Notes

Alkenes and Alkynes

Hydrocarbons that contain one or more double bonds (alkenes) or triple bonds (alkynes).

  • the central $C$ atoms in ethene are $sp^2$ hybridized $\to$ trigonal planar geometry; bond angles of 120
    • alkenes with one double bond $\to$ general formula $C_n H_{2n}$
  • central $\ce{C}$ in ethyne are $sp$ hybridized $\to$ linear geometry; bond angles of 180
    • alkynes with one triple bond $\to$ gen. formula $\ce{C_nH_{2n-2}}$

Configuration Isomerism

Acyclic alkenes may exhibit configurational isomerism.

  • double bond locks geometry and preserves spatial rel. between subs.
  • acyc. alkenes with 2 different groups on each $sp^2$ $C$ of the double bond, such as 2-pentene, are classified as cis-trans isomers.
Identifying a geometric isomer:
  1. is there a double bond
  2. are there 2 subs.
  3. are those 2 subs on each sp2 orbital (i.e. $\ce{C}$ atoms) of the double bond

  • cis implies groups (here 2 $H$ atoms) on the same side of the double bond
  • trans implies opposite

1-bromo-2-chloro-1-propene

  • cis-trans notation cannot be used to express the configuration of this stereoisomer because there are four different groups attached to the carbons of the double bond
Next Conformational Isomers