New Bouncing Curved Arrow Technique for the Depiction of Organic

Dec 1, 2009 - By removing this stumbling block and replacing it with a clear demonstration of the utility of curved arrows to describe regiochemistry ...
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In the Classroom

New Bouncing Curved Arrow Technique for the Depiction of Organic Mechanisms Andrei R. Straumanis Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424 Suzanne M. Ruder* Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284; *[email protected]

Traditionally, many organic chemistry students prepare for exams by memorizing copious quantities of material (1). This is done despite the instructor’s advice that students should strive for a conceptual understanding of reactions and mechanisms, rather than trying to memorize hundreds of reactions. The depiction of electron movement using curved arrows is a primary tool for developing this conceptual understanding. Curved arrow notation, first introduced in 1922 by Sir Robert Robinson (2), can reduce the quantity of rote memorization in an organic chemistry course by giving students a logical means to predict products and mechanisms. In pursuit of such understanding, mainstream textbooks (3, 4) and popular supplements (5–8), computer-aided tutorials (9, 10), and online homework systems (11) all emphasize the use of curved arrows. Despite this, there is evidence that many students have difficulty using the curved arrow notation (12), resulting in an inability to think through fundamental mechanisms and solve problems. Without this tool and the conceptual understanding that goes with it, organic chemistry can seem to students like a multitude of unrelated reactions. Thus, any technique that helps students use and understand curved arrows is expected to advance their conceptual understanding of the subject. This article describes a new curved arrow technique that helps clarify issues of regiochemistry in electrophilic addition and substitution mechanisms and serves as a way to explicitly depict carbocation rearrangements. Students, who nicknamed these curved arrows “bouncing” curved arrows, prefer them to conventional curved arrows for describing electrophilic addition reactions. This feedback, our success using these arrows, and the positive response we have received from instructors using them in the workbook Organic Chemistry: A Guided Inquiry (13) suggest that other faculty may find this new teaching tool useful in their classrooms.

The use of curved arrows becomes more complicated when reactions are introduced. Because sigma bonds are being broken and atoms are moving in relation to one another, students have the tendency to use curved arrows to show the more concrete movement of atoms rather than electrons. For example, when curved arrows are used to depict an acid–base reaction (correct representation shown in eq 2), many students incorrectly use the curved arrow to show where the hydrogen moves, as in eq 3.



H Br



∙ HO

∙

H Br

HOH ∙

∙ HO

Br

∙

∙

(3)

Bouncing Curved Arrow Notation We have found that students are especially confused by the traditional curved arrow representation of electrophilic addition reactions, particularly those involving an unsymmetrical alkene (eq 4). Formation of both of the two possible cationic intermediates shown in eq 4 can be described by the same curved arrow notation. This conventional arrow does not describe which carbon atom of the alkene makes a bond to the hydrogen. The misconception regarding atom movement described above, combined with student desire to use curved arrows to represent the regiochemistry of the reaction, commonly leads to errors such as the one shown in eq 5.

Background ∙ H Br

In a typical introductory organic chemistry course, curved arrows are first used to describe electron changes in the context of resonance structures, as in eq 1: O



O ∙



vs

∙

H ∙

H



∙

(2)

∙ H Br

∙

Br



∙

(4)

(5)

(1)

This use of curved arrows is comparatively simple for students to understand, because no sigma bonds are broken and atoms stay in the same relative positions.

A novel solution to this problem employs a curved arrow technique called “bouncing” curved arrows (see eq  6). The physical significance of bouncing curved arrows, along with all curved arrows (14), is open to debate. For most organic chemists, however, curved arrows have become standard practice

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In the Classroom

for explaining mechanisms of reactions. We have found that bouncing curved arrows more explicitly describe bond formation in reactions with the possibility of different regioisomeric products. The bouncing arrow in eq 6 indicates explicitly that the initial step in the addition of HBr involves bond formation between the less-substituted alkene carbon and the electrophile (H). Bouncing curved arrows provide a solution to the ambiguity of the traditional curved arrows in eq 4 and can even be used to discuss the complementary reaction pathway that is not favored (eq 7).

∙ H Br



∙ Br

∙

∙



(6)

∙



(7)

H

∙ H Br



H ∙

∙ Br

The bouncing curved arrows described above (e.g., eq 6) were invented to address student frustration regarding the traditional curved arrow representation of electrophilic addition reactions. Central to this frustration is confusion about what atoms are connected by the newly forming bond. If students had any prior experience with depiction of bond formation using curved arrows, it most likely involved acid–base or substitution reactions. In these reactions the curved arrow traces a path from one atom to the other atom that makes up the new bond. The reaction in eq 4 breaks this trend. Here the curved arrow indicates that a new bond forms to the H using pi electrons, but from which atom? The bouncing curved arrow in eq 6 answers this question by showing the new bond forming to the carbon the arrow “bounced” to. In our experience, bouncing curved arrows reinforce the value and versatility of curved arrow notation at a formative juncture early in the course. This leads to a greater ability and willingness to view curved arrow notation as a tool that is valuable for understanding subsequent organic reactions.

Traditional Curved Arrows ∙

∙

H

∙

H

H ∙

∙

∙

∙

∙

H ∙

Bouncing Curved Arrows

∙

∙

∙

Figure 1. Traditional and bouncing curved arrow depictions of carbocation rearrangements.

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Applications The application of bouncing curved arrows is not limited to electrophilic addition reactions. There are three key areas where we have found bouncing curved arrows useful: electrophilic addition (as described above), electrophilic aromatic substitution, and carbocation rearrangements. The latter two are discussed below. Recent research examining students’ use of curved arrow notation suggests that, although students might draw arrows to depict a mechanism, the curved arrows were meaningless to them. In other words, students simply added the arrows to make the mechanism “look real” or acceptable (15). We have observed evidence of this in our own students’ depiction of electrophilic aromatic substitution mechanisms. For example, in the mechanism for the ortho bromination of anisole (eqs 8a and 8b), if a student memorized that a methoxy group is an ortho–para director, they could correctly place the bromine in the ortho position. However, if they were relying on memory alone or if they were using a standard curved arrow as shown in eq 8a, they may incorrectly place the positive charge on the same carbon as the bromine. Bouncing curved arrows (eq 8b) help students keep track of bonds broken and bonds formed and make it clear that the pi electrons make a sigma bond between the ortho carbon and the electrophile (Br), leaving a lack of electrons on the MeO substituted carbon. OMe

OMe ∙ ∙

∙ Br Br FeBr3

OMe

∙

Br

∙ FeBr4∙

(8a)

∙ FeBr4∙

(8b)

OMe ∙ ∙

∙ Br Br FeBr3

∙

Br

Bouncing curved arrows are also useful for the depiction of carbocation rearrangements. Figure 1 shows traditional curved arrow use on the left and bouncing curved arrow use on the right for three types of carbocation rearrangements. As with electrophilic addition, the advantage of the bouncing arrow notation is that students are able to clearly see which atom is forming a new sigma bond to the cationic carbon. Student and Faculty Response To help gauge the value of bouncing curved arrows we surveyed both students and faculty. Though the results do not measure the impact on student learning, such data are in keeping with the purpose of this article: to inform instructors who may be interested in using this technique. Students in the sixth week of a second-quarter organic chemistry course were asked which of two correct representations of an electrophilic addition reaction they preferred. One representation showed ordinary curved arrows as in eq 4; the other showed a bouncing curved arrow as in eq 6. All of the students were taught the traditional curved arrow representation

Journal of Chemical Education  •  Vol. 86  No. 12  December 2009  •  www.JCE.DivCHED.org  •  © Division of Chemical Education 

In the Classroom

(eq 4) in their first-quarter organic chemistry course and had only recently, in their current course, been exposed to the bouncing curved arrow notation (eq  6). The survey was conducted anonymously during class using electronic polling devices: of 261 students who responded, 164 preferred the new bouncing curved arrow representation, 65 preferred the traditional notation that they had learned previously, and 32 had no preference. As part of our continued investigation of the effectiveness of this technique, we plan to collect data from students who are first exposed to the bouncing curved arrow notation in first-quarter organic chemistry and subsequently exposed to the traditional curved arrow notation in second-quarter organic chemistry. This will help us quantify the degree that opinion data are affected by the order in which the two types of curved arrows are introduced. Faculty who use both bouncing curved arrows and traditional curved arrows were also surveyed. Because curved arrow notation is used in the workbook Organic Chemistry: A Guided Inquiry (13), adopters of this book were asked to provide feedback about the use of bouncing curved arrows. The following are representative of the non-anonymous faculty comments. [Bouncing curved arrows] make it apparent to the student that the electrons must remain attached to one of the atoms in the original bond…. They provide a better bookkeeping method for regiochemistry. In electrophilic addition it is a way of getting [students] involved in Markovnikov’s rule.

Conclusion Our overall conclusion is that, in general, students find bouncing curved arrows to be more useful than traditional curved arrows for conceptualizing bond formation in regiospecific reactions such as electrophilic addition. Faculty surveyed agree that, for reactions including electrophilic aromatic substitutions and carbocation rearrangements, bouncing curved arrows can be a powerful teaching tool. For our own students, we find that introduction of bouncing curved arrows provides a more explicit way of describing bond formation at a critical juncture, which in turn increases student confidence in curved arrows as a valuable tool for understanding organic reactions. Given the central role of curved arrows to illustrate organic

reactions, the bouncing arrow method is expected to aid in the development of students’ conceptual understanding of organic chemistry. Acknowledgment We would like to thank FIPSE-P116B060026 for financial support of this work. Literature Cited 1. Karty, J. M.; Gooch, G.; Bowman, B. G. J. Chem. Educ. 2007, 84, 1209–1216. 2. Kermack, W. O.; Robinson, R . J. Chem. Soc. 1922, 121, 427–440. 3. Bruice, P. Y. Organic Chemistry, 5th ed.; Pearson Prentice Hall: Upper Sadler River, NJ, 2007. 4. Carey, F. A. Organic Chemistry, 7th ed.; McGraw Hill: New York, 2008. 5. Miller, A.; Solomon, P. H. Writing Reaction Mechanisms in Organic Chemistry, 2nd ed.; Academic Press: San Diego, 2000. 6. Scudder, P. H. Electron Flow in Organic Chemistry; J. Wiley and Sons: New York, 1992. 7. Klein, D. R. Organic Chemistry I as a Second Language; J. Wiley and Sons: New York, 2004. 8. Grossman, R. B. The Art of Writing Reasonable Reaction Mechanisms; Springer–Verlag: New York, 2003. 9. Turek, W. N. J. Chem. Educ. 1992, 69, 45–46. 10. Wentland, S. H. J. Chem. Educ. 1994, 71, 3–8. 11. ACE ORGANIC: Achieving Chemistry Excellence; Pearson Prentice Hall. http://aceorganic.pearsoncmg.com/nosession/about. html (accessed Oct 2009). 12. Bhattacharyya, G.; Bodner, G. M. J. Chem. Educ. 2005, 82, 1402–1407. 13. Straumanis, A. Organic Chemistry: A Guided Inquiry, 2nd ed.: Houghton Mifflin Co.: Boston, 2009. 14. Hosoya, H. J. Mol. Struc-Theochem. 1999, 461–472, 473–482. 15. Ferguson, R.; Bodner, G. M. Chem. Educ. Res. Pract. 2008, 9, 102–113.

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