Chapter 5: Structure and Preparation of Alkenes. Elimination Reactions

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, i.e. how many alkyl groups are attached to the C=C ? 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.

Structure:

The alkene functional group consists of two sp2 hybridised C atoms bonded to each other via a σ and a 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

Reactivity:

A  π bond  is a region of high electron density (red) so alkenes are typically nucleophiles. Alkenes typically undergo addition reactions in which the π bond  is converted to two new σ bonds.

© Dr. Ian Hunt, Department of Chemistry