Compounds that contain the group >C = C = C<, in which three carbon atoms are linked by two adjacent double bonds. The outer carbon atoms are each linked to two other atoms or groups by single bonds. The simplest example is 1,2-propadiene, CH₂CCH₂. Allenes are dienes with typical reactions of alkenes. Under basic conditions, they often convert to alkynes. In an allene, the two double bonds lie in planes that are perpendicular to each other. Consequently, in an allene of the type R₁R₂C=C=CR₃R₄, the groups R₁ and R₂ lie in a plane perpendicular to the plane containing R₃ and R₄. Under these circumstances, the molecule is chiral and can show optical activity.