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PENGANTAR REAKSI, KONTROL KINETIK DAN TERMODINAMIK
KIM
IA O
RG
AN
IK F
ISIK
Electronegativity of Some Common Elements
• Higher numbers indicate greater electronegativity
• Carbon bond to more elektronegative elements cause it has a partial positive charge (+)
2
Polarity3
4
And a few more:5
Polarizability• Polarization is a change in electron distribution as a
response to change in electronic nature of the surroundings
• Polarizability is the tendency to undergo polarization
• Polar reactions occur between regions of high electron density and regions of low electron density
6
Generalized Polar Reactions• An electrophile (like to electrons), an electron-
poor species (Lewis acid), combines with a nucleophile (like to nucleus), an electron-rich species (Lewis base)
• The combination is indicated with a curved arrow from nucleophile to electrophile
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Electrophiles & Nuclephiles8
Problem : Is BF3 electrophile or nucleophile?
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An Example of a Polar Reaction: Addition of HBr to Ethylene
10
An Example of a Polar Reaction: Addition of HBr to Ethylene
• HBr adds to the part of C-C double bond
• The bond is an electron-rich, allowing it to function as a nucleophile
• H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile
11
P-Bonds as Nucleophiles:1
2
Mechanism of Addition of HBr to Ethylene
• HBr electrophile is attacked by electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion
• Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bond
• The result is that ethylene and HBr combine to form bromoethane
• All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile
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Using Curved Arrows in Polar Reaction Mechanisms
• Curved arrows are a way to keep track of changes in bonding in polar reaction
• The arrows track “electron movement”
• Electrons always move in pairs in polar reactions
• Charges change during the reaction
• One curved arrow corresponds to one step in a reaction mechanism
15
Rules for Using Curved Arrows• The arrow goes from the nucleophilic reaction site to the
electrophilic reaction site
• The nucleophilic site can be neutral or negatively charged
• The electrophilic site can be neutral or positively charged
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Rule 1: electrons move from Nu to E1
7
Rule 2: Nu: can be negative or neutral
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Rule 3: E can be positive or neutral1
9
Rule 4: Octet rule!2
0
Practice Prob. : Add curved arrows to the reactions
21
Solution:22
Describing a Reaction: Equilibria, Rates, and Energy Changes
• Reactions can go in either direction to reach equilibrium
– The multiplication of concentrations of the products divided by the multiplied concentrations of the reactant is the equilibrium constant, Keq
– Each concentration is raised to the power of its coefficient in the balanced equation.
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aA + bB cC + dD
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b
Magnitudes of Equilibrium Constants
• Keq > 1 = most of materials are products
• Keq > 1 = most of materials are reactans
24
For example:2
5
Reactans Product
Free Energy and Equilibrium
• The ratio of products to reactants is controlled by their relative Gibbs free energy
• This energy is released on the favored side of an equilibrium reaction
• The change in Gibbs free energy between products and reacts is written as “DG”
• If Keq > 1, energy is released to the surrounding (exergonic reaction)
• If Keq < 1, energy is absorbed from the surroundings (endergonic reaction)
26
Free Energy and Equilibrium27
Numeric Relationship of Keq and Free Energy Change
• The standard free energy change at 1 atm pressure and 298 K is DGº
• The relationship between free energy change and an equilibrium constant is:
– DGº = - RT lnKeq where
– R = 1.987 cal/(K x mol) (gas constant)
– T = temperature in Kelvins
– ln = natural logarithm
• The exponential form: Keq = e-DG/RT
28
Energy Changes in Equilibrium Reactions
• Free energy changes (DGº) can be divided into– a temperature-independent part called entropy
(DSº) that measures the change in the amount of disorder in the system
– a temperature-dependent part called enthalpy (DHº) that is associated with heat given off (exothermic) or absorbed (endothermic)
• Overall relationship: DGº = DHº - TDSº
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Ethylene + HBr
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DGº = DHº - TDSº
Describing a Reaction: Bond Dissociation Energies
Bond dissociation energy (D): Heat change that occurs when a bond is broken by homolysis
The energy is mostly determined by the type of bond, independent of the molecule The C-H bond in methane requires a net heat input
of 105 kcal/mol to be broken at 25 ºC. Table 5.3 lists energies for many bond types
Changes in bonds can be used to calculate net changes in heat
32
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More homolytic BDE’s:3
4
Calculation of an Energy Change from Bond Dissociation Energies
35
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Describing a Reaction: Energy Diagrams and Transition States
37
Describing a Reaction: Energy Diagrams and Transition States
• The highest energy point in a reaction step is called the transition state
• The energy needed to go from reactant to transition state is the activation energy (DG‡)
38
39
First Step in the Addition of HBr
• In the addition of HBr the transition-state structure for the first step
• The bond between carbons begins to break
– The C–H bond begins to form
– The H–Br bond begins to break
40
Energy Diagram for step 14
1
Describing a Reaction: Intermediates
• If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product
• These are called reaction intermediates or simply “intermediates”
• Each step has its own free energy of activation
• The complete diagram for the reaction shows the free energy changes associated with an intermediate
42
Formation of a CarbocationIntermediate
• HBr, a Lewis acid, adds to the bond
• This produces an intermediate with a positive charge on carbon - a carbocation
• This is ready to react with bromide
43
Reaction Diagram for Addition of HBr to Ethylene
Two separate steps, each with a own transition state
Energy minimum between the steps belongs to the carbocationreaction intermediate.
44
Kinetic and Thermodynamic Control
• Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct
• At higher temperature, product ratio changes and 1,4 adduct predominates
Kinetic vs. Thermodynamic Control of Reactions