Authoring Questions in ACE

Any ACE instructor can author his or her own questions or modify database questions. If you modify a database question, the changes will propagate into all of your courses, but not any other instructor's courses, so they will be visible to you and your students but no others. Likewise, if you author a question from scratch, the question will be visible only to you and students in your courses. (Exception: In ACE 1.7, if another instructor designates you as a coinstructor of a course, you may assign the work you do in an authoring session to that instructor. In this case, the work that you do will be visible only to students in the courses created by your coinstructor.) However, you may share your questions with other instructors if you wish to do so.

Most instructors modify database questions simply to reword the question statement or feedback more to their taste. Occasionally, however, an instructor will find an error — a correct question marked as incorrect, or vice versa, or feedback completely irrelevant to a particular response. If you find an error, by all means, feel free to correct it yourself, but please contact Bob Grossman to alert him to the error so he can correct it in the database.

Some instructors are more ambitious, wanting to author their own questions in ACE. Authoring questions in ACE is an art. As such, learning how to do it requires practice and patience. We strongly suggest that beginners find questions in ACE to use as models for new questions.

A question consists of the following parts:

• the type of question, namely:
  • Lewis structure,
  • skeletal structure,
  • conformation,
  • mapping,
  • rank,
  • multiple-choice,
  • fill-in-the-blank, or
  • mechanism (or resonance structure),
  or, new to ACE 1.7:
  • R-group,
  • text,
  • numerical,
  • complete-the-table, or
  • multistep synthesis;
• the question provenance (book, chapter, remarks);
• keywords describing this question (not used);
• the question statement, automatically formatted by ACE;
• figures;
• question data, which may be:
  • items from which to choose in the case of multiple-choice and fill-in-the-blank questions;
  • items to order in the case of rank questions;
  • table size and captions in the case of complete-the-table questions;
  • collections of R groups from which to choose replacements for generic R groups in R-group questions;
  • reaction conditions from which a student may choose in the case of multistep synthesis questions.
• evaluators (at least one).

An evaluator determines whether a student's response has a particular property. If the evaluator is satisfied that the response has the property, the evaluator provides the associated feedback (also automatically formatted by ACE) to the respondent and assigns the associated points to the response.

The order in which an author lists evaluators can be very important, because the list of evaluators acts as a hierarchical sieve; that is, ACE tests the response against each evaluator in order, and the first evaluator that the response satisfies determines the student's grade and feedback. ACE does not use evaluators subsequent to the satisfied one to evaluate the response. Consequently, if evaluator 1 is, "If the total charge on the response is not 0," then an author can assume that subsequent evaluators will encounter no response with a positive or negative total charge.

Each evaluator has associated with it a grade between 0 and 1. A response that satisfies an evaluator earns the number of points associated with the evaluator and receives the feedback associated with that evaluator.

The question statement is exactly what its name says it is. A question must have either a statement or a figure for ACE to save it. ACE formats the question statement automatically.

The question statements of fill-in-the-blank questions require a special format.

If the question set in which a question resides has a common question statement, then, when ACE displays any question in that set to a student, it appends the common question statement to the beginning of the question's own statement. If you are writing a series of questions that all use the same statement, consider putting the entire series in its own question set and writing a common question statement for the entire set. That way, you don't need to write it out for every question, and, if you decide to change it, the change will be effective in every question simultaneously.

ACE applies its formatting rules to question statements, feedback, rank and multiple-choice options, fill-in-the-blank pulldown menus, structure names, and the titles of assignments. Authors who wish to learn more about ACE's formatting rules should consult this public-access page. It has an interactive feature that allows authors to type various expressions and see how ACE will format them.

Top.

MarvinSketch has four single-bond types to indicate tetrahedral stereochemistry: bold, hashed, wavy, and straight. The first two specify configuration as R or S, the third specifies a mixture of R and S, and the last leaves the configuration unspecified. (Note: In three-dimensional structures, MarvinSketch uses the structure's 3D coordinates to determine configurations. It ignores bold, hashed, and wavy bonds.)

Authors specify double bond stereochemistry by the physical coordinates of the substituents of the double bond. MarvinSketch provides three ways for an author to indicate unspecified double-bond configuration:

• a wavy bond from a double-bond atom to a substituent;
• a 180° bond angle at a double-bond atom;
• MarvinSketch's special "double cis or trans" bond.

The stereochemistry matching rules are:

• If a tetrahedral stereocenter or double bond in an author's structure has a specified configuration, then the configuration of the corresponding tetrahedral stereocenter or double bond in the student's structure must be the same for the two structures to match.
• If a tetrahedral stereocenter or double bond in an author's structure has an unspecified configuration, then the configuration of the corresponding tetrahedral stereocenter or double bond in the student's structure has no bearing on whether the structures match.

All evaluators that compare author structures to student structures (except the skeletons mode of Contains) use these matching rules. The most frequently used evaluators for this purpose are Is and Contains, but the rules also apply to MapProperty, MechProdStart, and a few others.

If a student's response should have a particular configuration at a stereocenter, it is useful to test both for particular configurations as well as for no configuration at all. For example, if the student is supposed to draw (S)-2-butanol, it is useful to write a correct-response evaluator for (S)-2-butanol and incorrect-response evaluators for (R)-2-butanol, (RS)-2-butanol (wavy bond), and 2-butanol with no configuration specified. Each incorrect-response evaluator should have different feedback, because the intellectual error that the student has made in each case is different. It is important to remember that the order of the evaluators matters. If the unspecified 2-butanol evaluator comes first, then no response will satisfy the (R)- and (RS)-2-butanol evaluators, because any response that would have satisfied one of those evaluators would have already satisfied the unspecified 2-butanol evaluator. The unspecified-configuration evaluator should always follow the specified-configuration evaluators.

Authors should also note the flag in the Is evaluator that allows them to determine how many times a particular structure or its enantiomer appears in a response. This flag is especially useful in draw-the-product questions when the reaction is diastereoselective but not enantioselective.

• Every atom and bond in S is present in A. (Hydrogen atoms in S must be explicit for ACE to search for them in A, but implicit H atoms in S may be explicit or implicit in A.)
• Each bond in S has the same order as the corresponding bond in A.
• Any stereochemical configurations indicated in S are the same in A.

  In ACE 1.6 and prior, all charges, isotopes, and radical states in S must be identical in A. Starting in ACE 2.0, the question author specifies whether charges, isotopes, or radical states indicated in S may differ from those in A.

Compound A contains a skeleton S if each of the following conditions is satisfied.

• Every atom and bond in S is also present in A. These atoms and bonds in A are called "skeletal". (Hydrogen atoms in S must be explicit for ACE to search for them in A, but implicit H atoms in S may be explicit or implicit in A.)
• Each skeletal bond in A has the same order as or a higher order than the corresponding bond in S. (ACE regards double and aromatic bonds as identical for this purpose.)
• For every skeletal C atom in A that is attached to another C atom, both the bond and the latter C atom are skeletal. (In other words, no skeletal C atom in A may be attached to a nonskeletal C atom, and every bond between skeletal C atoms in A must be present in S.)

  In ACE 1.6 and prior, charges, isotopes, and radical states in S may be different in A. Starting in ACE 2.0, the question author specifies whether charges, isotopes, or radical states indicated in S may differ from those in A.

If you wish to see whether a particular substructure is contained in a response, and you wish to exclude substitution at a particular atom in the substructure, draw explicitly all the H atoms attached to that atom.

One very useful feature of the Contains substructure search is that you may search for special atom and bond types such as "any" atoms, aromatic or nonaromatic C, single-or-double bonds, etc. See the JChem query guide for more information.

We have created an interactive Web page where you can draw a substructure or skeleton, enter various responses, and see which responses contain the substructures or skeletons.

In a Lewis structure question, a student responds not with ACE's standard structure-drawing applet, MarvinSketch, but with a stripped-down version that we wrote ourselves, which we call LewisSketch. LewisSketch is not capable of drawing stereochemistry, and it does not highlight valence errors or show automatically the appropriate number of H atoms on heteroatoms. However, LewisSketch has one feature that MarvinSketch lacks, and that is the ability to add unshared electrons to each atom in the structure.

ACE provides the following evaluators for Lewis structure questions.

• LewisIsomorph determines, "If a Lewis structure response is (or is not) identical to the Lewis structure provided by the author...." All atoms, bonds, charges, and unshared electrons must match exactly.
• Is determines, "If no (or exactly one) compound in the response is identical to the author's compound, either enantiomer of it, or a resonance structure of it, or has the same σ-bond network...." (This evaluator has six ways that the number can be measured, but the only ones of interest for Lewis structure questions are "no" or "exactly one".) The σ-bond network option is especially useful for Lewis structure questions. However, this evaluator cannot assess the unshared electrons in a LewisSketch response. Authors using Is in a Lewis structure question should check the "eschew normalization" option; otherwise, different resonance structures may be considered identical.
• LewisValenceTotal determines, "If the total number of valence electrons in a Lewis structure response equals (or does not equal) the number calculated from the structure and formal charges...."
• LewisFormalCharge determines, "If any (or no) atoms in a Lewis structure response have an incorrect formal charge...." The evaluator calculates the appropriate formal charge from the element's group number and its number of bonds and unshared electrons.
• LewisOuterNumber determines, "If the number of electrons in the outer shell of every atom in a Lewis structure response is (or is not) within the maximum (usually 8), OR if the number of electrons in the outer shell of any (or every) atom of element X (or any element) in a Lewis structure response is:
  • equal to
  • greater than
  • less than
  • not equal to
  • greater than or equal to
  • less than or equal to
  than a number chosen by the author...."
• LewisElecDeficientNumber determines, "If the number of electron-deficient atoms of a particular element (or any element) in a Lewis structure response is:
  • equal to
  • greater than
  • less than
  • not equal to
  • greater than or equal to
  • less than or equal to
  than a number chosen by the author...."
• Formula determines, "If the formula of the response is (or is not)...."
• The skeletal structure evaluators Atoms, Contains, and FnalGroup are also available, but authors rarely need them.

We have found the following order of evaluators useful when writing Lewis structure questions.

1. Use LewisIsomorph to see if the response is correct.
2. Use LewisIsomorph to see if the student merely forgot to add in the unshared electrons.
3. Use Formula to see if the formula is incorrect.
4. Use Is to see if the σ-bond network of the response is incorrect.
5. Use LewisValenceTotal to see if the student has placed the incorrect total number of electrons (bonds + unshared electrons) on the structure.
6. Use LewisOuterNumber to see if the student has violated the octet rule.
7. Use LewisFormalCharge to see if any atoms have an incorrect formal charge.

   At this point, if the student's response has still not satisfied any evaluators, it is a proper resonance structure of the correct answer.

8. Use LewisIsomorph or LewisElecDeficientNumber to see how the student's resonance form differs from the correct answer, and craft feedback accordingly. For example, you may wish the student to draw the best resonance structure of an iminium ion, and the student may have drawn the C(+) form.

• Is determines, "If
  • no
  • the only
  • exactly one
  • any
  • not every
  • every
  compound in the response is identical to the author's compound, either enantiomer of it, or a resonance structure of it, or has the same σ-bond network...."

  Some comments:

  • Any charges or isotopes in the author's structure or the response must be present in the other for a match to occur.
  • Stereochemical configurations must match as described here.
  • Authors may choose to eschew normalization. Normalization carries out the following steps on both the student's response and the author's structure to ensure that equivalent structures match:
  • • converts the bonds of aromatic rings to "toilet bowls";
    • ungroups shortcut groups such as Ph and Ac;
    • removes spin state information from divalent and trivalent radicals;
    • converts ylides written in their X⁺–Y^– resonance form to the X=Y resonance form;
    • inverts the direction of stereo bonds when the student has placed the wide end at the stereocenter and the narrow end at a nonstereocenter.
    Normalization has no effect when the author is comparing σ-bond networks only.
• Contains determines, "If
  • no
  • the only
  • exactly one
  • any
  • not every
  • every
  compound in the response contains the substructure or skeleton drawn by the author...."
• Atoms, FnalGroup, NumMols, Rings, Charge, and Weight respectively determine, "If
  • the number of times a particular element appears
  • the number of times a particular functional group appears
  • the number of molecules
  • the number of rings
  • the total charge of the compounds
  • the molecular weight (or exact mass) of the compounds
  in the response is
  • equal to
  • greater than
  • less than
  • not equal to
  • greater than or equal to
  • less than or equal to
  a number chosen by the author...."

  Additional comments:

  • Rings calculates the smallest set of smallest rings, the value that an organic chemist would use.
  • In the case of FnalGroup, the author chooses from a list of about 140 functional groups. (ACE 1.6 can determine only whether the functional group is present or absent.) Try our functional group finder to see which functional groups ACE can detect and how ACE defines them.
• Formula determines, "If the formula of the response is (or is not) a formula written by the author...." The formula may include wild cards, as in C₆H₁₀O_*, where * may indicate any number, including zero.
• Chiral (new to ACE 1.7) determines, "If
  • none
  • any
  • some but not all
  • all
  of the enumerated stereoisomers of the response are
  • chiral
  • achiral
  ...." ACE generates enumerated stereoisomers from a response molecule by elaborating stereocenters with unspecified configurations into all possible stereoisomers. (Configurationally specified stereocenters remain constant.) If there is more than one response molecule, ACE enumerates the stereocenters of all of them and determines the total number of chiral and achiral compounds.
• Any response satisfies the HumanReqd evaluator (new to ACE 1.7). When ACE encounters this evaluator, it marks the response as needing an instructor to assign a grade. The instructor can see in the gradebook which responses he or she needs to grade. The instructor clicks on the student's name and uses alter to assign a grade. Use this evaluator only when there are no alternatives! Even though the instructor grades the student's response, there is no provision for her to enter feedback. Instead, after she grades the student's response, ACE encourages the student to seek feedback from her directly.

For most skeletal structure questions, we find that it is most useful to make the first two incorrect-response evaluators,

1. If the number of molecules in the response is not equal to 1 ...
2. If the total charge is not equal to 0 ...

Beyond that, authors should think about the characteristics that a correct response would have and write evaluators that test for the absence of each of these properties in turn.

The R-group question (new to ACE 1.7) is a special kind of skeletal structure question that allows instructors to deliver a different version of the same question to each student.

An R-group question has the following features:

• "Skeletal structure" is selected from the question type menu, and the "uses R groups" checkbox is selected;
• the question has at least one figure, and Figure 1 contains one or more Rⁿ (R¹, R², etc.);
• there is one question datum for each Rⁿ in Figure 1, indicating what groups may be substituted for it.

When an author selects the "uses R groups" checkbox, ACE makes visible the question data table. The author should add as many question data as there are Rⁿ in Figure 1. For each Rⁿ, the author should select one or more R-group classes, such as "small alkyl groups" or "acyl groups". When a student first views a question, ACE replaces each Rⁿ in the student's Figure 1 with one of the groups in the R-group classes that the author has selected for Rⁿ. The groups now in Figure 1 are called instantiated R groups. Figure 1 subsequently remains ///fixed for that student and that question.

Authors should write evaluators for R-group questions as if the student's response will contain the original Rⁿ. ACE will automatically account for the instantiated R groups that the student's response ought to contain when it evaluates it. For example, if the question is, "Draw the product of the following reaction," and Figure 1 contains "R¹CH₂Br + NaOCH₂R² → ", then the evaluator for the correct response would be, "If the only compound in the response is R¹CH₂OCH₂R²...", and an evaluator for the incorrect formula would read, "If the response does not have the formula C₂H₄O..." (the formula omits the Rⁿ groups). The only evaluator that does not account for the instantiated R groups that ought to be present in a student's response is the Contains evaluator. Authors of R-group questions should search only for substructures and skeletons when the search will give the same result regardless of which instantiated R group ACE may have chosen in a particular case.

• Students can use one of the chair templates available from the MarvinSketch Insert menu (Insert → Templates → Conformers). These templates already contain the 3D coordinates, so when a student adds explicit H atoms (Edit → Add → Explicit H atoms), Marvin places the H atoms in the correct axial and equatorial positions.
• A student may draw cyclohexane in two dimensions, add the explicit H atoms, and then do a molecular mechanics minimization by typing control-3 or by choosing Edit → Clean → 3D → Clean in 3D. The student will then want to rotate the structure by typing the F7 button on the keyboard or by choosing View → Transform → Rotate in 3D, clicking and holding on the structure, and dragging the cursor. When the student has achieved a satisfying view, the student should cut the structure and paste it back to retain the view after each response submission.

In the case of single-bond conformation, students must use one of the ethane templates provided by MarvinSketch. The C atoms in the ethane template are marked so that ACE knows to analyze the conformation about their bond. Currently, ACE is able to analyze the conformations of C(sp³)–C(sp³) bonds only.

Important: Once a structure is in 3D, ACE ignores bold and hashed bonds. To make these bonds have effect, the user must convert back to 2D stereochemistry by typing control-2.

ACE provides one evaluator, ConformChair, for analyzing chair conformations, and one, ConformBond, for analyzing single-bond conformations, as well as most of the evaluators for skeletal structure questions. In both conformation evaluators, the author must provide the name of the group or groups whose orientation ACE must analyze. The group name must be either:

• a shortcut group name that Marvin recognizes when you type it (such as Me, iBu, or OPh); or,
• a SMILES definition such as O (for OH), Br, or C(C)C (for iPr).

(You can find the SMILES definition of a group by drawing it in MarvinSketch attached to Li, choosing Edit → Source, and choosing Format → SMILES. Delete the leading [Li] to get the group's SMILES definition. Note that SMILES definitions usually omit H atoms. ACE assumes that the first atom of the SMILES definition is attached to the C of the cyclohexane ring or rotatable bond. This assumption will be valid when you use the just-described method to craft your SMILES definition.)

ConformChair analyzes the number of times a group is axial or equatorial on a particular ring, and ConformBond determines the relative orientation (eclipsed, gauche, etc.) of two groups, one on each atom of the C–C bond that derives from the original ethane template. To choose the ring whose conformation it will analyze, ConformChair first looks for a six-membered ring in which all of the ring atoms are invisibly marked (i.e., that the student copied from the Templates menu.) In the absence of that, it looks for any saturated cyclohexane ring. (Exocyclic double bonds are permitted.) In the absence of that, it looks for a saturated six-membered ring with one heteroatom, and, in the absence of that, it looks for a saturated six-membered ring with any number of heteroatoms.

Your correct-response evaluator for a conformation question should be complex: for example, "If the response is X or its enantiomer AND if the response has one axial Br group...." If this evaluator is not satisfied, then write incorrect-response evaluators that test for various ways that the structure of the response may differ from X or its enantiomer. For example, you may use Is to test if the student has drawn an incorrect diastereomer, or if the student's response has the wrong formula. Your last evaluator in this section should be, "If the response is not X or its enantiomer...." If the response does not satisfy this evaluator, then the response must have the right structure, but be in the wrong conformation. You can use the conformation evaluators to determine how the conformation might be incorrect and provide appropriate feedback.

ACE provides one evaluator, MapProperty, for analyzing mapping, as well as most of the evaluators for skeletal structure questions. MapProperty requires that the student's structure, including its stereochemistry, matches the author's structure; if it does not, ACE returns the feedback, "Please draw the structure correctly." In most mapping questions, the author explicitly provides the compound that the student should map, so there is no question of drawing the wrong compound, but, if this is not the case, the author should have ACE ascertain that the compound's structure is correct before it analyzes its mapping.

In map questions, for a match to occur, all explicit H atoms in the response must be explicit in the evaluator structure, and all implicit H atoms in the response must be implicit in the evaluator structure. To be safe, give explicit instructions to the student about which H atoms should be explicit and which should be implicit.

The author enters the options as question data. ACE creates a place for the author to enter question data when the author chooses a question type that requires them. Each option may be text or a MarvinSketch structure. If it is a MarvinSketch structure, the author also enters a name for the structure. ACE uses the name for short displays of questions, as in the list of all questions in the assignment, the homework assembly tool, and the question set view in the authoring tool. Students will see the structure (and not the name) when they respond to a question or print out an entire assignment. If the option is text, ACE will format it according to its formatting rules.

In multiple-choice questions, an author decides whether to allow a student to choose multiple options. (ACE permits it by default.)

In ranking questions, in ACE 1.6, a student must rank every option, but in ACE 1.7, an author can choose whether to allow a student to leave some items unranked. The new feature is useful for synthesis questions, where a student must choose one or more reactions from a series and order them in such a way that they will convert starting material to target.

The evaluators for multiple-choice and rank questions are self-evident and need no explanation.

We strongly urge authors to use multiple-choice questions only when no other question type will do.

A fill-in-the-blank question is simply a multiple-choice question in which the author has chosen to organize the options into pulldown menus that appear in the text of the question statement. The question data that represent fill-in-the-blank options must be text. (ACE formats them according to its formatting rules.) The notation [[ ]] indicates where in the question statement ACE should insert a pulldown menu. The double brackets contain a comma-separated list of numbers, which correspond to the question data that that menu should contain. A typical fill-in-the-blank question statement and question data might be,

The most nucleophilic halide is [[1, 2, 3, 4]] because [[5, 6, 7, 8]].

1. F^-
2. Cl^-
3. Br^-
4. I^-
5. it is most basic
6. it is least basic
7. it is most solvated
8. it is least solvated

ACE has no restrictions on how many options the author may include in each menu or their sequence among or within menus, although it is easier to write evaluators when one lists the options in the same sequence that they appear in the menus.

ACE makes one evaluator, MultipleCheck, available for fill-in-the-blank questions. (ACE 1.6 also makes MultipleNumChosen available, but there is no need to use it, because ACE checks that a student has chosen one option from each pulldown menu before it even sends the student's response to the server.) MultipleCheck determines, "If the student has (or has not) chosen options that

• are exactly the same as
• are at least
• are all among
• only partly overlap with

If the student has chosen exactly the options {4, 8} ...

You will want to use the "at least" and "partly overlap" options of MultipleCheck, not necessarily the "exactly" option, when you write evaluators for wrong answers. For example, suppose you want to write particular feedback for when the student chooses F^–. You should use,

If the student has chosen at least the option {1} ...

because the student will have chosen not only option 1, but also one of options 5–8 from the second menu. If you want to write particular feedback for when the student cites basicity as a reason for increased nucleophilicity, you should use,

If the student has chosen options that only partly overlap the options {5, 6} ...

because the student cannot choose both 5 and 6, and the student must also choose one of options 1–4.

A complete-the-table question is a variation of a text or numerical question. Students must complete a table by entering text or numbers into the cells. If they enter numbers, the numbers may be integral, decimal, or in scientific notation (e.g., 1.25e-9). ACE provides six evaluators. TableTextVal and TableNumVal evaluate the contents of specific cells or entire rows and columns; TableTextText, TableNumText, TableTextNum, and TableNumNum evaluate the contents of cells in one column when another column in the same row has a particular value. These evaluators share many variables and methods with NumberIs and TextContains.

Authors can set any of the complete-the-table evaluators to ignore empty cells, treat them as containing the empty string "", or treat them as cause for displaying an error message to the student. They can also choose to set any of the evaluators that assess text to ignore case, and they can set any of the evaluators that assess numbers to ignore cells that contain nonnumeric values, treat them as containing the value 0, or treat them as cause for displaying an error message to the student.

• its form is correct:
  • every compound is inside a rectangle, and no rectangle is empty;
  • the rectangles are connected by reaction or resonance arrows in a linear or cyclic topology (the latter may have a linear part pointing to one of the rectangles in the cyclic part, as in the initiation part of a radical chain mechanism);
  • every rectangle that points to another rectangle (and hence does not contain only products) contains at least one electron-flow arrow;
  • if all of the arrows connecting boxes are resonance arrows, one terminal box has no electron-flow arrows, and all others have at least one;
• it arrives at the specified product;
• at least one of the products of the electron-flow arrows in each step appears in the subsequent step;
• any compounds not produced by electron-flow arrows are starting materials that are available in the reaction mixture;
• particular key intermediates are present in the mechanism;
• the mechanism breaks no heuristic rules, such as "no S_N2 at C(sp²)" or "no good bases under acidic conditions".

Below is an example of a correctly drawn ACE mechanism for the reaction, "acetone + EtONa + Br₂ → bromoacetone". Note that the student has omitted the EtOH and Br^– coproducts, as per the common convention, although students may draw them if they wish, and a question author may require students to draw them. Note also that the student has chosen to draw the enolate in both resonance forms. ACE would consider the mechanism to be correct if the student drew just one or the other resonance form as long as the student still drew the electron-flow arrows appropriately.

It can be difficult to draw mechanisms of free-radical chain reactions in the proper form in ACE. Here is an example of a correctly drawn radical chain mechanism for the reaction, "3-bromoTHF + Bu₃SnH + cat. (BzO)₂ → THF".

Note several aspects of this drawing:

• The drawing begins with a short linear part (the initiation), and the last box of the linear part points to a box in the cyclic part (the propagation).
• This particular mechanism consists of only two propagation steps, so the boxes in this part point to each other.
• In this particular drawing, the student has chosen to draw the stoichiometric products from the propagation steps appear in their own boxes outside of the cyclic part, with a separate reaction arrow leading to them from the midpoint of the main reaction arrow of that step. However, a student could equally well include the Bu₃SnBr product in the left-hand box of the propagation part and the tetrahydofuran in the right-hand box of the propagation part. (Of course, students may omit the coproduct Bu₃SnBr if the author of the question chooses not to write an evaluator that looks for it.)
• The drawing omits the termination steps of the mechanism.
• The electron-flow arrows of each step are bidirectional. Some organic chemists draw one-electron arrows flowing in one direction only and omit the electron-flow arrows pointing in the other direction, but ACE does not understand this convention.

An author does not need to write an evaluator to determine whether the form of a mechanism response is correct. ACE automatically evaluates the form of the response when a student submits it. If ACE is unable to parse the response, it provides appropriate feedback and may highlight the offending aspect of the response (for example, a rectangle that contains no electron-flow arrows).

The evaluators that ACE makes available to authors of mechanism questions are as follows.

• MechProdStart determines, "If
  • none
  • any
  • not all
  • all
  of the starting materials (or products) specified by the author are present among the response starting materials (or products), or if the response starting materials (or products)
  • are all among
  • are not all among
  • match one-to-one with
  • do not match one-to-one with
  the starting materials (or products) specified by the author...." This evaluator defines a response product as a compound that has no electron-flow arrows touching it, and a response starting material as one that it is not produced by electron-flow arrows in prior steps.
• MechFlowsValid determines, "If, given certain permissible starting materials, the electron-flow arrows in every step lead to (or in any step do not lead to) at least one of the compounds in the subsequent step...." This evaluator also checks that the products of the electron-flow arrows do not contain any valence errors (such as pentavalent C) or any half-bonds (from improper use of one-electron arrows) and that all compounds not generated by electron-flow arrows are permissible starting materials. This evaluator will automatically generate feedback if the response satisfies it and the evaluator is in its negative form ("... any step do not lead to ..."). ACE appends the author's feedback to the automatically generated feedback.
• MechRule determines, "If a particular heuristic rule is (or is not) violated by the response...." This evaluator generates automatic feedback if it finds a violation of either of two rules. ACE appends the author's feedback to the automatically generated feedback.
• MechSubstructure determines, "If the student's response contains (or does not contain) both the substructures and the electron-flow arrows drawn by the author?" The substructure may include atoms of unspecified elements.
• MechCounter determines, "If the number of boxes, reaction arrows, or resonance and reaction arrows is
  • equal to
  • greater than
  • less than
  • not equal to
  • greater than or equal to
  • less than or equal to
  than a number chosen by the author...."
• MechTopology determines, "If the mechanism is (or is not) linear (or cyclic)...."
• The skeletal structure evaluators Is, Contains, FnalGroup, Atoms, NumMols, Charge, and Formula are also available. (The latter three are useful only for resonance structure questions.)

We have found the following order of evaluators useful for mechanism questions.

1. Determine whether the specified product of the mechanism is actually present in the response as a product. Use MechProdStart to ask, "If, given these products, not all are present in the response...." Include only products that you would require a student to draw. For example, in the mechanism for RONa + CH₃I, you would probably not require a student to draw NaI or I^–.
2. Determine whether the student uses as starting materials all of the compounds you expect the student to use.
   • If there is no ambiguity about the way that the student might draw any of the starting materials, simply use MechProdStart to ask, "If, given these starting materials, not all of them are present in the response...." Check the "resonance-permissive" box if the student may use any resonance structure of the starting material.
   • On the other hand, if there is some ambiguity about the way that a student might draw certain starting materials, you will need to use a compound evaluator. For example, consider the reaction, Me₂C=CH₂ + HBr → Me₃CBr. The student may choose to draw the first step of the mechanism with HBr or with just H⁺. In this case, the appropriate evaluator structure is, "If, given these starting materials [isobutylene], not all are present in the response, OR if, given these starting materials [H⁺ and HBr], none are present in the response, OR if, given these starting materials [Br^– and HBr], none are present in the response...." If any of these evaluators is satisfied, then the response must be incorrect. If all are not satisfied, then this aspect of the mechanism is correct.
3. Use the MechRule evaluator to determine if the class of mechanism is incorrect. For example, if the mechanism should be polar, does the student use radicals? If the reaction conditions are acidic, does the student generate strong bases?
4. Use the Is evaluator to determine if any key intermediates are absent from the mechanism.
5. Use the MechFlowsValid evaluator to determine if the response uses electron-flow arrows correctly and if all of the compounds not produced by electron-flow arrows are permissible starting materials. In this evaluator, you should list every starting material that the student might permissibly use. For example, if one of the starting materials is HBr, you should list HBr, H⁺, and Br^–. Check the "resonance-permissive" box if the student may use any resonance structure of the starting materials.
6. Use the MechSubstructure evaluator to determine if there are any specific mechanistic steps that the student may have incorrectly included or omitted. For example, you may want to look for a concerted carbonyl-enol tautomerization or a four-centered TS for proton transfer.
7. Use the MechRule evaluator to determine if the student has broken any other relevant heuristic rules. For example, does the student propose an S_N2 substitution at an sp²-hybridized C atom? Does the response have a multiply charged intermediate? The last few incorrect-response evaluators should check for violations of mechanism conventions such as the inappropriate use or disuse of resonance arrows. We usually give 50% credit if the only error is a convention violation.
8. Finally, use the Atoms evaluator to determine if the response contains any C atoms. If it does, and it has not satisfied any prior evaluators, it is correct.

Resonance structure questions that ask the student to draw electron-flow arrows are a subset of mechanism questions. In a typical resonance structure question, the author provides two boxes connected by a resonance arrow, with a structure in one of the boxes, and the student must draw both a good resonance structure and the electron-flow arrows on the given structure that lead to the resonance structure. We have found the following sequence of evaluators useful for resonance structure questions.

1. Use Is to ascertain that the student did not alter the given structure.
2. Use NumMols to ascertain that there are two structures in the response.
3. Use Charge and Formula to ascertain that the total charge and formula of the response are twice those of the given structure.
4. Use Is to determine whether every structure in the compound is identical to the given structure (i.e., whether the student simply redrew the starting structure).
5. Use Is to determine whether every structure in the response is the given structure or a resonance structure of it. If not, the student has drawn a compound that is not a resonance structure. (You may wish to use this evaluator twice in combination with MechFlowsValid: once to see if the electron-flow arrows correctly lead to the nonresonance structure, and once to see if they do not.)
6. Use MechFlowsValid to determine whether the student's electron-flow arrows lead to their resonance structure.
7. Use Is to determine whether the student has not drawn the resonance structure you have asked them to draw.
8. Use MechRule to determine whether any atom simultaneously receives and supplies unshared electrons.
9. Use Atoms to write a correct-response evaluator, "If the number of C atoms is greater than 0...."

Multistep synthesis questions have several similarities to mechanism questions.

• Responses to both consist of boxed compounds connected in a logical sequence by straight arrows.
• ACE evaluates responses to both by checking for errors; if it finds none, the response must be correct.

There are also major differences between multistep synthesis and mechanism questions, as illustrated by the response reproduced below.

• In mechanism questions, the respondent uses electron-flow arrows to indicate how compounds in each step are transformed into compounds in the subsequent step, whereas in multistep synthesis questions, the respondent uses reaction conditions that he or she chooses from a list of built-in reaction conditions provided by ACE. In the example below, every synthetic step has a reaction condition associated with it, but, if any stage in a synthesis response does not have a reaction condition associated with it, ACE applies the default reaction condition, "[simply mix]". (If the number of a reaction condition does not correspond to any of the numbers in the MarvinSketch drawing, ACE simply ignores it.)
• Responses to mechanism questions must have either a linear or cyclic (or mixed) topology, whereas multistep synthesis questions must have a linear or linear-convergent topology. The example below has linear-convergent topology, with two separate branches leading to the two compounds that combine to give the target.
• The algorithms for calculating the products of each mechanism step are rigorously defined by convention, whereas the algorithms for calculating the products of a synthetic reaction are not. It can be difficult even for a knowledgeable chemist to predict the products of a reaction in every case; it is that much more difficult to write algorithms that allow ACE to predict the product for any substrate exposed to particular reaction conditions, even given that the compounds are unlikely to be very complex.
• Multistep synthesis questions usually have many fewer evaluators than do mechanism questions.

In general, a multistep synthesis is correct if,

• its form is correct:
  • every compound is inside a rectangle, and no rectangle is empty;
  • the rectangles are connected by reaction arrows in a linear or linear-convergent topology;
  • only one rectangle has no arrows pointing away from it, and it has only a single arrow pointing to it;
• it arrives at the specified synthetic target;
• at least one of the calculated products of the reaction in each stage appears in the subsequent stage;
• any compounds not produced by reactions are starting materials that fit the parameters defined by the question author;
• the reaction in each stage is structure-, diastereo-, or enantioselective for the compound in the subsequent stage;
• the respondent does not synthesize any compounds that are defined as permissible starting materials.

The author of a multistep synthesis question defines the permissible starting materials separately from the evaluators. Permissible starting materials may be specific compounds, may have a certain number of contiguous or total C atoms, may have one or more particular functional groups, may have a certain number of rings, or may be uncharged and metal-free. The author may combine these rules in any logical way. ACE considers all compounds chosen from the reaction conditions menu to be permissible starting materials.

The evaluators that ACE makes available to authors of synthesis questions are as follows.

• SynthTarget determines, "If the synthesis arrives at the indicated target...."
• SynthScheme determines, "If the reaction conditions acting on compounds in every step lead to (or in any step do not lead to) at least one of the compounds in the subsequent step...." This evaluator also determines whether any compounds not produced by a reaction of a previous step do not satisfy the author's rules about permissible starting materials or are too unstable to be allowed as starting materials (formyl halides, 1-haloalcohols, etc.). This evaluator automatically generates feedback if the response satisfies it and the evaluator is in its negative form ("... any step do not lead to ..."). ACE appends the author's feedback to the automatically generated feedback.
• SynthSelective determines, "If the reaction conditions acting on compounds in any step lead to (or in no steps lead to) structural isomers (usually regioisomers), diastereomers, or enantiomers of the compounds shown...." The question author chooses which type of selectivity ACE should examine. Because we have been unable to write reaction algorithms that predict the correct product selectivity in all cases, this evaluator gives the question author the option of excluding a particular synthetic reaction from consideration if it appears in the synthesis. This evaluator will automatically generate feedback if the response satisfies it and the synthetic scheme is unselective for one of the above-mentioned reasons. ACE appends the author's feedback to the automatically generated feedback.
• SynthEfficiency determines, "If the synthesis includes the preparation of compounds that are permissible starting materials...."
• SynthOneRxn determines, "If the synthesis includes a particular synthetic step...."
• SynthStart determines, "If
  • none
  • any
  • not all
  • all
  of the starting materials specified by the author are present among the response starting materials, or if the response starting materials
  • are all among
  • are not all among
  • match one-to-one with
  • do not match one-to-one with
  the starting materials specified by the author...." This evaluator defines a response starting material as one that it is not produced by the reaction of the prior step.
• SynthSteps determines, "If the number of linear or total synthetic steps is
  • equal to
  • greater than
  • less than
  • not equal to
  • greater than or equal to
  • less than or equal to
  than a number chosen by the author...."
• The skeletal structure evaluators Is, Contains, FnalGroup, NumMols, and Atoms are also available.

Instructors assembling an assignment that contains one or more synthesis questions may allow students to choose from any of the reaction conditions in the database, or they may restrict the reaction conditions available to the students.

When writing multistep synthesis questions, the author should make liberal use of the synthesis calculator to see if ACE's reaction definitions along the most likely synthetic route correctly predict the products of each step. The author may wish to use the SynthOneRxn evaluator to work around some of the selectivity prediction problems that may arise for particular reactions such as electrophilic aromatic substitutions.

If you want us to add a reaction condition to the menu, or if you find an error in the prediction of a particular reaction's products, please contact Bob Grossman.