Organic compound

An organic compound belongs to the conventional chemistry of carbon-based molecular frameworks, including hydrocarbons and their functionalized derivatives. The definition is practical and historical rather than a claim that every carbon-containing substance is organic.

Carbon dioxide and simple carbonates are usually treated as inorganic. Conversely, an organic compound need not have been produced by a living organism: a laboratory synthesis can give the same molecular identity as a natural source.

A diazine is an aromatic six-membered heterocyclic compound whose parent ring contains four carbon atoms and two nitrogen atoms. The three unsubstituted parent arrangements share formula ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$ but differ in connectivity: pyridazine is the $1,2$ arrangement, pyrimidine the $1,3$ arrangement, and pyrazine the $1,4$ arrangement.^[ref]

Why only three? Fix one nitrogen at position $1$ on a six-membered ring. The other can be adjacent, separated by one intervening ring atom, or opposite. Reflection identifies the remaining apparent alternatives: positions $6$ and $2$ describe the same adjacent arrangement, while $5$ and $3$ describe the same next-nearest arrangement. Thus the distinct shortest ring separations are one, two and three bonds.

The three diazines are constitutional isomers, not different orientations of one molecule. Rotating a drawing changes its appearance without changing its atom-to-atom connections. Moving a ring nitrogen to another connectivity position instead changes which chemical compound the drawing represents. This makes the diazine family a compact example of why a molecular formula alone does not specify chemical identity.

Pyridazine is the $1,2$-diazine: an aromatic six-membered heterocyclic compound with adjacent ring nitrogen atoms and formula ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$. The systematic ring position labels refer to connectivity, not to two added substituents on an unchanged hydrocarbon ring.^[ref]

Tracing once around the ring gives the atom sequence $\mathrm{N}-\mathrm{N}-\mathrm{C}-\mathrm{C}-\mathrm{C}-\mathrm{C}$. In pyrimidine, the shortest path between the two nitrogen atoms includes one carbon; in pyrazine, it includes two. Those are different constitutional isomers, although all three have the same elemental formula.^[ref]

For a structure-reading exercise, start with a hexagon marked with two adjacent $\mathrm{N}$ labels and rotate the entire page. It remains pyridazine. Move one $\mathrm{N}$ label across a ring vertex instead, while keeping the ring connectivity interpretation, and the identity can become pyrimidine. The useful distinction is between moving a picture and changing a molecule.

Pyrimidine is the aromatic organic compound ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$ with a six-membered ring containing nitrogen at positions $1$ and $3$. It is therefore a diazine, specifically 1,3-diazine.^[ref]

Changing the nitrogen positions gives different molecules: pyridazine has the $1,2$ arrangement, while pyrazine has the $1,4$ arrangement. The same elemental formula therefore does not uniquely specify ring connectivity. In the $1,3$ arrangement, the two nitrogen atoms have one ring carbon between them along the shorter path; in the $1,2$ arrangement they are adjacent.^[ref]

Cytosine, thymine and uracil contain a pyrimidine-derived ring framework, but their added groups and bonding make them chemically distinct from unsubstituted pyrimidine. This distinguishes a parent ring structure from the biological nucleobases named by association with that structure.

An organic compound containing a carbon–boron bond.

A six-membered alternating boron–oxygen ring, commonly $(\mathrm{RBO}{)}_3$. Three-coordinate boron has a vacant p orbital that accepts oxygen lone-pair donation. This stabilizing delocalization does not imply the same aromaticity or equal bond polarity as benzene.

An organic compound containing a carbon-halogen bond. The bonding environment matters: a vinylic halide has the halogen directly attached to a double-bonded carbon, whereas an allylic halide has it attached to the adjacent saturated carbon.

An allylic halide has a halogen atom bonded to a carbon adjacent to a carbon-carbon double bond. The halogen-bearing carbon is not itself one of the two alkene carbons.

The adjacent π system can stabilize developing charge or participate in delocalized intermediates. Allylic halides can therefore undergo substitution with patterns different from an otherwise comparable saturated alkyl halide.

A vinylic halide has a halogen atom bonded directly to a carbon of a carbon-carbon double bond. Vinyl chloride is the simplest common example.

Ordinary substitution at a saturated carbon cannot be transferred unchanged to this geometry and bonding. The distinction between a vinylic halide and an allylic halide is therefore mechanistically useful, even when both molecules contain an alkene and a halogen.

Vinyl chloride is chloroethene, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHCl}$. It is a small vinylic halide whose carbon-carbon double bond can participate in addition polymerization.

Polymerization converts many vinyl chloride molecules into polyvinyl chloride, with chlorine-bearing repeating units along the carbon backbone. The resulting polymer has physical properties very different from those of the molecular monomer.