alkaanide ühelt ja samalt süsiniku aatomilt kahe vesiniku aatomi eemaldamise teel moodustunud kahevalentsed rühmad, nt (CH3)2C= propaan-2-ülideen, the divalent groups formed from  alkanes  by removal of two hydrogen atoms from the same carbon atom, the free valencies of which are part of a double bond, e.g.  (CH3)2C=  propan-2-ylidene