Assigning Priorities: Priority Rules
In order to assign the stereochemistry of a stereocenter or double bond, the priority, or importance, of the groups attached to the stereocenter or double bond must first be be determined. Priority is assigned numerically, with 1 being the highest priority, 2 being next, etc. A stereocenter should have priorities 1, 2, 3, and 4 (because there should be four different groups at every stereocenter), while double bonds only use priorities 1 and 2 (because there are two groups attached to the atom on either side of the double bond).
There are three main priority rules.
Priority Rule #1: Priority increases as atomic number increases. In other words, the further to the right and down on the periodic table an atom is, the higher its priority will be.
The priorities of atoms commonly used in organic chemistry are shown below:
I > Br > Cl > S > O > N > C > H
This same trend can also be listed from lowest priority to highest priority, as follows.
H < C < N < O < S < Cl < Br < I
Although this trend should be sufficient for most molecules encountered in organic chemistry, it is always helpful to have a periodic table
at your disposal.
Using this trend or the periodic table, assign the priorities, 1 through 4, for the groups on the atom marked with an asterisk (*) below. For now, only consider the atoms directly bonded
to the stereocenter.
Using the trend above, Cl
has the highest priority, followed by O
, and H
. If hydrogen is one of the groups on an atom, it will always
have the lowest priority. The hydrogen was drawn in here to make it clear that it is the fourth group on the stereocenter. In future examples, you should draw in any missing hydrogens, keeping in mind the three-dimensional orientation that each hydrogen should have.
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