General Introduction (Solomons, 1996)
Olefins or alkenes are hydrocarbons structurally distinguished by their carbon-carbon double bond. They are generally represented by the molecular structure shown in Box 1.
Box 1. General molecular structure of olefins
Ethene or ethylene is the simplest olefin because both of its double bond carbon atoms are not substituted (i.e. they are only attached to hydrogen atoms). Propene or propylene is a gaseous mono-substituted olefin where a methyl group (-CH3) is attached to one of the double bond carbon atoms, thereby extending the hydrocarbon chain by one carbon atom. Disubstitution of both of the double bond carbon atoms leads to the emergence of two diastereomers: cis when both of the substituent groups are on the same side and trans when they are on opposite sides. Box 2 shows an example of the diastereomers (cis and trans) of 2-butene.
Box 2. Molecular structure of cis- and trans-2-butene
However, diastereomerism disappears when both of the substituent groups are attached to one carbon atom of the double bond, as in the case of isobutene or 2-methylpropene (Box 3).
Box 3. Molecular structure of 2-methylpropene
For trisubstituted and tetrasubstituted alkenes, alkene stereochemistry cannot be described anymore in terms of cis and trans; it is conveniently described by the Cahn-Ingold-Prelog convention, known as the (E)–(Z) system, on the basis of group priority. Both E and Z symbols come from German Language, i.e. the E comes from a German word “entgegen”, which means opposite, and also the symbol Z comes from a German word “zusammen”, which means together. Alkene stereochemistry is designated (E) when groups of higher priority are on opposite sides of the carbon atoms of the double bond. On the other hand, alkene stereochemistry is designated (Z) when groups of higher priority are on the same side of the carbon atoms of the double bond. Box 4 shows an example of the two diastereomers (E) and (Z) of 1-bromo-1-chloro-1-pentene.
Box 4. Molecular structure of (E)- and (Z)-1-bromo-1-chloro-1-pentene
The number of alkyl group substituents, which are connected to the carbon atoms of the double bond, affects the relative stabilities of alkenes. The greater the number of alkyl substituents on the carbon atoms of the double bond is, the greater the stability of alkene is owing to the electron-donating effect of alkyl substituents. Hence, 2-methyl-2-butene, a five-carbon trisubstituted alkene, is more stable than 2-methyl-1-butene, a five-carbon disubstituted alkene, which in turn is more stable than 3-methyl-1-butene, a five-carbon monosubstituted alkene (Box 5).
Box 5. Comparison of the relative stabilities of 2-methyl-2-butene, 2-methyl-1-butene, and 3-methyl-1-butene
Furthermore, the trans isomer of an alkene is more stable than the cis isomer because of the elimination of the steric effect in the trans isomer. The relative stabilities of alkenes can be evaluated by the heat of hydrogenation or combustion.
Alkenes occur naturally, e.g., β-pinene, a component of turpentine, and ethylene, a plant hormone for fruit ripening.
Ethylene, propylene, and butadiene are among the most important alkenes for polymer synthesis. Ethylene is also a raw material for making important chemicals such as ethanol and acetaldehyde. Propylene is used to synthesize acetone and cumene.