© prentice hall 2001chapter 31 thermodynamics consider the reaction if the products are more stable...

© Prentice Hall 2001 Chapter 3 1

Thermodynamics

[ ][ ]

[ ] [ ][ ] [ ]nm

ts

KBA

DC=

reactansproducts

=eq

Consider the reaction

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

mA + nB sC + tD


Thermodynamics

There is a quantitative relationship between the Gibbs standard free energy change and the equilibrium constant

G° = -RT lnKeq

G° = -2.303RT logKeq


Thermodynamics When G° is negative the reaction is

exergonic


Thermodynamics When G° is positive the reaction is

endergonic


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


H° from Bond Energies

bonds being broken

bond of ethylene DF = 61 kcal/mol

H - Br DF = 87 kcal/mol

DFtotal = 148 kcal/mol

bonds being formed

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

C C

H

H

H

H

+ H Br C C

H

H

H

H

H

Br


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


Solvation Effects


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


Free Energy of Activation


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

norientatioproper

withfractionx

energysufficient

withfractionxcollisionsofratereactionofrate


Kinetics You must distinguish between reaction rate

and rate constant

mA + nB sC + tD

[ ][ ][ ][ ][ ]{ }DCBAoffunction=

ΔtAΔ

m1

= kratereaction

rate constant


Kinetics Information relating to the energy barrier for

a reaction is obtained from measurement of the rate constant at different temperatures

RT

E

k=Aea

Ea must be distinguished from G‡

Ea does not include entropic terms; G‡ does


Thermodynamics and Kinetics

A Bk1

k-1

At equilibrium the rate of the forward reaction equals the rate of the reverse reaction

k1[A] = k-1[B][ ][ ]AB

==1

1eq k

kK


Reaction of 2-Butene withHydrogen Bromide


Rate-Determining Step Formation of the carbocation intermediate

is the slower of the two steps It is the rate-determining step


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


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


Transition States and Intermediates

An intermediate is at a local minimum energy in the reaction

coordinate diagram may be isolated in some cases


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


Addition of Hydrogen Halides to Alkenes

H2C CH2 + HCl CH3CH2Cl

H3C

C

H3C

C

CH3

CH3

+ HBr

CH3

CH

H3C C CH3

CH3

Br

+ HI

I


Addition of Hydrogen Halides to Alkenes

What about the following reaction?

Which sp2 carbon gets the hydrogen and which gets the chlorine?

CH3 C

CH3

CH2HCl CH3 C

CH3

Cl

CH3 CH3 CH

CH3

CH2Cl+ or


Addition of Hydrogen Halides to Alkenes The more substituted carbocation is

preferred


Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the

positive charge on the sp2 carbon of a carbocation


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


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 2001chapter 31 thermodynamics consider the reaction if the products are more stable...

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