priority rules

Cahn, Ingold, Prelog sequence rules

The cis/trans naming systems works well for all disubstituted alkenes. The cis geometric isomer as we have seen has the two groups of higher priority on the same side of the molecule and in the trans geometric isomer the two groups of higher priority are opposite each other in the alkene molecule. The order of priority of the attached groups was simply based on the atomic number of the attached element. In the example shown below the two groups attached to each carbon atom in the carbon carbon double bond (C=C) are hydrogen and chlorine. Chlorine having a larger atomic number than hydrogen will be ranked higher in priority than hydrogen. So in the cis isomer the two atoms of higher priority, the chlorine atoms, are on the same side of the molecule and in the trans isomer the two higher ranking chlorine atoms are on opposite sides of the molecule or the carbon carbon double bond (C=C).

The cis/trans naming system works well for all disubstituted alkenes, but what about trisubstituted and tetrasubstituted alkene molecule? Here we use the E/Z naming system, this system of nomenclature is based on the Cahn-Ingold-Prelog rules which uses an order of priority to rank each substituent on the carbon atoms in the carbon carbon double bond (C=C).

Cahn-Ingold-Prelog sequence rules

Rule 1

Look at each atom in turn which is attached directly to each of the carbon atoms in the carbon-carbon double bond (C=C) and rank them in order of their atomic number. The higher the atomic number the higher the priority of the substituent. In the example below the first carbon atom in the alkene molecule has a bromine and an iodine atom attached to it. Since iodine has a larger atomic number than bromine it will rank higher in the priority rule. Similarily on the other carbon atom in the alkene molecule chlorin has a larger atomic number than hydrogen and so will be ranked above it in order of priority. So in the first molecule the two high priority groups are on the same side of the alkene molecule, so this will be the Z-isomer. Whereas in the other isomer the two high priority groups are on opposite side of the alkene molecule so this will be the E-isomer.

naming a tetrasubstituted alkene

If two isotopes of the same element are present then the isotope of higher mass takes higher priority. So for example deuterium (2D or 2H) has a higher priority hydrogen (1H)

Cahn-Ingold-Prelog sequence rules

Rule 2

If the two atoms attached to the carbon atom in the carbon carbon double bond (C=C) are identical then we use the atomic numbers of the next attached atoms in the chain to decide on the order of priority. All you need to do is move along the attached chain of atoms until you find a difference, at this point the atom with with highest atomic number will take priority. As an example consider the trisubstituted alkene molecule shown below. Here one of the carbon atoms in the carbon carbon double bond has a methyl (-CH3) group and an ethyl group (-CH2CH3) attached. Following the above rule will result in the ethyl group having a higher priority than the methyl group.

Cahn Ingold Prelog rules

Cahn-Ingold Prelog sequence rules

Rule 3

If atoms attached to the carbon carbon double bond (C=C) contain groups with either double or triple bonds then these groups are given a single bond equivalency, this simply means that each double or triple bonded atom is duplicated, for example:

Cahn Ingold Prelog rules

Key Points

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