PENGANTAR REAKSI, KONTROL KINETIK DAN TERMODINAMIK

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Electronegativity of Some Common Elements

• Higher numbers indicate greater electronegativity

• Carbon bond to more elektronegative elements cause it has a partial positive charge (+)

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Polarity3

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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

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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

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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

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P-Bonds as Nucleophiles:1

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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

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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

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Rule 2: Nu: can be negative or neutral

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Rule 3: E can be positive or neutral1

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Rule 4: Octet rule!2

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Practice Prob. : Add curved arrows to the reactions

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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

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For example:2

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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)

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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

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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

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More homolytic BDE’s:3

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Calculation of an Energy Change from Bond Dissociation Energies

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Describing a Reaction: Energy Diagrams and Transition States

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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‡)

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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

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Energy Diagram for step 14

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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

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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

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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.

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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