What is conjugation in alkenes?

“The phenomenon of the existence of alternate double and single bonds among carbon atoms is called conjugation.”

-C=C-C=C-C=C-

Example of Conjugated Compound:

e.g. 1,3-butadiene CH₂ = CH – CH = CH₂ is a conjugated compound. In it all the four carbons are sp² hybridized. Also, all carbons and hydrogens in this compound are in the same plane. All four carbons have an unhybridized p-orbital which is perpendicular to the plane of δ-bonds. The p-orbitals of carbon 2 and 3 overlap on either side so π-electrons are not restricted to two carbons rather they are attracted by all the four carbons. Thus delocalized π-bonds are formed in such conjugated compounds.
The delocalization of π-electrons is responsible for the greater stability of 1,3-butadiene. The C – C single bond length is 1.48A^o which is shorter than the normal C – C single bond length (1.54 A^o). Similarly, C = C double bond length is 1.37 A^o which is longer than normal C = C double bond (1.34A^o).