https://doi.org/10.1351/goldbook.D01753
Electrically neutral molecules carrying a positive and a negative charge in one of their major canonical descriptions. In most dipolar compounds the charges are delocalized; however the term is also applied to species where this is not the case. 1,2-Dipolar compounds have the opposite charges on adjacent atoms. The term 1,3-dipolar compounds is used for those in which a significant canonical @R05326@ form can be represented by a separation of charge over three atoms (in connection with 1,3-dipolar cycloadditions). Subclasses of 1,3-dipolar compounds include:
  1. Allyl type X=Y+–Z\(\leftrightarrow\) X–Y+=Z\(\leftrightarrow\) X+–Y–Z\(\leftrightarrow\) X=Y–Z+ (X, Z = C, N, or O; Y = N or O)
    See:
    azo imides
    ,
    azomethine imides
    ,
    azomethine ylides
    ,
    azoxy compounds
    ,
    carbonyl imides
    ,
    carbonyl oxides
    ,
    carbonyl ylides
    ,
    nitro compounds
    ,
    nitrones
  2. Propargyl type X≡N+–Z\(\leftrightarrow\) X=N+=Z\(\leftrightarrow\) X=N–Z+\(\leftrightarrow\) X–N=Z (X = C or O, Z = C, N, or O)
    See:
    azides
    ,
    diazo compounds
    ,
    nitrile imides
    ,
    nitrile oxides
    ,
    nitrile ylides
    ,
    nitrilium betaines
  3. Carbene type X:–C=Z\(\leftrightarrow\) X+=C–Z (X = C or N; Z = C, N, or O)
    See:
    acyl carbenes
    ,
    betaines
    ,
    imidoyl carbenes
    ,
    vinyl carbenes
Source:
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1333 [Terms] [Paper]