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 Most synthetically useful One-step and requires a strong base Best transition state is anti-coplanar. Stereospecific 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
requires trans-diaxial configuration in a cyclic alkyl halide.
When drawing this mechanism, you must show the trans-diaxial (anti) configuration.
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.
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.
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?
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
alkenefrom petroleum
Dehydrogenation of Alkanes
Similar to catalytic cracking Catalyst = metal such as Pt Mixture of products makes it
unsuitable for the lab.