what are allenes, commulenes, biphenyls, spiran derivativs....& how are they able to show optical activity even when they don't have optically active carbon

Dear student,

The simplest cumulated diene is 1,2-propadiene, CH₂=C=CH₂, also known as allene. Indeed, cumulated dienes are often called allenes. The central carbon in such compounds is sp-hybridized (it has only two bonding partners), and the double bond array is linear as a result. Since the π-bonds of allenes are orthogonal, the planes defined by the end carbon substituents are also orthogonal. As shown in the following diagram, the overall configuration of allenes resembles that of an elongated tetrahedron. An interesting consequence of this configuration is that allenes having two different substituents on each of the terminal carbon atoms are chiral.

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