Unit 5 - 2 Synthesis of Alkenes 4/16/2017 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 Unit 5 - 2 Synthesis of Alkenes 4/16/2017 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 Unit 5 - 2 Synthesis of Alkenes 4/16/2017 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 29% 71%

E2 Dehydrohalogenation bulky base 72% 28%

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

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

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 ?

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.

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.