Stereo and Conformational Isomery

Cis-Trans System for Stereo-Isomers

Used when the two geometric forms do not interconvert, such as double-bonded structures. The cis-isomer is when the identical groups are on the same side of the double bond and the trans-isomer is when the identical groups are on the opposite sides of the double bond.

E-Z System for Stereo-Isomers

Used when the two geometric forms do not interconvert, such as double-bonded structures - similar to the cis-trans system. However, when an alkene or an unsaturated molecule has no two identical groups, cis-trans cannot be used, but E-Z still works. The IUPAC nomemclature gives the priority of the groups, and E-Z specifies wether the first two are Entgegen (opposite) or Zusammen (same side).

Syn/Anti for Conformational Isomers

Used when the two geometric forms easily interconvert, such as most open-chain single-bonded structures (rotamers).

Enantiomers

D/L System

This system pre-dates the R/S System for Enantiomers. It is based on the relationship to enantiomers of glyceraldehyde. Looking at the Fisher projection of the molecule, if the func. group is on the left side, the enantiomer is L-, as L-glucose. If the func. group is instead on the right side, then the enantiomer is D-, as D-glucose. Most natural sugars are D- and most natural amino acids are L- .

R/S System (Rectus/Sinister)

Based on the molecule's geometry with respect to a chiral center. Assigned to a molecule based on the priority rules assigned by Cahn–Ingold–Prelog priority rules (atom/group prioritized by atomic number). The prefixes ar/es are used to qualify the handedness, using ar = Rectus and es = Sinister for resp. right and left handedness, e.g. ketamine -> arketamine, esketamine

(+)/(-) System (Dextro/Laevo)

The (+) or (−) symbol specifies the optical rotation i.e. polarity, clockwise or CCW, of light as it passes through a solution of the molecule.

Dextrorotatory (+): rotates light plane clockwise.

Laevorotatory (-): rotates light plane counter-clockwise.

Constitutional / Structural Isomers

Position Isomers / Regioisomerism

Isomers differing only on the position of a functional group, substituent, or some other feature on the same "parent" structure, e.g. C₅H₁₂O describes three position isomers of a pentane parent depending which carbon of the chain of five receives the -OH: Pentan-1-ol, Pentan-2-ol and Pentan-3-ol.

Functional Isomers

Isomers which have different functional groups, resulting in significantly different chemical and physical properties, e.g. C₃H₆O describes functional isomers propanal H3C–CH2–C(=O)-H (an aldehyde), and acetone H3C–C(=O)–CH3 (a ketone).

Structural Isotopomers

Isomerism from isotope substitution when neutron count is the same, but the position of the isotope differ. e.g. ethenes with a deuterium at different positions. To not be confused with Isotopologues, where neutron count differ, e.g. ethenes with zero vs. one deuterium.

IUPAC Nomenclature of Organic Chemistry

Method of naming organic chemical compounds.

1. Identification of the most senior group. If more than one functional group, if any, is present, the one with the highest precedence should be used.
2. Identification of the ring or chain with the maximum number of senior groups.
3. Identification of the ring or chain with the most senior elements. (In order: N, P, Si, B, O, S, C).
4. Identification of the parent compound. Rings are senior to chains if composed of the same elements.
5. Identification of the side-chains. Side chains are the carbon chains that are not in the parent chain, but are branched off from it.
6. Identification of the remaining functional groups, if any, and naming them by their ionic prefixes.
7. Identification of double/triple bonds.
8. Numbering of the chain.
9. Numbering of the various substituents and bonds with their locants. If there is more than one of the same type of substituent/double bond, a prefix is added showing how many there are.