2,3-pentadiene, a chiral allene

Alkenes with 2 C=C double bonds in a row are called cumulated dienes or allenes. When the two outer doubly bound carbons each have two different groups attached then the molecule lacks a plane of symmetry and is chiral. Mirror-image forms of CH₃CH=C=CHCH₃ are shown below. No matter how you try to line them up a methyl group superimposes on a hydrogen (and vice versa) - they are non-superimposable and, therefore, enantiomers. For an interactive version of these models click here.