10 - Chapter 10 Describing the Pathway of a Chemical...

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Chapter 10 – Describing the Pathway of a Chemical Reaction A change in structure corresponds to a chemical reaction. Reactivity is the term we use to describe the potential of a structure to undergo chemical change. Some structures are more prone to react than others. Structures that are likely to undergo chemical change are said to be reactive. Structures that are resistant to chemical change are said to be unreactive. We sometimes choose to speak about the complementary property known as stability. Structures that are highly reactive are generally unstable. Structures that are unreactive are generally stable. We want to begin to view structure from the perspective of chemical changes. Here we will begin to teach the rules that will enable you to examine a structure and make predictions about its chemical characteristics. Just like a doctor diagnoses patients, you will learn to associate certain features of structure with the tendency to undergo chemical change – it’s called the structure-reactivity relationship. structure potential energy reactivity Potential energy is the link between structure and reactivity. The greater is a structure’s potential energy, the greater is its reactivity. Understanding this link between structure and reactivity will help us decide which among several possible pathways for change is the most likely. The pathway of change describes the energetics of each structure from reactants to products. In general, reactions follow the pathway of least resistance. An example illustrating this point will emerge by the end of this chapter. More about curved arrow notation In the last chapter, we learned that the electron reconfiguration that occurs in each step of a chemical reaction is described by curved arrow notation illustrating the movement of electron pairs. Here we will learn about a connection between the set of arrows that describe this electron reconfiguration and the pathway by which chemical change takes place. CH 3 H HB r CH 3 H H Br + source sink It is sometimes the case that for certain steps of a reaction mechanism, a single curved arrow is all that is needed to fully describe the electron reconfiguration. Other steps may require a set of arrows to describe the reconfiguration. Consider our example above for the addition of H-Br across a double bond. In the first step of this mechanism, a pair of arrows was used to describe how the electrons of the double bond “attack” and form a bond with the hydrogen atom of H-Br ( first arrow ), followed by breakage of the H-Br bond (relocation of electrons from within this bond onto Br, second arrow ) to form the resulting bromide anion. The tail of the first
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arrow is placed at the source of electrons. This may be a lone pair, pi bond or sigma bond. The head of the last arrow indicates the destination or sink . The electron sink tends to either be an electronegative atom – that is, an atom that can stabilize negative charge – or an atom with an empty orbital.
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This note was uploaded on 10/02/2008 for the course CHEM 232 taught by Professor Vanderdonk during the Fall '08 term at University of Illinois at Urbana–Champaign.

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10 - Chapter 10 Describing the Pathway of a Chemical...

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