Reactions of Alkenes. Thermodynamics and Kinetics

Reactions of Alkenes. Thermodynamics and Kinetics

Thermodynamics Consider the reaction mA + nB sC + tD [ products] [ C] [ D] t [ reactans] [ A] [ B] n s K eq = = m

If the products are more stable than the reactants, (i.e. at a lower standard free energy) then reaction favors the products and Keq > 1 Prentice Hall 2001 Chapter 3 1 Thermodynamics There is a quantitative relationship between the Gibbs standard free energy change and the equilibrium constant G = -RT lnKeq G = -2.303RT logKeq

Prentice Hall 2001 Chapter 3 2 Thermodynamics When G is negative the reaction is exergonic Prentice Hall 2001 Chapter 3 3 Thermodynamics

When G is positive the reaction is endergonic Prentice Hall 2001 Chapter 3 4 Thermodynamics G = H - TS H is the standard change in enthalpy or heat exchanged at constant pressure S is the standard change in entropy or disorder

Note the standard here usually refers to 1 molar concentration of dissolved molecules and ions 1 atm pressure for gases Prentice Hall 2001 Chapter 3 5 H from Bond Energies H H C H

+ C H Br H H H H C C H

H Br bonds being formed bonds being broken bond of ethylene DF = 61 kcal/mol H - Br DF = 87 kcal/mol DFtotal = 148 kcal/mol C - H DF = 101 kcal/mol C - Br DF = 69 kcal/mol DFtotal = 170 kcal/mol H = DF (bonds broken) - DF (bonds formed) H = 148 kcal/mol - 170 kcal/mol = - 22 kcal/mol Prentice Hall 2001 Chapter 3

6 Things to Consider When Using H as an Approximation for G If H is significantly negative, as in the case of the addition of HBr to ethylene (22 kcal/mol), S not likely to have much effect Such approximations are most reliable when considering gas phase reactions In solution there can be significant S effects as polar solvent molecules orient themselves around reactants and/or products

Prentice Hall 2001 Chapter 3 7 Solvation Effects Prentice Hall 2001 Chapter 3 8 Kinetics Knowing the G of a reaction will not tell us how fast it will occur or if it will

occur at all We need to know the rate of reaction The rate of a reaction is related to the height of the energy barrier for the reaction, G, the free energy of activation Prentice Hall 2001 Chapter 3 9 Free Energy of Activation Prentice Hall 2001 Chapter 3 10 Kinetics

The rate of a reaction depends on The rate collisions take place between reactant molecules The fraction of collisions that occur with sufficient energy to react The fraction of collisions that occur with the proper orientation to react fraction with fraction with x rate of reaction rate of collisions x sufficient energy proper orientation Prentice Hall 2001

Chapter 3 11 Kinetics You must distinguish between reaction rate and rate constant mA + nB sC + tD 1 t[ A] reactionrate= = k functionof { [ A][ B][ C][ D] } m tt rate constant Prentice Hall 2001

Chapter 3 12 Kinetics Information relating to the energy barrier for a reaction is obtained from measurement of the rate constant at different temperatures k=Ae E a RT Ea must be distinguished from G

Ea does not include entropic terms; G does Prentice Hall 2001 Chapter 3 13 Thermodynamics and Kinetics A k1 k-1 B At equilibrium the rate of the forward reaction equals the rate of the reverse reaction [ B] k1 K eq = =

k1[A] = k-1[B] k 1 [ A] Prentice Hall 2001 Chapter 3 14 Reaction of 2-Butene with Hydrogen Bromide Prentice Hall 2001 Chapter 3 15 Reaction of 2-Butene with Hydrogen Bromide Prentice Hall 2001

Chapter 3 16 Rate-Determining Step Formation of the carbocation intermediate is the slower of the two steps It is the rate-determining step Prentice Hall 2001 Chapter 3 17 Rate-Determining Step

Carbocation intermediates are consumed by bromide ions as fast as they are formed The rate of the overall reaction is determined by the slow first step Prentice Hall 2001 Chapter 3 18 Transition States and Intermediates It is important to distinguish between a transition state and a reaction intermediate

A transition state is a local maximum in the reaction coordinate diagram has partially formed and partially broken bonds has only fleeting existence Prentice Hall 2001 Chapter 3 19 Transition States and Intermediates An intermediate

is at a local minimum energy in the reaction coordinate diagram may be isolated in some cases Prentice Hall 2001 Chapter 3 20 Mechanism for Electrophilic Addition to Alkenes

Reaction of 2-butene with hydrogen bromide is typical of electrophilic addition to alkenes The reaction starts with thee slow addition of an electrophile to an sp2 carbon, resulting in formation of a carbocation The next step is the rapid addition of a nucleophile to the other sp2 carbon Prentice Hall 2001 Chapter 3 21 Addition of Hydrogen Halides to Alkenes H2C CH2 H3C

CH3CH2Cl HCl CH3 C H3C + C + HBr CH3 H3C CH3

CH3 C H C CH3 Br + HI I Prentice Hall 2001 Chapter 3 22

Addition of Hydrogen Halides to Alkenes What about the following reaction? CH3 CH3 CH3 C CH2 + HCl CH3 C

CH3 CH3 or CH3 CH CH2Cl Cl Which sp2 carbon gets the hydrogen and which gets the chlorine? Prentice Hall 2001 Chapter 3 23

Addition of Hydrogen Halides to Alkenes The more substituted carbocation is preferred Prentice Hall 2001 Chapter 3 24 Stability of Carbocations Alkyl groups (Rs) tend to stabilize the positive charge on the sp2 carbon of a carbocation Prentice Hall 2001

Chapter 3 25 Stability of Carbocations Alkyl groups are more polarizable than hydrogen (i.e. they tend to release electrons more easily than does hydrogen) Also, alkyl groups can release electrons via hyperconjugation Prentice Hall 2001 Chapter 3 26 Stability of Carbocations

Alkyl groups bonded to the sp2 carbon of a carbocation tend to spread out the positive charge, thereby stabilizing the carbocation Prentice Hall 2001 Chapter 3 27

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