© Prentice Hall 2001Chapter 31 Thermodynamics Consider the reaction If the products are more stable...
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Transcript of © 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
© Prentice Hall 2001 Chapter 3 2
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 5
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 6
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
© Prentice Hall 2001 Chapter 3 7
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 9
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 11
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
© Prentice Hall 2001 Chapter 3 12
Kinetics You must distinguish between reaction rate
and rate constant
mA + nB sC + tD
[ ][ ][ ][ ][ ]{ }DCBAoffunction=
ΔtAΔ
m1
= kratereaction
rate constant
© Prentice Hall 2001 Chapter 3 13
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
© Prentice Hall 2001 Chapter 3 14
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
© Prentice Hall 2001 Chapter 3 17
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 18
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 19
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 20
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 21
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 22
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
© Prentice Hall 2001 Chapter 3 23
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
© Prentice Hall 2001 Chapter 3 24
Addition of Hydrogen Halides to Alkenes The more substituted carbocation is
preferred
© Prentice Hall 2001 Chapter 3 25
Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the
positive charge on the sp2 carbon of a carbocation
© Prentice Hall 2001 Chapter 3 26
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