[chemical substances]
   reactive intermediates
     Class diradicals [X:, X•Y•]
       Class carbenes [X2C:, X=C:]
         1 Class alkylidenes [R2C:]
         2 Class alkylidynes [RC:•]
         3 Class acyl carbenes [acyl-C:R]
         4 Class imidoyl carbenes [RC(=NR)C:R]
         5 Class vinyl carbenes [R2C=CRC:R <-> R2C+CR=C-R]