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Alkene Synthesis - Manu's Adventures

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Alkene Synthesis Dr. Sapna Gupta
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Page 1: Alkene Synthesis - Manu's Adventures

Alkene SynthesisDr. Sapna Gupta

Page 2: Alkene Synthesis - Manu's Adventures

Review – Synthesis of Alkenes

• Most synthesis of alkenes are elimination reactions.• Elimination – dehydrohalogenation

• Elimination – dehydration

• Elimination gives two kinds of products• Zaitsev’s product

• Non-Zaitsev’s product (Hoffman product)

• Anti periplanar elimination occurs in very specific conformations

• Rearrangements during carbocation transition state formation

Dr. Sapna Gupta/Synthesis of Alkenes 2

Page 3: Alkene Synthesis - Manu's Adventures

Dehydrohalogenation

• Reactions are mostly E2 mechanism, not E1

• E2 reaction are favored by:• Secondary or tertiary alkyl halides

• Alkoxide bases such as sodium ethoxide or potassium tert-butoxide

• Bulky bases such as potassium tert-butoxide should be used for E2 reactions of primary alkyl halides

Dr. Sapna Gupta/Synthesis of Alkenes 3

Page 4: Alkene Synthesis - Manu's Adventures

Zaitsev’s Rule• Formation of the more substituted alkenes by a small base. When

two alkenes are formed the major product is the more substituted (Zaitsev’s product)

• When the base is CH3CH2O- the major product is (a)

• Formation of the least stable alkene (Hoffman product) is formed when a bulky base is used.

Dr. Sapna Gupta/Synthesis of Alkenes 4

Page 5: Alkene Synthesis - Manu's Adventures

Stereochemistry (Anti-coplanar elimination)

• All four atoms involved must be in the same plane• Anti coplanar orientation is preferred because all atoms are staggered

• In a cyclohexane ring the eliminating substituents must be diaxial to be anti coplanar

Dr. Sapna Gupta/Synthesis of Alkenes 5

Page 6: Alkene Synthesis - Manu's Adventures

Stereochemistry …. Contd.• Neomenthyl chloride and menthyl chloride

give different elimination products because of this requirement

• In neomenthyl chloride, the chloride is in the axial position in the most stable conformation. (Two axial hydrogens anti to chlorine can eliminate; the Zaitzev product is major)

• In menthyl chloride the molecule must first change to a less stable conformer to produce an axial chloride (Elimination is slow and can yield only the least substituted (Hoffman) product from anti elimination)

Dr. Sapna Gupta/Synthesis of Alkenes 6

Page 7: Alkene Synthesis - Manu's Adventures

Dehydration of Alcohols

• Elimination is favored over substitution at higher temperatures

• Typical acids used in dehydration are sulfuric acid and phosphoric acid

• The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol• Primary alcohols are most difficult to dehydrate, tertiary are the easiest

• Rearrangements of the carbon skeleton can occur

Dr. Sapna Gupta/Synthesis of Alkenes 7

Page 8: Alkene Synthesis - Manu's Adventures

Mechanism of Dehydration

• E1 mechanism; the acid is a catalyst and needed only in small quantity

Dr. Sapna Gupta/Synthesis of Alkenes 8

Page 9: Alkene Synthesis - Manu's Adventures

Carbocation Stability in Dehydration

• Recall the stability of carbocations is:

• The second step of the E1 mechanism in which the carbocation forms is rate determining

• Tertiary alcohols react the fastest because they have the most stable tertiary carbocation-like transition state in the second step

• The mechanism for primary alcohols is E2 because primary carbocations are not stable

Dr. Sapna Gupta/Synthesis of Alkenes 9

Page 10: Alkene Synthesis - Manu's Adventures

Rearrangement in Dehydration Reaction• Rearrangements of carbocations

occur if a more stable carbocation can be obtained

Dr. Sapna Gupta/Synthesis of Alkenes 10

Page 11: Alkene Synthesis - Manu's Adventures

Some More Rearrangements

• Shifts can be hydride (hydrogen), methyl, phenyl

• Shifts occur only once

• Rearrangements can also cause change in the ring size

Dr. Sapna Gupta/Synthesis of Alkenes 11

Page 12: Alkene Synthesis - Manu's Adventures

Key Words/Concepts

• Dehydrohalogenation

• Dehydration of alcohols

• Zaitsev’s and Hoffman’s product

• Anti coplanar elimination

• Stability of carbocation

• Rearrangement of carbocation intermediate

• Hydride, methyl and phenyl group shifts

Dr. Sapna Gupta/Synthesis of Alkenes 12


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