Curved Arrow Press

The following are the results of a midterm survey of an organic chemistry class at the start of a second semester of organic chemistry. While this sample size is small, the class average on the ACS organic chemistry examination was near the 50th percentile (48th). The class had been using a functional group based textbook (Wade) and had covered several, but not all of the reactions in the survey.

My objective in performing the survey was to learn how well the curved arrow convention was understood by students. I had introduced a convention in which it should be possible to correctly answer all of the questions. I had performed this survey on an earlier class and I already had a similar result. I surmised that students were attempting to predict the products of the reactions despite what the curved arrows may indicate. Consequently, I modified the survey to include problems without curved arrows. Thus problems 2 and 7 in column one do not have correct and incorrect answers as there are no curved arrows to predict a correct product. Here, the students are guessing what the product could be. The same problems are repeated in columns two and three with curved arrows.

The survey was administered by going down each column and in the midst of giving the survey, I introduced ‘pre-bonds’. 'Pre-bonds" are simply a dashed line indicating where a new bond is to form.

Curved Arrow Survey Table
1		87%		87%		80%
2		3A/12B		7%		33%
3		13%		33%		53%
4		53%		53%		67%
5		27%		60%		73%
6		60%		60%		53%
7		7A/8B		73%		73%
8				73%		67%
9				27%		53%
10				87%		73%

The following explains the rational for the examples.

	Example	Interpretation
1		This is a typical Markovnikov addition reaction. The correct answer is A. This is the answer I expected students may give.
2		This is a rearrangement reaction. The correct answer is B. While this has been written clearly, one can find examples in which the end of the arrow may not point to an atom as clearly.
3		This is a typical Markovnikov-like reaction. The correct answer is B. This example uses the same curved arrow as in Example 1, but leads to a different product.
4		This is an electrophilic aromatic substitution reaction. The correct answer is A. The objective here was to get equal responses for A and B. The curved arrow itself does not distinguish between the products. This was expected to be an advanced problem and that no student was expected to know the answer and thus bias the survey.
5		This is a reduction reaction. The correct answer is A. The objective of this question was to give a product (B) that looked plausible despite the curved arrows.
6		This is a Grignard addition reaction. The correct answer is B. The objective of this question was to use a bond that students may not know to which atom the electrons remain attached. .
7		This is a counterpoint to the earlier rearrangement reaction, Example 2. The correct answer is A. While this has been written clearly, and there should not be any ambiguity, the answer reveals how well students understand the curved arrows.
8		The following reactions do not involve any ambiguity. They show whether students know how curved arrows should be used. The correct answer is B.
9		The correct answer is B. A is not correct as curved arrows do not start with carbon atoms, but many textbooks use it. While B is not the typical way this might be written, some textbooks do so, B is frequently found with dashed lines to help understand where bonds are to form.
10

With the three sets of problems, one can see little effect of the curved arrows. Even though pre-bonds remove the ambiguity of these problems, students may have been confused by them or ignored the curved arrows. The class as a whole was not correctly predicting the products. If students are expected to learn reaction mechanisms, they must first understand the curved arrow convention. It is the language of organic chemistry. From other experiments, I thought many students failed to grasp the logic of the curved arrows. My analysis of this failure led to two possible reasons. In the first, I found some uses of the curved arrows are ambiguous. Examples 1-7 are all inherently ambiguous. For example, in order to interpret the Markovnikov addition reaction in Example 1, the product of that reaction must be known a priori. The curved arrows themselves do not distinguish between the possible products. Thus, when presented with Example 3, the students are unprepared to provide a correct answer. When the class could not rely upon Markovnikov’s rule, the class gave equal weight to the choices in Example 4.

Examples 5 and 6 present students with another choice. To which of the atoms must a new bond be formed? While another rule can be applied to predict the answer, the curved arrows do not provide a clear answer.

Examples 2 and 7 both represent a correct use of the curved arrows. In Example 2, this form of the curved arrow represents a rearrangement reaction and Example 7 an elimination. However, the problem provides an option of forming an equally stable secondary carbocation. Thus, even though the curved arrow clearly indicates a rearrangement reaction, the majority of students did not use this information. The pre-bond example shows some improvement, but overall, the pre-bonds resulted in little change.

There is no agreement on how curved arrows must be used. They may be ambiguous for predicting the products of reactions. In addition, the drawing of curved arrows may be inconsistent, careless, or poorly drawn. Therefore, for many students, the curved arrows are not contributing to an understanding of the product of the reaction and the students learn a mechanism without them. Consequently, students are not learning the electron movements that guides a mechanism.

I believe the absence of clear meaning from curved arrows can have a negative impact on student learning. It teaches students that the curved arrows do not help in understanding a reaction and may be safely ignored.

If you wish to see the chemistry schemes more clearly, view the survey.pdf file and zoom in on the schemes.