compounds by combining with the electrophilic reagents. Strength of the double bond (bond enthalpy, kJ mol – ) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, kJ mol – ). Orbital diagrams of ethene molecule are shown in Figs. .
Nomenclature For nomenclature of alkenes in IUPAC system, the longest chain of carbon atoms containing the double bond is selected. Numbering of the chain is done from the end which is nearer to the double bond. The suffix ‘ ene ’ replaces ‘ ane ’ of alkanes. It may be remembered that first member of alkene series is : CH (replacing n by in C n H 2n ) known as methene but has a very short life.
As already mentioned, first stable member of alkene series is C H known as ethylene (common) or ethene (IUPAC). IUPAC names of a few members of alkenes are given below : Structure IUPAC name CH – CH = CH Propene CH – CH – CH = CH But – l - ene CH – CH = CH–CH But- -ene CH = CH – CH = CH Buta – , - diene CH = C – CH -Methylprop- -ene CH = CH – CH – CH -Methylbut- -ene Problem . Write IUPAC names of the following compounds: (i) (CH ) CH – CH = CH – CH – CH CH – CH – CH C H (ii) (iii) CH = C (CH CH CH ) (iv) CH CH CH CH CH CH CH – CHCH = C – CH – CHCH (i) , -Dimethyl- , -decadiene; (ii) , , , Octatetraene; (iii) - n -Propylpent- -ene; (iv) -Ethyl- , -dimethyl-dec- -ene; Problem . Calculate number of sigma ( σ ) and pi ( π ) bonds in the above structures (i-iv).
σ bonds : , π bonds : σ bonds : , π bonds