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DEDUCTIVE REASONING IN ORGANIC CHEMISTRY


by Ray A. Gross, Jr.
(Associate Professor, Physical Sciences)

This article summarizes an activity initiated in the spring 2004 semester to enhance the reasoning skills of students taking the second-semester organic chemistry laboratory course, CHM 204. A current trend in chemical education is to provide students with raw data and have them analyze the data to arrive at a conclusion. The concept is not new, but many instructors are now emphasizing this kind of approach in their teaching. Accordingly, a novel set of problems that require students to apply logic and reasoning in arriving at their solutions was developed.  Students were given a minimal amount of data about an unknown hydrocarbon and tasked to determine its structure (i.e., the precise arrangement or connectivity of the carbon and hydrogen atoms).

Consider the structural formulas in Figure 1. The structure at the left shows the arrangement of the carbon and hydrogen atoms, and the simplified or bond-line structure at the right shows the connectivity of the carbon atoms. The carbon atoms are indicated by solid dots and the hydrogen atoms are omitted, because each carbon atom must have four bonds and each hydrogen atom one bond. Thus, a carbon atom (dot) with only one line emanating from it has three hydrogen atoms bonded to it; a carbon with two lines has two hydrogen atoms, and so on.

Figure 1. Structural formulas of a C10 hydrocarbon

Carbon atoms may be joined to other carbon atoms by a single bond, double bond, or triple bond. Single bonds are shown in bond-line structures by one line, double bonds by two lines, and triple bonds by three lines joining two dots. Ozone is unreactive with single bonds but cleaves double bonds (db) and triple bonds (tb) in hydrocarbons, as shown in Figure 2 by the red dashed lines   (- - - - -). The structures of the products depend upon the steps taken by the analyst to isolate them (i.e., the reaction workup). 

Figure 2. Ozonolysis of the hydrocarbon by a reductive workup

A complete cleavage by ozone (ozonolysis reaction) involves two steps: an initial addition of ozone (ozonation) followed by a workup conducted under either reductive or oxidative conditions. The analyte or compound analyzed in Figure 2 is cleaved into three new compounds by a reductive workup. The new hydrogen and oxygen atoms in the products arising from the workup are shown in red. The black partial structures of the products are anathons or degradation pieces derived from the analyte. The reductive reaction is repeated below, showing only the molecular formulas of the reactant and products.

         C10H16    C3H160   +   C6H10O3   +   CH2O2   

As a problem, students are given the latter equation and told the analyte follows the isoprene rule. From this data, they must determine the structure of the unknown to be that of the Figure 1 hydrocarbon. The isoprene rule allows the skeleton of the hydrocarbon to be determined from the number of carbon atoms, ten in this case. No single methodology exists for solving problems of this type. Most of them can be solved by multiple approaches, including trial and error. In a systematic approach, the numbers of triple bonds, double bonds, and rings, if any, in the unknown are determined by the numbers of new oxygen and hydrogen atoms that appear on the right side of the equation. Knowledge of ozonolysis reactions allows the structures of the oxygenated products as well as the structures of the underlying anathons to be deduced. The anathons are then rejoined on paper in a manner consistent with the isoprene rule to arrive at the unknown’s structure. Some problems require an analysis of both kinds of workups. Once the structure is known, it must be named by the systematic rules of organic nomenclature (e.g., the compound in Figure 1 is 3,7-dimethyl-6-octen-1-yne).

Students were given twelve graded unknowns, starting with those with a single double bond. The structural complexity of the unknowns was gradually increased, allowing students to build upon their previous week’s experience and not be overwhelmed by the immediate problem. The idea was to engage students over the entire semester and allow them to discover for themselves how to solve the problems by employing Bloom’s higher order cognitive skills of analysis, synthesis, and evaluation. Thus, students were given the content necessary to solve the problems but not the methodology.

Results were mixed. Of the 52 problems with unique solutions, one or more students solved 46, or 88%. Of 265 total problems tendered, including duplicates, only 94 were solved. Thus, the problems were difficult but solvable. Student success jumped from 11% for the first problem to 72% for the last problem of this type. Every student who commented on the usefulness of the problems stated that the exercises helped them develop problem-solving skills in some fashion. One student remarked, “…I ask myself, what combination of factors is necessary to arrive at the answer. If I know more than one way to solve a problem, I will try them all.”

The Journal of Chemical Education is reviewing an article documenting this activity, while this semester’s CHM 204 course is evaluating a new set of over 100 problems, in which the unknowns contain oxygen as well as carbon and hydrogen.

 

The Instructional Area Newsletter, Volume 20, No. 2 

Spring 2005