Chapter 4 The Study of Chemical Reactions

Chapter 4

The Study of

Chemical Reactions

Jo Blackburn

Richland College, Dallas, TX

Dallas County Community College District

Organic Chemistry, 5th Edition

Tools for Study

• To determine a reaction’s

mechanism, look at:

Equilibrium constant

Free energy change

Enthalpy

Entropy

Bond dissociation energy

Kinetics

Chlorination of Methane

• Requires heat or light for initiation.

• _{The most effective wavelength is blue, which} is absorbed by chlorine gas.

• Lots of product formed from absorption of only one photon of light (chain reaction).

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

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

=>

Propagation Step (1)

Propagation Step (2)

The methyl free radical collides with

Termination Steps

• _{Collision of any two free radicals} • Combination of free radical with

contaminant or collision with wall.

C

Can you suggest others?

Equilibrium constant

• K_eq = [products]

[reactants]

• For chlorination K_eq = 1.1 x 1019

• _{Large value indicates reaction “goes to}

completion.”

Free Energy Change

• DG = free energy of (products -

reactants), amount of energy available to do work.

• Negative values indicate spontaneity. • _DGo = -RT(lnK

eq)

where R = 1.987 cal/K-mol

and T = temperature in kelvins

• _{Since chlorination has a large K}

eq, the free

energy change is large and negative.

Problem

• Given that -X is -OH, the energy difference for the following reaction is -1.0 kcal/mol.

• _{What percentage of cyclohexanol molecules} will be in the equatorial conformer at

equilibrium at 25°C?

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

• 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

• 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.

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.

A B A ₊ B

Kinetics

• _{Answers question, “How fast?”}

• Rate is proportional to the concentration

of reactants raised to a power.

• _{Rate law is experimentally determined.}

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 = -E_a + lnA

RT

Activation Energy

• _{Minimum energy required to reach}

the transition state.

• At higher temperatures, more molecules

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

Rate-Determining Step

• _{Reaction intermediates are stable as long}

as they don’t collide with another molecule or atom, but they are very reactive.

• Transition states are at energy maximums. • _{Intermediates are at energy minimums.}

• The reaction step with highest E_a will be the

Rate, E

_a

,

and Temperature

X + CH_{4 HX + CH3}

X E _a Rate @ 300K Rate @ 500K

F 1.2 kcal 140,000 300,000

Cl 4 kcal 1300 18,000

Br 18 kcal 9 x 10-8 0.015

I 34 kcal 2 x 10-19 2 x 10-9

Conclusions

• With increasing E_a, 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

• There are six 1 H’s and two 2 H’s. We

expect 3:1 product mix, or 75%

1-chloropropane and 25% 2-1-chloropropane.

• Typical product mix: 40% 1-chloropropane

and 60% 2-chloropropane.

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

Reactivity of Hydrogens

• To compare hydrogen reactivity, find

amount of product formed per hydrogen: 40% 1-chloropropane from 6 hydrogens and 60% 2-chloropropane from 2

hydrogens.

• 40% _ 6 = 6.67% per primary H and

60%  2 = 30% per secondary H

• Secondary H’s are 30% _ 6.67% = 4.5

Predict the Product Mix

Given that secondary H’s are 4.5 times as reactive as primary H’s, predict the

percentage of each monochlorinated product of n-butane + chlorine.

Free Radical Stabilities

• _{Energy required to break a C-H bond}

decreases as substitution on the carbon increases.

• Stability: 3 > 2 > 1 > methyl

DH(kcal) 91, 95, 98, 104

Chlorination Energy Diagram

• _{There are six 1} _{H’s and two 2} H_{’s. We}

expect 3:1 product mix, or 75%

1-bromopropane and 25% 2-1-bromopropane.

• Typical product mix: 3% 1-bromopropane

and 97% 2-bromopropane !!!

• _{Bromination is more selective than}

• _{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 and}

97%  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

• _{Note larger difference in E}

a

• Why endothermic?

Endothermic and

Exothermic Diagrams

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

• 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

Reactive Intermediates

• _{Carbocations (or carbonium ions)} • Free radicals

• Carbanions • _Carbene

Carbocation Structure

• Carbon has 6 electrons,

positive charge.

• _{Carbon is sp}2 hybridized

with vacant p orbital.

Carbocation Stability

• Stabilized by alkyl substituents 2 ways: • (1) Inductive effect:

donation of electron density along the

sigma bonds.

• _{(2) Hyperconjugation:} overlap of sigma

bonding orbitals with empty p orbital.

Free Radicals

• Also electron-deficient

• _{Stabilized by alkyl} substituents

• _{Order of stability:}

3 > 2 > 1 > methyl

Carbanions

• Eight electrons on C: 6 bonding + lone pair • _{Carbon has a negative}

charge.

• _{Destabilized by alkyl} substituents.

• _{Methyl >1} _{> 2}  _{> 3}

Carbenes

• Carbon is neutral. • _{Vacant p orbital, so}

can be electrophilic. • Lone pair of

electrons, so can be nucleophilic.