Structure and Bonding in Alkenes
1. Drawing Skeletal Formulae
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2. Nomenclature
Methylpropene (or 2-methylpropene)
Hex-2-ene
3,3-dimethylbut-1-ene
Pent-2-ene
3. Stereoisomerism
Definition:
Stereoisomerism is when two or more molecules have the same structural formula but a different arrangement of atoms in space.
Restricted Rotation
There is restricted rotation around the C=C double bond (due to the presence of the π-bond).
4. Structural Isomerism & Properties
Chain Isomerism
Surface Area and Van der Waals Forces
Although both molecules have the same molecular formula (and same number of electrons), hexene is a straight chain while 2-methylpentene is branched.
Hexene molecules can pack closer together, resulting in a larger surface area of contact. This leads to stronger Van der Waals forces between hexene molecules compared to the branched 2-methylpentene, requiring more energy to overcome.
Solubility Energetics
Alkenes are non-polar molecules and can only form weak Van der Waals forces with other molecules.
Dissolving them in water would require breaking the strong hydrogen bonds between water molecules. The energy required to break these Hydrogen bonds is not compensated for by the formation of the weak Van der Waals forces between the alkene and the water. Therefore, the process is energetically unfavorable.