notes_14C_str&react - Structure and Reactivity...

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Structure and Reactivity: Acidity and Basicity In this section we will analyze how structure affects acidity and basicity. Acids and bases are vital to many chemical reactions and we need to understand what properties contribute to their strength. First we will begin by understanding how a reaction occurs. When two molecules collide with proper orientation and sufficient energy they can react with one another. For example, in the self-ionization of water we see that two water molecules collide to react and form a hydroxide ion and a hydronium ion. Each reaction begins first by an electrostatic attraction between an electron rich and an electron poor region of the reactants. Hydrogen bonding occurs between the electron deficient hydrogen of one water molecule with the electron rich oxygen of another water molecule. The products form as the electron from one O-H bond becomes a lone pair on oxygen. Additionally the lone pair on the oxygen of another water molecule becomes shared in a new O-H covalent bond. Reaction Mechanism: The reaction mechanism is a step by step account of bond and electron changes. Reaction mechanisms are very useful because they provide us with predictive power and organizational information. For example, based on the previous reaction, we can predict that any oxygen, and hence the alcohol functional group, can take away a hydrogen atom. The reaction mechanism also shows us where the electrons move to and what bonds they form giving us organizational power. In chemical reactions, we see that electrons move from regions of excess to regions where they are deficient. Now, we will introduce some terms that will illustrate this topic. Electron Deficient Region: The electron deficient region serves as the electron acceptor in the reaction because it is electron poor. It may have a formal positive charge, a partial positive charge, or an open octet all of which illustrate electron deficiency. The species which gains a pair of electrons from another atom to form a new covalent bond is termed the electrophile (“electron loving”). Based on this definition this species can also be termed a
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Lewis acid. It is important to note that a new covalent bond must be formed for the species to be considered an electrophile. You may remember from general chemistry that in a proton transfer reaction the species which donates the proton is termed acid. Therefore based on this definition a Bronsted acid is a proton donor. Electron Rich Region: The electron rich region serves as the electron donor in the reaction because it is willing to share its electron density. The electron rich species can be identified based on a negative formal charge, a partial negative charge, a pi bond, or a lone pair. This species which shares a pair of electrons with another atom to form a new covalent bond is termed the nucleophile . Based on this definition this species can also be termed a Lewis base . In a proton transfer reaction this species is the proton acceptor and is termed the
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This note was uploaded on 07/27/2009 for the course CHEM 14C-14D taught by Professor Hardinger during the Fall '08 term at UCLA.

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notes_14C_str&react - Structure and Reactivity...

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