Alkenes

Alkenes 

The alkenes are the unsaturated hydrocarbons that contain one double bond. They have the general formula C_nH_2n, and the double bond is known as the ‘olefinic bond’ or ‘ethylenic bond’.

The name olefin arose from the fact that ethylene was called ‘olefiant gas’ (oil-forming gas), since it formed oily liquids when treated with chlorine or bromine. The original name given to this homologous series was olefin; but it was later decided to reserve the suffix –ine for basic substances only. Since the name olefine had gained wide usage, it was decided to compromise and called the series the olefins.

 One method of nomenclature is to name the olefin from the corresponding alkane by changing the suffix –ane of the later into –ylene, e.g., (methylene), ethylene, propylene, etc.

Another method of nomenclature is to consider ethylene as the parent substance and the higher members derivatives of ethylene. If the compound is a monosubstituted derivative of ethylene, then no difficulty is encountered in naming it; if the compound is a disubstituted derivative of ethylene isomerism is possible, since the alkyl groups can be attached to the same or different carbon atoms.

According to the I.U.P.A.C. system of nomenclature, the class suffix of the olefins is –ene, and so the series becomes the alkene series. The longest carbon chain containing the double bond is chosen as the parent alkene, the name of which is obtained by changing the suffix –ane of the corresponding alkane into –ene.

General method of preparation of the alkenes

By the action of concentrated sulphuric acid, at 160-170^oC, on primary alcohols. The acid acts as a dehydrating agent, e.g., ethylene from ethanol :

C₂H₅OH + H^+   H2SO4→ C₂H₅OH₂⁺  →  H₂O + C₂H₅+  →  C₂H₄ + H⁺

Dehydration of secondary and tertiary alcohols is best carried out using dilute sulphuric acid, since the alkenes produced from these alcohols (particularly tertiary alcohols) tend to polymerise under the influence of the concentrated acid. The yields of alkenes from secondary and tertiary alcohols are very good. Also, the ease of dehydration of alcohols is t > s >primary.

Acid-catalysed dehydration of primary alcohols normally gives 1-alkenes, but s- and t-alcohols give mixtures of alkenes which arise from removal of a proton from either adjacent carbon atom and also from rearrangements undergone by the various carbonium ion intermediates. Rearrangements, however, may be avoided by dehydrating the alcohol over heated alumina in the presence of a little pyridine.