![clockwise rotation rules clockwise rotation rules](https://i.ytimg.com/vi/mmEnieXc2Q4/hqdefault.jpg)
If the lowest priority bond is not pointing to the back, mentally rotate it so that it is. IF YOU DO NOT HAVE A MODELING KIT: remember that the dashes mean the bond is going into the screen and the wedges means that bond is coming out of the screen. Then determine the direction from the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S). When using a model, make sure the lowest priority is pointing away from you. USE YOUR MODELING KIT: Models assist in visualizing the structure. Determine if the direction from 1 to 2 to 3 clockwise or counterclockwise.(it is helpful to draw or imagine an arcing arrow that goes from 1-> 2->3) Put the lowest priority substituent in the back (dashed line).If the two substituents have similar branches, rank the elements within the branches until a point of difference.Īfter all your substituents have been prioritized in the correct manner, you can now name/label the molecule R or S. If there is branching, choose the branch that is higher in priority. When looking for the first point of difference on similar substituent chains, one may encounter branching. After the first point of difference, the rest of the chain is irrelevant. Keep in mind that priority is determined by the first point of difference along the two similar substituent chains. In such a case, follow the same method as above. Remember that being double or triple bonded to an atom means that the atom is connected to the same atom twice. Therefore, 1-methylethyl ranks higher in priority than ethyl, as shown below: Connected to the first carbon atom, ethyl only has one other carbon, whereas the 1-methylethyl has two carbon atoms attached to the first this is the first point of difference. Dashes indicate pointing away from the viewer.Ī 1-methylethyl substituent takes precedence over an ethyl substituent.Wedges indicate coming towards the viewer.When looking at a problem with wedges and dashes, if the lowest priority atom is not on the dashed line pointing away, the molecule must be rotated. If it is going clockwise, then it is an R-enantiomer If it is going counterclockwise, it is an S-enantiomer.
![clockwise rotation rules clockwise rotation rules](https://mathbitsnotebook.com/JuniorMath/Transformations/clockcounter1.jpg)
Because the 4th highest priority atom is placed in the back, the arrow should appear like it is going across the face of a clock. Then, draw an arrow from the highest priority atom to the 2nd highest priority atom to the 3rd highest priority atom.Attach the pole to the back of the clock, so that when when looking at the face of the clock the pole points away from the viewer in the same way the lowest priority substituent should point away. To understand how this works or looks, imagine that a clock and a pole. When visualizing the molecule, the lowest priority substituent should always point away from the viewer (a dashed line indicates this).When dealing with isotopes, the atom with the higher atomic mass receives higher priority.Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. The method for this is formally known as R/S nomenclature.įirst, examine at the atoms directly attached to the stereocenter of the compound. To name the enantiomers of a compound unambiguously, their names must include the "handedness" of the molecule. Sequence rules to assign priorities to substituents.