The more weakly held electrons in the π-bond are more polarizable and the vinylic bonds tend to be slightly polar. Alkyl groups are slightly electron donating toward a double bond that slightly polarizes the vinylic bond, with a small partial positive charge on the alkyl group and a small negative charge on the double-bond carbon atom. For example, propene has a small dipole moment of 0.35 D. In a cis -disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans -disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans- substituted, the dipole moment is zero. Cis -2-butene and trans -2-butene have similar van der Waals attractions, but only the cis- isomer has dipole-dipole attractions (Figure 4).

Figure 4

The effect of bond polarity is even more apparent in the 1,2-dichloroethenes, where carbon-chlorine bonds are strongly polar. The cis- isomer has a large dipole moment (µ = 2.4 D), giving it a boiling point 12 degrees higher than that of the trans- isomer (Figure 5).

Figure 5

4.5 Synthesis of Alkenes

Alkenes can be synthesized by elimination reactions. Dehydrohalogenation of alkyl halides takes place by E1 or E2 elimination mechanisms. E2 elimination of dehydrohalogenation takes place in one step, in which base abstracts a proton from one carbon and leaving group leaves the adjacent carbon.

Saytzeff Rule : A more substituted alkene is favored with small base. For example, (2-bromoethyl)cyclopentane in the presence of ethoxide (a small base) follows Saytzeff rule to give more substituted alkene as major product (Scheme 4).

Scheme 4

Hoffman Rule : A less substituted alkene is favored with bulky base. Dehydrohalogenation with a bulky base such as tert- butoxide ( t -BuOK) in tert -butyl alcohol ( t -BuOH) favours the formation of less substituted alkene. The large tert -butoxide ion seems to have difficulty in removing a β -Hydrogen atom because of greater crowding (Scheme 5).

Scheme 5

Dehydration of alcohol is another method of making alkene. Most alcohols undergo dehydration to form an alkene when heated with a strong acid. Concentrated sulfuric acid or concentrated phosphoric acid are often used as reagents. Alcohols that form stable carbocations can easily undergo dehydration. The relative ease with which alcohols undergo dehydration is as follows:

Tertiary alcohol undergoes dehydration easily as it form relatively stable tertiary carbocation. For example, cyclopentanol, 2-methylcyclohexanol and cycloxanol give the corresponding alkenes on dehydration (Scheme 6).