across the -bond of an alkene to give an alcohol;
opposite of dehydration
H3C
C
H3C
CH2
H2SO4, H2O
H3C
H3C
H3C
C
OH
This addition reaction follows Markovnikov’s rule The more
highly substituted alcohol is the product and is derived from
The most stable carbocation intermediate.
Reactions works best for the preparation of 3° alcohols
138
Mechanism is the reverse of the acid-catalyzed dehydration
of alcohols: Principle of Microscopic Reversibility
139
Thermodynamics of Addition-Elimination Equlibria
H3C
H2SO4
C
CH2
+ H2O
H3C
Bonds broken
C=C -bond 243 KJ/mol
H–OH
497 KJ/mol
H3C
C
H3C
H3C
OH
Bonds formed
H3C-H2C–H -410 KJ/mol
(H3C)3C–OH -380 KJ/mol
calc. H° = -50 KJ/mol
G° = -5.4 KJ/mol
H° = -52.7 KJ/mol
S° = -0.16 KJ/mo
How is the position of the equilibrium controlled?
Le Chatelier’s Principle - an equilibrium will adjusts to any stress
The hydration-dehydration equilibria is pushed toward hydration
(alcohol) by adding water and toward alkene (dehydration) by
140
removing water
The acid catalyzed hydration is not a good or general method for
the hydration of an alkene.
Oxymercuration: a general (2-step) method for the Markovnokov
hydration of alkenes
H
H
C
C4H9
H
1) Hg(OAc)2, H2O
C
H
Hg(OAc)
C
H H
O
C
H3C
C
C4H9
H
Ac= acetate =
OH
O
2) NaBH4
OH
C
C4H9
H
C
H H
NaBH4 reduces the C-Hg
bond to a C-H bond
141
Hydroboration-Oxidation of Alkenes - Anti-Markovnikov
addition of H-OH; syn addition of H-OH
CH3
1) B2H6, THF
2) H2O2, NaOH, H2O
H
HO
CH3
H
Stereochemistry of Hydroboration-Oxidation
Mechanism of Hydroboration-Oxidation Step 1: syn addition of the H2B–H bond to the same face of the
-bond in an anti-Markovnikov sense; step 2: oxidation of the
B–C bond by basic H2O2 to a C–OH bond, with retention of
stereochemistry
142
Addition of Halogens to Alkenes
X2 = Cl2 and Br2
X2
X
X
(vicinal dihalide)
C C
C C
alkene
1,2-dihalide
Stereochemistry of Halogen Addition - 1,2-dibromide
has the anti stereochemistry
Br
Br
+
+
Br2
Br
Br
not observed
CH3
Br
Br2
H
CH3
Br
143
Mechanism of Halogen Addition to Alkenes:
Halonium Ions - Bromonium ion intermediate explains the
stereochemistry of Br2 addition
144
Conversion of Alkenes to Vicinal Halohydrins
"X-OH"
X
OH
C C
C C
alkene
halohydrin
X2, H2O
X
+ HX
OH
anti
stereochemistry
Mechanism involves a halonium ion intermediate
145
For unsymmterical alkenes, halohydrin formation is
Markovnikov-like in that the orientation of the addition of
X-OH can be predicted by considering carbocation stability

CH3
Br 
+
more + charge on the
more substituted carbon

H2O adds in the second step and adds to the
carbon that has the most + charge and ends
up on the more substituted end of the double bond
CH3
HO
Br2, H2O
CH3
+ HBr
H
Br
Br adds to the double bond first (formation of
bromonium ion) and is on the least substituted
end of the double bond
146
Organic molecules are sparingly soluble in water as
solvent. The reaction is often done in a mix of organic
solvent and water using N-bromosuccinimide (NBS) as
he electrophilic bromine source.
O
+
N Br
O
O
OH
DMSO, H2O
Br
N H
+
O
Note that the aryl ring does not react!!!
Epoxidation of Alkenes - Epoxide (oxirane): threemembered ring, cyclic ethers.
Reaction of an alkene with a peroxyacid:
peroxyacetic acid
O
H3C
O
O
peroxyacetic
acid
O
H3C
O
H
H3C
OH
acetic
acid
HO OH
peroxide
H
O
OH
O
H3C
O
+
O
Stereochemistry of the epoxidation of alkenes: syn
addition of oxygen. The geometry of the alkene is
preserved in the product
Groups that are trans on the alkene will end up trans on
the epoxide product.
Groups that are cis on the alkene will end up cis on the
epoxide product.
H
H
R
R
H3CCO3H
R
R
H
trans-alkene
O
H
R
R
cis-epoxide
cis-alkene
H
H
H3CCO3H
H
O
R
H
R
trans-epoxide
Ozonolysis of Alkenes - oxidative cleavage of an alkene
to carbonyl compounds (aldehydes and ketones). The and -bonds of the alkene are broken and replaced with
C=O double bonds.
C=C of aryl rings, CN and C=O do not react with ozone,
CC react very slowly with ozone
Ozone (O3):
3 O2
electrical
discharge
+
O
2 O3
1) O3
2) Zn
O
O
+
1) O3
2) Zn
_
O
H
H
+
O C
H
O
1) O3
2) Zn
O
O
H
O
Introduction to Organic Chemical Synthesis
Synthesis: making larger, more complex molecules out of
less complex ones using known and reliable reactions.
devise a synthetic plan by working the problem backward from
the target molecule
OH
??
H2SO4
H2, Pd/C
OH
??
152
CH3
CH3
Br
??
H
Br