Chapter 1Review – Synthesis of Alkenes
• Elimination – dehydration
• Non-Zaitsev’s product (Hoffman product)
• Anti periplanar elimination occurs in very specific conformations
• Rearrangements during carbocation transition state formation
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Dehydrohalogenation
• 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
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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.
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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
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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)
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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
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Mechanism of Dehydration
• E1 mechanism; the acid is a catalyst and needed only in small quantity
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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
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Rearrangement in Dehydration Reaction • Rearrangements of carbocations
occur if a more stable carbocation can be obtained
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Some More Rearrangements
• Shifts occur only once
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Key Words/Concepts
• Anti coplanar elimination
• Stability of carbocation