|Chapter 5: | |Structure and Preparation of Alkenes. | |Elimination Reactions |
Alkenes contain the C=C functional group which can be prepared by 1,2-elimination reactions such as: • dehydration of alcohols (- H2O) or
• dehydrohalogenation of alkyl halides (- HX).
Zaitsev's rule indicates that the preferred product is the more highly substituted, and therefore more stable alkene. But the rule is not always obeyed and the anti-Zaitsev (or Hofmann) product can dominate. Alkenes
Preparation of Alkenes
• Dehydration of alcohols
• Dehydrohalogenation of alkyl halides
1,2 Elimination Reactions
• E1 mechanism
• E2 mechanism
• Sample questions
• More practice problems ?
• Reaction Drills
Functional group suffix = -ene (review)
Simple 1,2-alkenes can be described as cis- or trans- (review) More complex alkenes are best described as E- or Z- based on the Cahn-Ingold-Prelog priority rules (review) Stability:
There are 3 factors that influence alkene stability:
1. Degree of substitution: more highly alkylated alkenes are more stable, so tetra > tri > di > mono-substituted. 2. Stereochemistry: trans > cis due to reduced steric interactions when R groups are on opposite sides of the double bond. 3. Conjugated alkenes are more stable than isolated alkenes.
|[pic] |[pic] |more stable than |[pic] |[pic] | |trans-2-butene | |cis-2-butene |
|[pic] |The alkene functional group consists of two sp2 hybridised C atoms bonded to each other via a σ and a|[pic]| | |π bond. | | | |In ethene, four other σ bonds between the sp2 hybridised C and H 1s make 4 C-H bonds to complete the | | | |σ bond framework. | | | |The π bond is produced by the side-to-side overlap of the p-orbitals not utilised in the hybrids (see| | | |left). | | | |The 2 C of the C=C and the 4 atoms attached directly to the C=C are all in the same plane. | | | |For more details, you should review sp2 hybridisation in Chapter 2 | |
|A π bond is a region of high electron density (red) so alkenes are typically nucleophiles. |[pic] | |Alkenes typically undergo addition reactions in which the π bond is converted to two new σ bonds. | |
Preparation of Alkenes
Dehydration of Alcohols
Reaction type: 1,2- or ß-Elimination
• When heated with strong acids catalysts (most commonly H2SO4, H3PO4), alcohols typically undergo a 1,2-elimination reactions to generate an alkene and water. • Also known as dehydration since it involves the removal of a molecule of water. • Alcohol relative reactivity order : 3o > 2o > 1o
• Regioselectivity : major product is usually the more highly substituted alkene (alkene stability) Zaitsev's Rule. • Stereoselectivity : trans- > cis- again controlled by stability • Reaction usually proceeds via an E1 mechanism which proceeds via a carbocation intermediate, that can often undergo rearrangement. • Primary alcohols will...
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