35. Applying Chemical Principles

The script opens with a statement about the content being provided under a Creative Commons license and a call for donations to support MIT OpenCourseWare. The lecturer, Catherine Drennan, introduces the topic of converting CO2 into biofuels, emphasizing the importance of chemistry in solving real-world problems such as climate change. She reviews the course objectives, highlighting the goal of applying chemistry knowledge to research and everyday life. The lecture will focus on the chemistry behind CO2, its reactivity, and how it can be transformed into biofuels using both catalysts and biological pathways found in nature.

This paragraph delves into a review of the Lewis structure of CO2, discussing the formal charge and the importance of electronegativity in determining the correct structure. The correct Lewis structure for CO2 is identified, and the audience is guided through the process of elimination for incorrect structures. The lecture then transitions to the molecular geometry of CO2, using Valence Shell Electron Pair Repulsion (VSEPR) theory and Valence Bond Theory to predict the molecule's linear shape and bond angles. The polarity of CO2 is also discussed, noting that despite having polar bonds, the molecule is non-polar due to its symmetrical linear geometry.

The script continues with an exploration of CO2's stability, using the concept of Gibbs free energy (ΔG) to determine that CO2 is a stable molecule due to its negative ΔG of formation. The stability is attributed to the strong double bonds between carbon and oxygen, which require a significant amount of energy to break. The paragraph also touches on the challenges of converting CO2 into biofuels, given its non-polar and stable nature, setting the stage for the discussion of potential solutions.

The lecturer introduces the concept of using microbes and bacteria, which are adept at performing complex chemistry, to address the challenge of CO2 conversion. The process of acetogenesis is highlighted as one of the six known pathways for CO2 fixation, which can produce Acetyl-CoA, a crucial precursor for biofuels. The script suggests that understanding and potentially mimicking these natural processes could be key to developing effective CO2 conversion methods.

This section discusses the biochemical process of reducing CO2, noting the significant reduction in electron count required for the conversion to Acetyl-CoA. It is mentioned that this process involves multiple enzymes, each contributing to the stepwise reduction of CO2. The script uses the analogy of a 'kindergarten soccer game' to describe the challenge of transferring one-carbon units between enzymes, emphasizing the complexity of the process.

The role of folic acid, or folate, in the reduction of CO2 is explained, highlighting its function as a carrier for one-carbon units through the series of enzymes involved in the reduction process. The script discusses the challenges of removing the methyl group from folic acid once the reduction is complete, introducing the concept of acid-base chemistry and the importance of the protonated form of folic acid in facilitating this removal.

The script examines the impact of pH on the reactivity of folic acid, using the Henderson-Hasselbalch equation to illustrate the predominance of the deprotonated form of folic acid at physiological pH. This form is less reactive, posing a challenge for the removal of the methyl group. The need for enzymatic catalysis to overcome this challenge is emphasized, introducing the role of Vitamin B-12 in facilitating this reaction.

The script concludes with a summary of the key points discussed regarding the chemistry of CO2 and its potential conversion into biofuels. It wraps up with the announcement of the winners of the 2014 T-Shirt Clicker Competition, celebrating the educational aspect of the lecture and the engagement of the students in the topic of CO2 utilization.