11 - Chapter 11 Making Predictions Based on Mechanistic...

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Chapter 11 – Making Predictions Based on Mechanistic Reasoning: Reactions of alkenes Our journey through organic chemistry is aimed at understanding chemical reactivity based on mechanistic models. These models facilitate chemical predictions, which is what makes this approach so meaningful. Although our course cannot possibly teach you all of the world’s organic chemistry, we can provide you with a logical, analytical approach that will allow you to understand all of the world’s organic chemistry. In this chapter we build upon our mechanistic model of the addition of electrophiles to alkenes. This model will help us to rationalize and even predict outcomes that we have not explicitly been taught. The content of this chapter is a prime example that employs “reasoning by analogy” as a means of solving problems that we have never seen before. Predicting transition state energies and transition state structures The relative energies of transition state structures (TS ) provide insight into relative reaction rates, and as we saw in the last chapter, they enable predictions as to which of two possible pathways is followed. Because of this, transition states are important in understanding and predicting reactivity. Although TS and their relative energies can accurately be calculated with modern computational tools, they cannot be studied experimentally because they have to finite lifetime. For many purposes it is impractical to apply sophisticated calculations when qualitative analysis will serve the purpose; thus, it would be worthwhile to have some rules to facilitate our reasoning about transitions states. In the TS , bonds are being made and bonds are being broken. How far along are the bond-making and bond-breaking processes at the point the transition state is crossed? Are bonds nearly fully developed or just beginning to form? Are bonds nearly fully broken, or are they just beginning to weaken? The Hammond postulate helps to answer these questions. Remember that the transition state provides a bridge between two minima (i.e., states) on a potential energy surface. We’ll call these two minima beginning state and ending state. The Hammond postulate tells us that of the two minima connected via the TS , the structure of the transition state will more closely resemble the minima that it is closer to in energy. This might sound complicated, but in reality there are only two possibilities to consider. Let’s examine them with reference to the diagram below. Case 1: the beginning state is higher than the ending state 1 – In this case, the Hammond postulate tells us that the TS will more closely resemble the beginning state. The “bump” in energy associated with the TS must be closer to the beginning state. There’s just no other way to draw the diagram. We call this TS “early” since the structure of the transition state has not evolved far from its starting point.
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11 - Chapter 11 Making Predictions Based on Mechanistic...

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