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WELCOME TO MODERN ORGANIC CHEMISTRY. Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 4 The Study of Chemical Reactions. WHAT IS A REACTION MECHANISM . A DESCRIPTION OF STRUCTURES AN ENERGIES OF STARTING MATERIALS AND PRODUCTS OF A REACTION AS WELL AS ANY REACTION INTERMEDIATES.

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Chapter 4 the study of chemical reactions l.jpg

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chapter 4The Study of Chemical Reactions


What is a reaction mechanism l.jpg
WHAT IS A REACTION MECHANISM

A DESCRIPTION OF STRUCTURES AN

ENERGIES OF STARTING MATERIALS AND

PRODUCTS OF A REACTION AS WELL AS

ANY REACTION INTERMEDIATES

IN ADDITION, ALL OF THE TRANSITION

STATES (ENERGY MAXIMA) SEPARATING

THE REACTANTS FROM THE PRODUCTS

(ENERGY MINIMA) MUST BE DETERMINED.


Product determination l.jpg
PRODUCT DETERMINATION

?

1. ISOLATE THEM

2. TAKE THEIR SPECTRA

a. IR - FUNCTIONAL GROUPS

b. NMR - environment of hydrogen and carbon atoms

c. MS - actual MW

d. X-RAY



Structure of transition state l.jpg
STRUCTURE OF TRANSITION STATE?

CAN'T MEASUREMENT DIRECTLY

MEASURE THE RATE LAW TELLS NUMBER AND KIND OF MOLECULES INVOLVED IN TS

CERTAIN ENZYMES RECOGNIZE TS -

THAT IS THEY BOND WITH IT!!


How far does a reaction go l.jpg
HOW FAR DOES A REACTION GO?

THERMODYNAMICS

= -2.3xRxTxLogKeq

DGo = DHo-TDSo

HOW FAST DOES A REACTION GO? KINETICS

RATE IS INVERSELY PROPORTIONAL TO ACTIVATION ENERGY (EA)


Chlorination of methane l.jpg
Chlorination of Methane

Requires heat or light for initiation.

  • The most effective wavelength is blue, which is absorbed by

  • chlorine gas.

  • High quantum yield

  • Lots of product formed from absorption of only one photon of

  • light (chain reaction).

Some ethane is formed

THESE FACTORS ARE CHARACHERISTICS OF FREE-

RADICAL REACTIONS


Free radical chain reaction l.jpg
Free-Radical Chain Reaction

  • Initiation generates a reactive intermediate.

  • Propagation: the intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule).

  • Termination: side reactions that destroy the reactive intermediate. =>


Initiation step l.jpg
Initiation Step

A chlorine molecule splits homolytically into chlorine atoms (free radicals)

THE CHLORINE ATOM IS ELECTROPHILIC - LEWIS ACID

SEEKS AN ELECTRON TO REGAIN OCTET OF ELECTRONS

=>

USUALLY ABSTRACTS A HYDROGEN ATOM


Propagation step 1 l.jpg
Propagation Step (1)

The chlorine atom collides with a methane molecule and abstracts (removes) a H, forming another free radical and one of the products (HCl).

Methyl radicals is also electrophilic -seeks e-

=>

Methyl radical is a member of the highly reactive

intermediates gang


Propagation step 2 l.jpg
Propagation Step (2)

The methyl free radical collides with another chlorine molecule, producing the other product (methyl chloride) and regenerating the chlorine radical.

chlorine atom then attacks another methyl step 1 and 2 repeat over and over again

UNTIL -------

=>


Overall reaction l.jpg

=>

Overall Reaction


Termination steps l.jpg
Termination Steps

  • Collision of any two free radicals

  • Combination of free radical with contaminant or collision with wall.

Can you suggest others? =>


Equilibrium constant l.jpg
Equilibrium constant

  • Keq = [products] [reactants]

  • For chlorination Keq = 1.1 x 1019

  • Large value indicates reaction “goes to completion.”=>


Free energy change l.jpg
Free Energy Change

  • DG = free energy of (products - reactants), amount of energy available to do work.

  • Negative values indicate spontaneity.

DGo = -RT(lnKeq) where R = 1.987 cal/K-mol and T = temperature in kelvins

Since chlorination has a large Keq, the free energy change is large and negative.


Factors determining g l.jpg
Factors Determining G

  • Free energy change depends on

    • enthalpy

    • entropy

  • H = (enthalpy of products) - (enthalpy of reactants)

  • S = (entropy of products) - (entropy of reactants)

G = H - TS


Enthalpy l.jpg
Enthalpy

  • DHo = heat released or absorbed during a chemical reaction at standard conditions.

  • Exothermic, (-DH), heat is released.

  • Endothermic, (+DH), heat is absorbed. =>

  • Reactions favor products with lowest enthalpy

  • (strongest bonds). =>


Entropy l.jpg
Entropy

  • DSo = change in randomness, disorder, freedom of movement.

  • Increasing heat, volume, or number of particles increases entropy.

  • Spontaneous reactions maximize disorder and minimize enthalpy.

  • In the equation DGo = DHo - TDSo the entropy value is often small. =>


Problems l.jpg
PROBLEMS

Which entropy value shown below is correct?

POSSIBLE ANSWERS

A. 0

B. -35

c. +35

c. +18

B. -18

A. 0


Bond dissociation energy l.jpg
Bond Dissociation Energy

  • Bond breaking requires energy (+BDE)

  • Bond formation releases energy (-BDE)

Table 4.2 gives BDE for homolytic cleavage of

bonds in a gaseous molecule.

We can use BDE to estimate H for a reaction.=>


Determination of delta h for overall reaction l.jpg
Determination of delta H for overall reaction

-103 kcal

58 kcal

-84 kcal

104 kcal

-187kcal

162 kcal

-25 kcal

EXOTHERMIC


Individual steps l.jpg
INDIVIDUAL STEPS

RDS

DH

104 kcal

-103 kcal

+1

-26

58 kcal

-84 kcal

-25

THIS IS THE OFFICIAL MECHANISM


Which is more likely l.jpg

104

103

58

84

104

84

=>

103

58

Which is more likely?

Estimate DH for each step using BDE.

+1 kcal

+20 kcal


Kinetics l.jpg
Kinetics

  • Answers question, “How fast?”

  • Rate is proportional to the concentration of reactants raised to a power.

  • Rate law is experimentally determined.=>


Reaction order l.jpg
Reaction Order

  • For A + B  C + D, rate = k[A]a[B]b

    • a is the order with respect to A

    • a + b is the overall order

  • Order is the number of molecules of that reactant which is present in the rate-determining step of the mechanism.

  • The value of k depends on temperature as given by Arrhenius: ln k = -Ea + lnART =>


Activation energy l.jpg

=>

Activation Energy

  • Minimum energy required to reach the transition state.

  • At higher temperatures, more molecules have the required energy.


Reaction energy diagrams l.jpg

=>

Reaction-Energy Diagrams

  • For a one-step reaction:reactants  transition state  products

  • A catalyst lowers the energy of the transition state.


Energy diagram for a two step reaction l.jpg

=>

Energy Diagram for a Two-Step Reaction

  • Reactants  transition state  intermediate

  • Intermediate  transition state  product


Rate determining step l.jpg
Rate-Determining Step

  • Reaction intermediates are stable as long as they

  • don’t collide with another molecule or atom,

  • but they are very reactive.

Trapping agents are used to determine intermediate

Transition states are at energy maxima.

The reaction step with highest Ea will be the slowest,

therefore rate-determining for the entire reaction.


Rate e a and temperature l.jpg
Rate, Ea,and Temperature

=>


Conclusions l.jpg
Conclusions

  • With increasing Ea, rate decreases.

  • With increasing temperature, rate increases.

  • Fluorine reacts explosively.

  • Chlorine reacts at a moderate rate.

  • Bromine must be heated to react.

  • Iodine does not react (detectably). =>


Chlorination of propane l.jpg
Chlorination of Propane

1 C

  • There are six 1 H’s and two 2 H’s. We expect 3:1 product mix, or 75% 1-chloropropane and 25% 2-chloropropane.

  • Typical product mix: 45% 1-chloropropane and 55% 2-chloropropane.

  • Therefore, not all H’s are equally reactive. =>

2 C


Reactivity of hydrogens l.jpg
Reactivity of Hydrogens

  • To compare hydrogen reactivity, find amount of product formed per hydrogen: 45% 1-chloropropane from 6 hydrogens and 55% 2-chloropropane from 2 hydrogens.

  • 45%  6 = 7.5% per primary H and55%  2 = 27.5% per secondary H

  • Secondary H’s are 27.5%  7.5% = 3.7 times more reactive toward chlorination than primary H’s. =>


Chlorination of isobutane methylpropane l.jpg
Chlorination of Isobutanemethylpropane

36%

64%

reactivity of 3o = 36/1 = 36% per 1 H

Rel reactive of 3o/ 1o = 36/7.1 =

5.1

reactivity of 1o = 64/9 =

RelativeReactivity = 3o > 2o > 1o


Free radical stabilities l.jpg
Free Radical Stabilities

  • Energy required to break a C-H bond decreases as substitution on the carbon increases.

  • Stability: 3 > 2 > 1 > methylDH(kcal) 91, 95, 98, 104 =>


Chlorination energy diagram l.jpg
Chlorination Energy Diagram

Lower Ea, faster rate, so more stable intermediate is formed faster.

=>


Bromination of propane l.jpg
Bromination of Propane

1 C

2 C

  • There are six 1 H’s and two 2 H’s. We expect 3:1 product mix, or 75% 1-bromopropane and 25% 2-bromopropane.

  • Typical product mix: 3% 1-bromopropane and 97% 2-bromopropane !!!

  • Bromination is more selective than chlorination. =>


Reactivity of hydrogens39 l.jpg
Reactivity of Hydrogens

  • To compare hydrogen reactivity, find amount of product formed per hydrogen: 3% 1-bromopropane from 6 hydrogens and 97% 2-bromopropane from 2 hydrogens.

  • 3%  6 = 0.5% per primary H and97%  2 = 48.5% per secondary H

  • Secondary H’s are 48.5%  0.5% = 97 times more reactive toward bromination than primary H’s. =>


Bromination energy diagram l.jpg
Bromination Energy Diagram

  • Note larger difference in Ea

  • Why endothermic?

=>



Endothermic and exothermic diagrams l.jpg
Endothermic and Exothermic Diagrams

=>


Hammond postulate l.jpg
Hammond Postulate

  • Related species that are similar in energy are also similar in structure. The structure of a transition state resembles the structure of the closest stable species.

  • Transition state structure for endothermic reactions resemble the product.

  • Transition state structure for exothermic reactions resemble the reactants.=>


Radical inhibitors l.jpg
Radical Inhibitors

  • Often added to food to retard spoilage.

  • Without an inhibitor, each initiation step will cause a chain reaction so that many molecules will react.

  • An inhibitor combines with the free radical to form a stable molecule.

  • Vitamin E and vitamin C are thought to protect living cells from free radicals. =>


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