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Graphic of two carbons facing each other. One says: My achiral twin! The other states: I don't think so.

Welcome to Config R|S Config.

This tool is intended to teach you how to recognize a stereocentre, and assign its absolute configuration (R/S). Please select the Level of Difficulty, as well as the Mode in which you will respond.

Level of Difficulty

     

Response Mode

 
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warning iconYou must select a response mode and a level of difficulty.

Creative Commons

Config R|S Config is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Co-created by: Jeffrey W. Keillor (University of Ottawa) & Christophe Pardin (Cégep de l'Outaouais).

Design and Production by: Centre for Innovative Pedagogies and Digital Learning (University of Ottawa).

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Instructions

The Config R|S Config online tool allows you to practise assigning the absolute configuration (R or S) to organic molecules.

To begin, you must first select the level of difficulty (where 1 is the easiest and 4 is the most difficult) and the practice mode, either Rookie or Pro. In Rookie mode, you must complete several sequential steps of analysis before assign the configuration as your final answer. In Pro mode, you must answer directly if the molecule is achiral, R or S.

Rookie Mode

Step 1:

Indicate if the molecule is chiral or achiral by clicking on your choice.

Reminder

A chiral molecule is one that is not superimposable on its mirror image. In organic chemistry, this is often due to the presence of an asymmetric carbon, one that bears four different substituents.

Your response will be evaluated, and the correct answer will be given immediately after you submit your response.

If the molecule is achiral, you can analyse another molecule by clicking on “Next question”. If the molecule is chiral, you can advance to Step 2.

Step 2:

Assign the relative priorities of the four substituents, on the basis of the Cahn-Ingold-Prelog rules. To do this, click on the dropdown menu beside each substituent and select the priority number (1 = highest priority, 4 = lowest priority). Submit your response by clicking on “Submit”.

Reminder

To determine the relative priorities, apply the Cahn-Ingold-Prelog rules in order.

  • Rule 1. Assign the highest priority (#1) to the atom connected directly to the central carbon (the asymmetric carbon) that has the highest atomic number, and successive priorities based on descending atomic number.
  • Rule 2. When two (or more) substituents are connected directly to the central carbon by identical atoms, consider the next atoms in each substituent, atom by atom, until you find a point of difference. At this point, the atom having the higher atomic number signifies that the entire substituent is assigned the higher priority number.
  • Rule 3. In the case of double or triple bonds, the bonding atoms are considered to be bound two or three times to the same atom with which they form the double or triple bond.
Step 3:

Indicate if an arrow that passes from the priority 1 substituent to priority 2, to priority 3, is in the clockwise (CW) or counter-clockwise direction (CCW).

Your response will be evaluated, and a drawing of the correct arrow will appear on the molecule immediately after you submit your response.

Step 4:
Reminder

To determine the absolute configuration, R or S, you must visualize the molecule as a three-dimensional model, by looking at the central carbon with the priority 4 substituent oriented behind. If the arrow is clockwise, from this perspective, the configuration is R; If the arrow is counter-clockwise, the configuration is S.

Tip: If you visualize the molecule by looking at the central carbon with the priority substituent in front, rather than behind, you can simply “invert” the convention : a clockwise arrow corresponds to S and a counter-clockwise arrow corresponds to R.

Your response will be evaluated, and the correct answer will be given immediately after you submit your response.

Step 5:

You can analyse another molecule by clicking on “Next question”.

Pro Mode

Step 1:

Indicate if the molecule is achiral, R or S, by clicking on your choice. (If you need a reminder of how this is done, see the Rookie mode!)

Your response will be evaluated, and the correct answer will be given immediately after you submit your response.

You can see how you have performed by clicking on “Success rate” in the top right of the screen. Or, you can return to the selection of difficulty level and response mode by clicking on the X in the top right corner.

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