Organic Chemistry 1

|Chapter 5: |
|Structure and Preparation of Alkenes. |
|Elimination Reactions |

Summary
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
• Nomenclature • Stability • Structure • Reactivity
Preparation of Alkenes • Dehydration of alcohols • Dehydrohalogenation of alkyl halides
1,2 Elimination Reactions • Overview • E1 mechanism • E2 mechanism • Selectivity • Sample questions
Problems
• More practice problems ? • Reaction Drills

Alkenes
Nomenclature:
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 |

Structure: |[pic] |The alkene functional group consists of two sp2 hybridised C atoms bonded to each other via a σ and a|[pic]|
| |π bond.