Mechanism Rules evaluator to grade chemistry answers

Updated 30 Aug 2024

Use the Mechanism Rules answer evaluator (expressionOchemEvalMechanismRules) to check whether a particular heuristic rule is or is not violated by the student response in a mechanism question. When this expression is triggered, the step box that contains an error changes to red.

1. Select Grade as Correct or Grade as Incorrect, then enter the conditions you want to be checked in student answers.
2. Choose violates or does not violate.
3. Select the rule category you want evaluated.
   • general chemical rule
   • reaction-condition-specific rule
   • mechanism drawing convention
4. Select the specific rule from the category.
   General chemical rules

   Menu option	Description
   No SN2 at sp- or sp2- hybridized atom	Does the student answer include curved arrows from a nucleophile to an atom incompatible with an SN2 mechanism (SN2 reactions must occur at sp3-hybridized atoms)?
   No ionization of sp- or sp2- hybridized atom	Does the student answer result in a product that places a charge on an sp- or sp2-hybridized atom?
   No CH3+ or unstabilized 1° carbocations	Does the student answer result in a product that places a charge on a primary carbon atom?
   Violation of pKa rule (best base pKb – best acid pKa ≤ 8)	Does the student answer contain unbalanced acids and bases? Select a checkbox to ignore the first or last step of the mechanism. Check here to omit the first step of the mechanism (e.g., because a strong base initiates the reaction). Check here to omit the last step of the mechanism (e.g., because it is a workup step).
   Cationic shifts must be 1,2-shifts	Does cation rearrangement contain shifts other than 1,2-shifts?
   No zwitterions	Does the student answer include a zwitterion (such as an amino acid)?
   No multiply charged atoms	Does the student answer include an atom with a charge greater than +1 or less than -1?
   No multiply charged molecules	Does the sum of the charges on the atoms in a molecule result in a total charge greater than +1 or less than -1?
   No similarly charged compounds react with one another	Do two molecules with the same charge (i.e., both positive) react with one another? Select the checkbox to allow proton (H+) transfer steps such as:
   No X–H bond as nucleophile	Does the student indicate that an X-H bond, where X is a halogen, acts as a nucleophile? For example:
   No four-membered TS (transition states) for H+ transfer	Does the student indicate that a step involves a four-membered transition state during a deprotonation? For example:
   No dyotropic rearrangements (A–B–C–D → D–B–C–A)	Does the student indicate a step that involves multiple rearrangements in the connectivity of atoms in a single molecule? For example:
   No σ-bond metatheses or dyotropic rearrangements	Does the student indicate that a step involves multiple rearrangements in the connectivity of atoms in multiple molecules or a single molecule? For example:
   No termolecular steps	Does the student answer contain a step requiring three molecules to come together?
   No four-atom pericyclic reactions	Does the student answer contain a transition state that has a cyclic geometry and only four atoms?
   No even-electron pericyclic reactions	Does the student answer contain a pericyclic reactions (other than electrocyclic reactions) that involves an even number of pairs of electrons?

   Reaction-condition-specific rules

   Menu option	Description
   No radicals under polar conditions	Does the student answer contain a radical under conditions that do not support a radical?
   No carbocations (except iminium ions) under basic conditions	Does the student answer contain a radical under conditions that do not support a radical?
   No bases under these acidic conditions should have pKb >	Does the student answer contain a base, explicit or implied, with pKb greater than that specified? Enter a pKb value. (The student response may show the bases explicitly or may produce them in a mechanistic step but omit them from the drawing.)
   No acids under these basic conditions should have pKa <	Does the student answer contain an acid, explicit or implied, with pKa less than that specified? Enter a pKa value. (The student response may show the acids explicitly or may produce them in a mechanistic step but omit them from the drawing.)
   No SN2 under acidic conditions.	Does an SN2 reaction occur in the absence of a strong base?
   No E2 under acidic conditions.	Does an E2 reaction occur in the absence of a strong base?

   Mechanism drawing convention rules

   Menu option	Description
   No atom should receive & supply electrons	Does the student answer include curved arrows to and from the same atom?
   No resonance arrow where there should be one	Does the student use a reaction arrow to indicate movement of electrons between two resonance contributors? Select the checkbox to skip the first step of a photochemical mechanism.
   Resonance arrow where there shouldn't be one	Does the student use a resonance arrow to indicate an organic reaction?

 

See also:

Answer evaluators list (for correct or incorrect answers) | How grading evaluators work | Properties for the Marvin JS drawing tool setup