Group priority

When the protonated epoxide undergoes ring-opening, the CH,OH molecule attacks from the backside of the C. The nearby, newly formed OH group hasn t moved out of the way and blocks approach from the frontside. This leads to inversion at the chiral carbon. Since there was no change in group priorities, the configuration in the product is (S). 

In certain polyfunctional compounds, an aldehyde or ketone group can also be named as a substituent on a molecule with another functional group as its root. The aldehyde carbonyl is given the prefix formyl-, and the ketone group is named 0x0- with a number to show its position in the molecule. Compounds with both an aldehyde and ketone are named as aldehydes, because aldehydes have functional group priority over ketones. A ketone containing a benzene ring is named as a -phenone. 

The group priorities are chlorine = I, ethyl = 2, methyl = 3, and hydrogen = 4. Viewing from the right side, so that the bond from the carbon to the hydrogen is pointed away from the observer, I — 2 — 3 —> I is counterclockwise, so the configuration is S. This is easier to see if the structure is rotated. 

Step [2] Orient the molecule with the lowest priority group (4) b k (on a dash), and visualize the relative positions of the remaining three groups (priorities 1,2, and 3). 

CP Stereochemistry. Cahn-Ingold-Prelog stereochemistry conventions. An lUPAC approved and widely used set of rules for assigningstereo-isomers based on atom and group priorities (see http //www.chein.qmw.ac.uk/iupaq/stereo/). 

Aromatic" compounds are those derived from benzene and similar ring systems. As with aliphatic nomenclature described above, the process is determining the root name of the parent ring determining priority, name, and position number of substituents and assembling the name in alphabetical order. Functional group priorities are the same in aliphatic and aromatic nomenclature. See p. 676 for the list of priorities. 

A system developed by chemists Cahn, Ingold, and Prelog, uses a series of steps to determine group priorities, and a definition of position based on the relative arrangement of the groups. In alkenes, the system is relatively simple ... 

As with alkenes, the orientation around an asymmetic carbon can be only one of two choices. In three dimensions, clockwise and counterclocWise are the only two directions that are definite, and even that description requires a fixed reference point. To designate configuration, the lowest (fourth) priority group is always placed farthest away from the viewer (indicated by a dashed line), and the group priorities will follow 1 to 2 to 3 in either a clockwise or a counterclockwise direction. 

The group priorities of the original drawing are shown and then they are interchanged (if necessary) in pairs to obtain the perspective with the lowest priority group oriented back. 

The R or S specification of configuration is summarized nicely by models in which group priorities based on atomic numbers follow the order of decreasing size of ball 4,3,2,1 (Fig. L-la). 

Functional group priorities carboxylic acid > acid chloride > aldehyde > nitrile > ketone > alcohol > amine > halide... 

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