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11.5 Halogenation Regioselectivity
With substrates more complex than ethane, multiple monohalogenation products are possible If the halogen were indiscriminant, predict the product ratio? Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
For the CHLORINATION process, the actual product distribution favors 2-chloropropane over 1-chloropropane Which step in the mechanism determines the regioselectivity? Show the arrow pushing for that key mechanistic step Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
In one reaction, a 1° free radical forms, and in the other a 2° radical forms Is the chlorination process thermodynamically or kinetically controlled? WHY? Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
For the BROMINATION process, the product distribution vastly favors 2-bromopropane over 1-bromopropane Which step in the mechanism determines regioselectivity? Show the arrow pushing for that key mechanistic step Copyright 2012 John Wiley & Sons, Inc.
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11.4 Halogenation Thermodynamics
Is the key step in the bromination mechanism kinetically or thermodynamically controlled? WHY? First step is endothermic Both steps are exothermic Second step is exothermic Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
Focus on the H abstraction step, and consider the Hammond postulate: species on the energy diagram that are similar in energy are similar in structure Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
When the bromine radical abstracts the hydrogen, the carbon must be able to stabilize a large partial radical When the chlorine radical abstracts the hydrogen, the carbon does not carry as much of a partial radical Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
Which process is more regioselective? WHY? Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
Bromination at the 3° position happens 1600 times more often than at the 1° position Copyright 2012 John Wiley & Sons, Inc.
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11.5 Halogenation Regioselectivity
Which process is least regioselective? WHY? What is the general relationship between reactivity and selectivity? WHY? Practice with SkillBuilder 11.5 Copyright 2012 John Wiley & Sons, Inc.
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11.6 Halogenation Stereochemistry
The halogenation of butane or more complex alkanes forms a new chirality center 2-chlorobutane will form as a racemic mixture Which step in the mechanism is responsible for the stereochemical outcome? Copyright 2012 John Wiley & Sons, Inc.
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11.6 Halogenation Stereochemistry
Whether the free radical carbon is sp2 or a rapidly interconverting sp3, the halogen abstraction will occur on either side of the plane with equal probability Copyright 2012 John Wiley & Sons, Inc.
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11.6 Halogenation Stereochemistry
Three monosubstituted products form in the halogenation of butane Draw all of the monosubstituted products that would form in the halogenation of 2-methylbutane including all stereoisomers Copyright 2012 John Wiley & Sons, Inc.
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11.6 Halogenation Stereochemistry
In the halogenation of (S)-3-methylhexane, the chirality center is the most reactive carbon in the molecule. WHY? Name the product and predict the stereochemical outcome Practice with SkillBuilder 11.6 Copyright 2012 John Wiley & Sons, Inc.
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11.6 Halogenation Stereochemistry
Draw all of the monosubstituted products that would form in the halogenation below including all stereoisomers Classify any stereoisomer pairs as either enantiomers or diastereomers Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation When an C=C double bond is present it affects the regioselectivity of the halogenation reaction Given the bond dissociation energies below, which position of cyclohexene will be most reactive toward halogenation? Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation When an allylic hydrogen is abstracted, it leaves behind an allylic free radical that is stabilized by resonance Based on the high selectivity of bromination that we discussed, you might expect bromination to occur as shown below What other set of side-products is likely to form in this reaction? Hint: addition reaction Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation with NBS
To avoid the competing halogenation addition reaction, NBS can be used to supply Br• radicals Show how resonance stabilizes the succinimide radical Heat or light initiates the process Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation with NBS
Propagation produces new Br• radicals to continue the chain reaction Where does the Br-Br above come from? The amount of Br-Br in solution is minimal, so the competing addition reaction is minimized Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation with NBS
The succinimide radical that is produced in the initiation step can also undergo propagation when it collides with an H-Br molecule. Show a mechanism + H-Br Give some examples of some termination steps that might occur Copyright 2012 John Wiley & Sons, Inc.
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11.7 Allylic Halogenation with NBS
Give a mechanism that explains the following product distribution. Hint: resonance Practice with SkillBuilder 11.7 Copyright 2012 John Wiley & Sons, Inc.
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11.8 Atmospheric Chemistry and O3
Ozone is both created and destroyed in the upper atmosphere O3 molecules absorb harmful UV radiation O3 molecules are recycled as heat energy is released Copyright 2012 John Wiley & Sons, Inc.
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11.8 Atmospheric Chemistry and O3
O3 depletion (about 6% each year) remains a serious health and environmental issue Compounds that are most destructive to the ozone layer Are stable enough to reach the upper atmosphere Form free radicals that interfere with the O3 recycling process Chlorofluorocarbons (CFCs) fit both criteria Atmosphere O3 is vital for protection, but what effect does O3 have at the earth’s surface? Copyright 2012 John Wiley & Sons, Inc.
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11.8 Atmospheric Chemistry and O3
CFC substitutes that generally decompose before reaching the O3 layer include hydrochlorofluorocarbons Hydrofluorocarbons don’t even form the chlorine radicals that interfere with the O3 cycle Copyright 2012 John Wiley & Sons, Inc.
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11.8 Combustion and Firefighting
Like most reactions, combustion involves breaking bonds and forming new bonds A fuel is heated with the necessary Eact to break bonds (C-C, C-H, and O=O) homolytically The resulting free radicals join together to form new O-H and C=O bonds Why does the process release energy overall? What types of chemicals might be used in fire extinguishers to inhibit the process? Copyright 2012 John Wiley & Sons, Inc.
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11.8 Combustion and Firefighting
Water deprives the fire of the Eact needed by absorbing the energy CO2 and Argon gas deprive the fire of needed oxygen Halons are very effective fire-suppression agents Halons suppress combustion in three main ways Copyright 2012 John Wiley & Sons, Inc.
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11.8 Combustion and Firefighting
Halons suppress combustion in three main ways As a gas, they can smother the fire and deprive it of O2 They absorb some of the Eact for the fire by undergoing homolytic cleavage The free radicals produced can combine with C• and H• free radicals to terminate the combustion chain reaction Copyright 2012 John Wiley & Sons, Inc.
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11.8 Combustion and Firefighting
How do you think the use of halons to fight fires affects the ozone layer? FM-200 is an alternative firefighting agent Practice with conceptual checkpoint 11.18 Copyright 2012 John Wiley & Sons, Inc.
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11.9 Autooxidation Autooxidation is the process by which compounds react with molecular oxygen The process is generally very slow Copyright 2012 John Wiley & Sons, Inc.
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11.9 Autooxidation Mechanism
The mechanism illustrates the need for a more refined definition of initiation and propagation. HOW? Copyright 2012 John Wiley & Sons, Inc.
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11.9 Autooxidation Mechanism
Propagation can be more precisely defined as the steps that add together to give the net chemical equation Steps in the mechanism that are not part of the net equation must be either initiation or termination Copyright 2012 John Wiley & Sons, Inc.
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11.9 Autooxidation Light accelerates the autooxidation process
Dark containers are often used to store many chemicals such as vitamins In the absence of light, autooxidation is usually a slow process Compounds that can form a relatively stable C• radical upon H abstraction are especially susceptible to autooxidation. WHY? Consider the autooxidation of compounds with allylic or benzylic hydrogen atoms Copyright 2012 John Wiley & Sons, Inc.
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11.9 Antioxidants Triglycerides are important to a healthy diet
Autooxidation can occur at the allylic positions causing the food to become rancid and toxic Copyright 2012 John Wiley & Sons, Inc.
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11.9 Antioxidants Foods with unsaturated fatty acids have a short shelf life unless preservatives are used Copyright 2012 John Wiley & Sons, Inc.
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11.9 Antioxidants Preservatives can undergo H abstraction to quench the C• radicals that form in the first step of autooxidation One molecule of BHT can prevent thousands of autooxidation reactions by stopping the chain reaction How does BHT’s structure make it good at taking on a free radical? Consider resonance and sterics Copyright 2012 John Wiley & Sons, Inc.
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11.9 Natural Antioxidants Vitamins C is hydrophilic
Vitamin E is hydrophobic What parts of the body do these vitamins protect? For each vitamin, show its oxidation mechanism, and explain how that protects the body from autooxidation Copyright 2012 John Wiley & Sons, Inc.
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Study Guide for Sections 11.5-11.9
DAY 2, Terms to know: Sections Hammond postulate, regioselective, stereoselective, NBS, combustion, autooxidation, antioxidant DAY 2, Specific outcomes and skills that may be tested on exam 1: Sections Be able to give a complete description of the kinetic analysis of radical halogenations processes with either Cl, Br, or I. Be able to explain the regioselectivity for a bromination reaction and WHY bromination is the most selective of the free radical halogenations reactions Be able to predict the stereochemical outcome of halogenations reactions where appropriate Be able to explain why NBS is often used rather than Br2 in radical reactions and draw a complete mechanism and predict products for an NBS reaction Given reactants and products, be able to describe appropriate reagents that could complete the radical reaction Be able to describe the basics of atmospheric ozone chemistry Be able to describe the basics of chemistry in firefighting Be able to describe the basics of autooxidation and how antioxidants work
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Extra Practice Problems for Sections 11.5-11.9
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period b and d a-c a and b
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Prep for Day 3 Must Watch videos:
(radical hydrobromination, JP McCormick) (radical halogenation, Dave) (radical polymerization, JP McCormick) (synthesis practice, Dr K) Read Sections , 11.13,
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