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Chapter 6: Chemical Reactivity and Mechanisms

by: Rebeka Jones

Chapter 6: Chemical Reactivity and Mechanisms CHMY 321-001

Marketplace > Montana State University > CHMY 321-001 > Chapter 6 Chemical Reactivity and Mechanisms
Rebeka Jones
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Chapter reading notes.
Organic Chemistry I
Holmgren, Steven
Class Notes
Organic Chemistry
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This 7 page Class Notes was uploaded by Rebeka Jones on Sunday October 16, 2016. The Class Notes belongs to CHMY 321-001 at Montana State University taught by Holmgren, Steven in Fall 2016. Since its upload, it has received 4 views.


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Date Created: 10/16/16
Chapter 6: Chemical Reactivity and Mechanisms Enthalpy is used to measure an exchange of energy. The change in enthalpy ( H) for any process is defined as the exchange of kinetic energy, also called the heat (q), between a system and its surroundings under constant pressure. For bond breaking reactions DH is primarily determined by the amount of energy necessary to break the bond homolytically. Homollytic bond cleavage generates two uncharged species, called radicals each of which bear an unpaired electron. - Fishhook arrows used to break bond Heterolyic bond cleavage is illustrated with two headed arrows generat ing charged species called ions. Bond dissociation energy – energy required to break a covalent bond. Heat of reaction (DH°) – total change in enthalpy for a reaction - Sign of DH° indicates the direction in which the energy is exchanged. +DH° indicates the system has increased in energy -DH° indicated the system has decreased in energy exothermic – system gives energy to the surroundings endothermic – system receives energy from the surroundings. Entropy is the ultimate deciding factor of if a reaction will happen Entropy – measure of disorder associated wits system A process that involves an increase in entropy is said to be spontaneous Gibbs free energy – a repackaged way of expressing total entropy Second Law of thermodynamics – In order for a process to be spontaneous DG for that process must be negative Exergonic – any process with a negative DG will be spontaneous Endergonic – any process with a positive DG will not be spontaneous At the point in a reaction where no further change is observed the system is said to have reached equilibrium. The exact position for any reaction is described by the equilibrium Keq. ???????????? = [????????????????????????????????] [????????????????????????????????????] related to DG DG = −RTlnKeq R = 0.008314 DG determines the maximum yield of products -DG products favored (Keq > 1) +DG reactants favored (Keq <1) in order for a reaction to be useful DG must be negative 2 Theremodynamics – study of how energy is distributed under the influence of entropy. The term spontaneous does not mean that the reaction wil l occur suddenly rather that it is thermodynamically favorable; that is that the reaction favors the products Kinetics – study of reaction rates ???????????????????????????????? ???????????????? = ???? ???????????????????????????????????? k = rate constant which is specific for each reaction ▯ ▯ Rate = ???? [????] [????] = experimentally Exponents x and y are determined by how the rates are affected with different concentrations of A and B First order Second Order Third Order Doubling A doubles the Doubling A double rate Doubling A quadruples rate and B has no affect as does doubling B the rate while double B has the effect of doubling the rate (exponent of A is 2 and of B is 1) Sum of Exponents is 1 Sum of exponents is 2 Sum of exponents is 3 The rate constant depends on three factors Energy of Activation (Ea) – the energy between the reaction and products Temperature – raising temperature will make the rate increase. Rule of thumb raising the temperature by 10°C causes the rate to double. 3 Steric Considerations – the geometry of the reactants and the orientation of their collusions can have an impact on the frequency of collisions that lead to reactions. Catalyst is a component that can speed up the rate of a reaction without itself being consumed in the reaction - Lowers activation energy Kinetic refers to the rate of a reaction, while thermodynamics refers to the equilibrium concentration of reactant and products When look at an energy diagram the valleys are intermediates, while the peaks are transition states. A transition state is a state though which the reaction passes. Transition states cannot be isolated. Bonds are in the process of being broken, and or formed simultaneously. Intermediates have a certain by short lifetime. An intermediate is not the process of forming for breaking. In an exothermic process the transition state is closer in energy to the reactants than to the products and vice versa. This principal is called the Hammond Postulate. Ionic or polar reactions involve participation of ions as reactant, intermediates, or products. Ionic reactions happen when one reactant has a site of high electron density and the other reactant has a site of low electron density. 4 Nucleophilic – electron rich center - Characterized by its ability to react with a positive or partial positive charge. Electrophilic – electron deficient center - Characterized by its ability to react with a negative or partial negative charge A nucleophilic center is an electron rich atom that is capable of donation a pair of electrons (Lewis Base) - Any atom that possess a localized lone pair - Pi bonds Polarizability – described the ability of an atom to distribute its electron density evenly in response to external influences *effects strength of nucleophilic An electrophilic center is an electron-deficient atom that is capable of accepting a pair of electrons (Lewis Acid) Carbocation – positively charged carbon - Has an empty p orbital o Functions as a site for electrons Nucleophiles Electrophiles Inductive effects Inductive effects Lone pair Empty P orbital Pi bond 5 A mechanism shows how a reaction takes place. The tail of every curved arrow shows where the electrons are coming from and the h ead of every curved arrow shows where the electrons are going. Patterns of electron flow - Nucleophilic attack: characterized by a nucleophile attacking a electrophile - Loss of a leaving group: something leaves or breaks off - Proton Transfers: something is being protonated - Rearrangements – hydride or methyl shift All ionic mechanisms, regardless of how complex, are just different combinations of the four characteristic patterns seen. Concerted process – using two patterns at once. In both types of rearrangements, a secondary carbocation is converted into a more stable tertiary carbocation. In order to determine when a carbocation rearrangement will occur, we must determine whether the carbocation will become more stable via rearrangement. *carbocation rearrangements generally do not occur when the carbocation is already tertiary unless a rearrangement will produce a resonance- stabilized carbocation <- called a allylic carbocation Nucleophilic attack - Reversible arrow generally used. If the nucleophile is capable of functioning as a good leaving group after the attack has occurred 6 while an irreversible arrow is used if the nucleophile is a poor leaving group. Loss of leaving group - A reversible arrow is generally used if the leaving group is capable of functioning as a good nucleophile Proton Transfer - All proton transfers are technically reversible - Generally speaking, irreversible reactions arrows are used for reactions in which the acids differ in strength by more than 10 pKa units. - When the different in pKa values is between 5 and 10 pKa units, either reversible or irreversible reaction arrows may be used, depending on the context of the discussion Carbocation ion rearrangement - Technically an equilibrium will be establishing in which all the possible carbocations are present, with the most stable dominating the equilibrium - The different in energy between the possible carbocations is often significant, so carbocation arrangements will generally be drawn as an irreversible process. 7


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