Transcript Alkene - Synthesis

Synthesis of Alkenes
 E2 dehydrohalogenation
 Debromination of vicinal
dibromides
 E1 dehydrohalogenation
 Acid-catalyzed dehydration of an
alcohol
 Dehydrogenation of alkanes
 Reduction of alkynes
E2 Dehydrohalogenation
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Most synthetically useful
One-step and requires a strong base
Best transition state is anti-coplanar.
Stereospecific
Example: t-butyl bromide + methoxide
E2 Dehydrohalogenation
 Example: t-butyl bromide + methoxide
E2 Dehydrohalogenation
 Works best with bulky 2° alkyl halides and
3° halides.
 For 2° alkyl halides, a bulky base can
 minimize the SN2 product.
 give rise to the Hoffman product.
bulky bases
E2 Dehydrohalogenation
base not bulky
71%
29%
E2 Dehydrohalogenation
bulky base
28%
72%
E2 Dehydrohalogenation
 is stereospecific.
E2 Dehydrohalogenation
 requires trans-diaxial configuration in a
cyclic alkyl halide.
 When drawing this mechanism, you must
show the trans-diaxial (anti) configuration.
E2 Dehydrohalogenation
Br
CH3
H3C
N
CH3
?
heat
Debromination of Vicinal
Dibromides (a reduction)
 E2 mechanism: one-step and best
transition state is anti-coplanar
 Stereospecific
 Rarely used to make alkenes
Debromination of Vicinal
Dibromides
 NaI/acetone or Zn/acetic acid
 Acetone can dissolve both the iodide
and the alkyl halide (if small).
 If Zn is used, reaction is
heterogeneous and takes place on
the surface of the Zn.
 Reduction because “Br2” is
removed.
Debromination of Vicinal
Dibromides
Br
Br
Br H
Br
C C CH CH
2
3
H
NaI / acetone
NaI / acetone
E1 Dehydrohalogenation
 2° or 3° alkyl halides
 requires a good ionizing solvent:
alcohol or water.
 no strong nucleophile or base
 Rearrangements can occur.
 will be accompanied by SN1
products.
E1 Dehydrohalogenation
CH3OH
?
Cl
heat
Acid-Catalyzed Dehydration of
Alcohols
 Common method for making
alkenes.
 Reversible, water must be removed as it
forms by using a dehydrating agent. Or
you can distill the alkene as it is
formed…it will be lower boiling than the
alcohol…why?
 Conc H2SO4 or conc H3PO4 act as both acid
catalyst and dehydrating agent.
 After protonation of the alcohol group, the
reaction is E1.
Acid-Catalyzed Dehydration of
Alcohols
 Step 1: protonation of the alcohol
 Fast equilibrium
 Converts -OH to a good leaving group
Acid-Catalyzed Dehydration of
Alcohols
 Step 2: ionization to a carbocation
 slow, rate-limiting
 leaving group is H2O
+ H2O
Acid-Catalyzed Dehydration of
Alcohols
 Step 3: deprotonation to give alkene
 fast
 The carbocation is a strong acid: a weak
base like water or bisulfate can abstract
the proton.
What else forms?
Acid-Catalyzed Dehydration of
Alcohols
 Write the mechanism for the product
shown.
OH
H2SO4, heat
Catalytic Cracking of Alkanes
 Common industrial method for making
small alkenes from petroleum.
 Catalyst = aluminosilicates
 Mixture of products makes it unsuitable
for the lab.
smaller
alkane
from petroleum
alkene
Dehydrogenation of Alkanes
 Similar to catalytic cracking
 Catalyst = metal such as Pt
 Mixture of products makes it
unsuitable for the lab.
Reduction of Alkynes
 This will be addressed in reactions
of alkynes.