Editing Structural isomer (section)

==Isomer enumeration and counting==
Enumerating or counting structural isomers in general is a difficult problem, since one must take into account several bond types (including delocalized ones), cyclic structures, and structures that cannot possibly be realized  due to valence or geometric constraints, and non-separable tautomers.

For example, there are nine structural isomers with molecular formula [[C3H6O|C<sub>3</sub>H<sub>6</sub>O]] having different bond connectivities. Seven of them are air-stable at room temperature, and these are given in the table below.

{| class="wikitable sortable skin-invert-image"
|+Compounds with molecular formula C<sub>3</sub>H<sub>6</sub>O
! Name !! class="unsortable" | [[Molecular structure]] !!  [[Melting point|Melting<br>point]] (°C) !! [[Boiling point|Boiling<br>point]] (°C) !!  class="unsortable" | Comment 
|-
|valign=top| [[Allyl alcohol]]
|valign=top| [[File:Allyl alcohol v2.svg|80px]]
|valign=top| –129
|valign=top| 97
|valign=top| 
|-
|valign=top| [[Cyclopropanol]]
|valign=top| [[File:Cyclopropanol.svg|40px]]
|valign=top| 
|valign=top| 101–102
|valign=top| 
|-
|valign=top| [[Propionaldehyde]]
|valign=top| [[File:Propanal-skeletal.png|60px]]
|valign=top|  –81
|valign=top| 48
|valign=top| Tautomeric with prop-1-en-1-ol, which has both [[Cis–trans isomerism|''cis'' and ''trans'' stereoisomeric forms]]
|-
|valign=top| [[Acetone]]
|valign=top| [[File:Acetone-2D-skeletal.svg|60px]]
|valign=top|  –94.9
|valign=top| 56.53
|valign=top| Tautomeric with propen-2-ol 
|-
|valign=top| [[Oxetane]]
|valign=top| [[File:Oxetane.png|40px]]
|valign=top|  –97
|valign=top| 48
|valign=top| 
|-
|valign=top| [[Propylene oxide]]
|valign=top| [[File:PropyleneOxide.png|60px]]
|valign=top| –112
|valign=top| 34
|valign=top| Has two [[enantiomer|enantiomeric forms]]
|-
|valign=top| [[Methyl vinyl ether]]
|valign=top| [[File:Methyl vinyl ether.svg|80px]]
|valign=top|  –122
|valign=top| 6
|valign=top| 
|}

Two structural isomers are the [[enol]] [[tautomer]]s of the [[carbonyl]] isomers (propionaldehyde and acetone), but these are not stable.<ref>[[CRC Handbook of Chemistry and Physics]] 65th ed.</ref>