Stereochemical labels such as R/S and E/Z require a reproducible way to rank substituents. The Cahn-Ingold-Prelog (CIP) priority rules provide that ranking from atomic connectivity.

Rule 1: compare the directly attached atoms

Higher atomic number receives higher priority.

For substituents beginning with

$$\mathrm{Br},\quad \mathrm O,\quad \mathrm C,\quad \mathrm H,$$

the order is

$$\boxed{\mathrm{Br>O>C>H}}.$$

Different isotopes of the same element are ranked by mass number, with the heavier isotope receiving higher priority.

Rule 2: move outward to the first point of difference

If two substituents begin with the same element, compare the atoms attached to those first atoms. List those attached atoms in decreasing atomic-number order and compare the lists term by term. If they are still tied, continue one bond farther outward until the first difference appears.

For example, compare an ethyl group, $\mathrm{-CH_2CH_3}$, with a methyl group, $\mathrm{-CH_3}$. Both begin with carbon. Excluding the atom through which each group is attached to the stereochemical center, the first carbon of ethyl is connected to

$$\mathrm{C,H,H},$$

while the methyl carbon is connected to

$$\mathrm{H,H,H}.$$

Because C outranks H at the first difference,

$$\mathrm{-CH_2CH_3>-CH_3}.$$

Rule 3: represent multiple bonds with duplicated connections

For priority comparisons, a multiple bond is treated as though each multiply bonded atom were connected to duplicate copies of the atom at the other end.

Thus a carbonyl carbon in C=O is compared as if it were attached to

$$\mathrm{O,O,\ldots},$$

whereas an ordinary alcohol-bearing carbon may be attached to

$$\mathrm{O,C,H}$$

or another appropriate set. The duplicate atoms are bookkeeping devices for ranking; they are not real additional atoms in the molecule.

Example

Compare $\mathrm{-CHO}$ and $\mathrm{-CH_2OH}$. Both substituents begin with carbon. For the aldehyde carbon, C=O is treated as connections to O, O and H, giving

$$\mathrm{O,O,H}.$$

For the alcohol-bearing carbon,

$$\mathrm{O,H,H}.$$

The first entries tie at O, but the second comparison is O versus H, so

$$\boxed{\mathrm{-CHO>-CH_2OH}}.$$

CIP rules rank substituents; they do not by themselves assign R, S, E or Z. Once priorities are known, the spatial arrangement determines the stereochemical descriptor.