Swimming Upstream: Relating Trapped Energy in Organic Hydrogenations to Use of Reduced Hydrocarbons as Energy Sources

Shane E. Hendrickson, Wenatchee Valley College

Summary

The following teaching-and-learning activity has been constructed to allow students to gain valuable experience in carrying out reactions in a major branch of organic synthesis while simultaneously developing perspective on the energetics of organic oxidations and reductions as pertains to global energy concerns. The teaching activity divides into four overlapping sections and is intended to encompass four three-hour second year organic laboratory sessions, four one-hour class sessions for student presentations, and approximately 20 to 35 hours of out of class student research. The extent of student involvement makes the activity suitable as a capstone project for the second year organic chemistry series for science majors. Selected topics will fall under the broad categories of current use of saturated hydrocarbons as supplies decline, generation of ethanol from corn or more cellulosic feedstock, coal gasification, utilization of methane from biological waste and alternatives to oxidation of reduced carbon.

Learning Goals

The high energy potential of reduced organic compounds has been utilized extensively since humans first discovered fire. In a modern context with a world population nearing of 6.82 billion, the consequences of the resulting oxidized products are of particular concern as global energy demand continues to increase.

A principal focus of this project then is to inform the student of the relationship between reduction status and energy potential by researching the energy required to generate common reducing agents then utilizing one of these reactive reagents. Hopefully, a major benefit of investigating the requirements for the generation of reducing agents prior to utilizing them will be a uniting of global and local perspectives - an improved capacity to view the use of materials for a particular purpose in terms of broadly inclusive total cost.

Context for Use

The teaching activity divides into four overlapping sections and is intended to encompass four three-hour second year organic laboratory sessions, four one-hour class sessions for student presentations, and approximately 20 to 35 hours of out of class student research. The extent of student involvement makes the activity suitable as a capstone project for the second year organic chemistry series for science majors.


As a part of a discussion of sustainability issues, the activity will be part of a discussion of global energy generation and use and couched in a form similar to the US energy flow trends.

In addition to an improved fundamental understanding of the trapping and flow of energy from a chemical perspective, the activities should prove useful to students in a number of discipline areas, since the downstream oxidation of ethanol to acetic acid via acetaldehyde is fundamental to understanding the health effects of ethanol and there is great agricultural and industrial interest in the generation of ethanol as a biofuel. As mentioned in the summary, the project will divide into four overlapping parts and is intended to encompass four three hour second year organic laboratory sessions, four one hour class sessions for student presentations, and approximately 20 to 35 hours of out of class student research. Again, the extent of student involvement makes the activity suitable as a capstone project for the second year organic chemistry series for science majors.

Description and Teaching Materials

The high energy potential of reduced organic compounds has been utilized extensively since humans first discovered fire. In a modern context with a world population nearing of 6.87 billion, the consequences of the resulting oxidized products are of particular concern as global energy demand continues to increase.

The downstream flow of energy in organic chemistry is in the direction of increased oxidation, culminating in CO2 with C in the +4 oxidation state. Conversely, CH4 has the highest energy potential with an oxidation number of -4. Organic compounds of intermediate oxidation have intermediate energy potentials (e.g. ΔHf° for C2H6(g) = -16.5 kcal/mol, for CH3CH2OH(g) = -56.2 kcal/mol and for CH3COOH(g)= -103.3 kcal/mol). If one wishes to lower the oxidation state of carbon there will be a cost energetically, and a number of reactive reagents have been developed for this purpose. A principal focus of this project then is to inform the student of the relationship between reduction status and energy potential by researching the energy required to generate common reducing agents then utilizing one of these reactive reagents. Hopefully, a major benefit of investigating the requirements for the generation of reducing agents prior to utilizing them will be a uniting of global and local perspectives - an improved capacity to view the use of materials for a particular purpose in terms of broadly inclusive total cost.

Another major concern in global energy is the efficiency of energy generation and use. An understanding of losses in the flow of energy from supplier to consumer is necessary if one is to be a conscientious inhabitant of Earth and is also a focus of this project. By making the synthetic target ethanol, students will be able to undertake a fermentation reaction and compare the efficiency of the hydride reduction of acetic acid to ethanol to the ultimate disproportionation that occurs when glucose (overall ox. = 0) is transformed to CH3CH2OH (ox. = -1, -3) and CO2 (ox. = +4). Combined with the research on reactive reducing species generation, changes in heats of combustion between acetic acid and ethanol, and supported by didactic lecture, the student may gain valuable insight into the critical concept of efficiency of energy conversion, and be able to apply it in the selected topics in global energy section. By finishing the project with research on related topics of energy generation and its consequences, the student will be further encouraged to unite the microscopic perspective of organic oxidation and reduction to the macroscopic realities of the world in which we reside.

In addition to an improved fundamental understanding of the trapping and flow of energy from a chemical perspective, the activities should prove useful to students in a number of discipline areas, since the downstream oxidation of ethanol to acetic acid via acetaldehyde is fundamental to understanding the health effects of ethanol and there is great agricultural and industrial interest in the generation of ethanol as a biofuel. As mentioned in the abstract, the project will divide into four overlapping parts and is intended to encompass four three hour second year organic laboratory sessions, four one hour class sessions for student presentations, and approximately 20 to 35 hours of out of class student research. Again, the extent of student involvement makes the activity suitable as a capstone project for the second year organic chemistry series for science majors.

Section (1) introduces students to the energy requirements for generating common organic reducing agents by researching current practices. Section (2) allows students to utilize a common hydride reducing agent to carry out a reduction of acetic acid to ethanol. By then comparing literature data for heat of combustion between reactants and products having just researched the energy requirements for the generation of common organic reducing agents, students will be able to describe the trapping of energy in the reduction process - swimming upstream energetically. Section (3) affords students an opportunity to carry out a fermentation study as one of two large groups. Since both the synthetic approach and fermentation will focus on the generation of ethanol, students will gain comparative insight into different energy pathways leading to the same molecule/energy potential. In the final section (4) students will select related topics in global energy production and present their results to the class. Selected topics will fall under the broad categories of current use of saturated hydrocarbons as supplies decline, generation of ethanol from corn or more cellulosic feedstock, coal gasification, utilization of methane from biological waste and alternatives to oxidation of reduced carbon.

As a part of a discussion of sustainability issues, the activity will be part of a discussion of global energy generation and use and couched in a form similar to the US energy flow trends shown below.

Estimated U.S. Energy Use in 2006 (Microsoft Word 2007 (.docx) 322kB Nov10 11)

As a part of the organic chemistry series, the activity will more specifically discuss organic oxidations and reductions, including mechanistic considerations needed to rationalize the variability of reactivity towards the different classes of reducing agents as a function of functional group.

The reducing agent employed will be sodium borohydride, NaBH4. While NaBH4 does not usually reduce acids, the reaction can be promoted by addition of an electrophile such as I2.

NaBH4 (Microsoft Word 2007 (.docx) 15kB Nov10 11)

The choice of NaBH4 is primarily one of safety and handling requirements for success since the more traditional reagents for reduction of acids (LiAlH4 or B2H6) are highly reactive and require careful treatment to maintain an inert environment. As a hydride reducing agent stable in an aqueous environment, NaBH4 allows direct comparison to reduction of glucose by yeast which utilizes the hydride reducing agent NADH. This opens the door to investigation of dihydropyridine derivatives generally. Such an approach is not new, and Hantzsch esters have been extensively investigated
Since sodium borohydride is a sodium derivative, the students will be directed to investigate the Downs Cell. A refresher in electrolytic chemistry will first be provided

Downs Cell (Microsoft Word 2007 (.docx) 37kB Nov10 11)

Along with a literature investigation of the Downs Cell, students will consider the conversion of elemental Na to NaH, and NaH to NaBH4. In particular, students will be instructed to focus on sources of H2, as this is a matter of great practical concern.

Conversion of Na to NaH and NaH to NaBH4 (Microsoft Word 2007 (.docx) 15kB Nov10 11)

In undertaking a small laboratory scale fermentation the students will divide into two groups and compare corn as a source of sugars against fruit not suitable for market donated from local orchardists. This will afford each group some measure of success (see extra credit immediately below) and address a central issue in biofuel generation, namely the use of highly productive farmland and grain crops for ethanol production. The fermentation from traditional sources will take a "home brewer's" carboy approach, will begin at the start of the quarter, and will proceed approximately 6-7 weeks. At this time the students will have an opportunity to distill the ethanol generated. As an extra credit option, students may attempt to generate ethanol from cellulosic materials by digesting starting material of their choosing with concentrated H2SO4. A cellulose source will be provided so students are able to proceed with fermentation.

As a final component to the project, students will select form a variety of related topics, spend on the order of 10 to 20 hours researching them, and then provide a factually supported, concise 10-12 minute perspective to the class. Allowing room for discussion there should be time for four students per session. Alternatively, the presentations could be extended to 15-20 minutes if one of the final lab sessions were devoted to them. To reiterate, considering the time requirements for initial research into energy requirements for the generation of reducing agents (10-15 hr), the laboratory portion (estimated as 4 x 3 hour lab periods) the individual presentations would culminate the project and the project would do nicely as a capstone for the second year organic chemistry series.

Topic List for Individual Presentations (Microsoft Word 27kB Oct31 11)
Spring Quarter Timeline for Activities (Microsoft Word 32kB Oct31 11)

Teaching Notes and Tips

The teaching and learning activities described above have not yet been employed in the classroom. Once implemented, the description above will be amended and reposted. I would be happy to work with anyone interested in employing the above exercises - in part or in their entirety - to streamline their implementation.

Assessment

References and Resources

Evergreen State College