1. free radical substitution

Radica l React ionsRadica l React ions

Ms. Anjelyn del Rosario

Chemistry 31Chemistry 31

U N IV E R S ITY OF THE PH ILIPP IN E S M AN ILAPadre Faura, E rmita, Manila

A.Y. 2009 – 2010, S econd S emester

A lkanesA lkanes– s aturated hydroc arbonss aturated hydroc arbons

– fairly unreactive; (old family name, fairly unreactive; (old family name, paraffinparaffin , meant little , meant little reactivity)reactivity)

– have relatively strong, have relatively strong, almost almost NON-POLAR, S INGLE NON-POLAR, S INGLE covalent covalent bondsbonds

– have only have only strong strong δδ bonds bonds , they have no real sites that will , they have no real sites that will encourage substances to attack themencourage substances to attack them

– Due to lack of reactivity, alkanes need a very reactive species Due to lack of reactivity, alkanes need a very reactive species to persuade them to reactto persuade them to react

Introduction: AlkanesIntroduction: Alkanes

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There are There are 3 ways to split 3 ways to split the shared electron pair in an the shared electron pair in an unsymmetrical unsymmetrical covalent bond.covalent bond. U N E QUAL S PLITTIN GU N E QUAL S PLITTIN Gproducesproduces ION SION Sknown asknown as H E TE ROLYS ISH E TE ROLYS IS

E QUAL S PLITTIN GE QUAL S PLITTIN Gproducesproduces R AD IC ALSR AD IC ALSknown asknown as H OM OLYS ISH OM OLYS IS

• • If several bonds are present the If several bonds are present the weakest bond is usually broken firstweakest bond is usually broken first • • Energy to break bonds can come from a variety of energy sources - Energy to break bonds can come from a variety of energy sources - heat / lightheat / light • • In the reaction between methane and chlorine either can be used, In the reaction between methane and chlorine either can be used, however...however... • • In the laboratory a source of UV light (or sunlight) is favoured.In the laboratory a source of UV light (or sunlight) is favoured.

Breaking of Covalent BondsBreaking of Covalent Bonds

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• A A R adic a lR adic a l is a reactive intermediate with a is a reactive intermediate with a single unpaired single unpaired electronelectron, formed by , formed by homolysis of a covalent bondhomolysis of a covalent bond..

• A radical contains an atom that A radical contains an atom that does not have an octet of does not have an octet of electronselectrons ..

• Half-headed arrowsHalf-headed arrows are used to show the are used to show the movement of movement of electroelectrons in radical processes.ns in radical processes.

• A carbon radical is A carbon radical is spsp22 hybridized hybridized and and trigonal planartrigonal planar

Free Radicals : DefinitionFree Radicals : Definition

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Free Radicals: StabilitiesFree Radicals: Stabilities

Relative S tabilities of A lkyl Relative S tabilities of A lkyl R adic a lsR adic a ls

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S tabilized by S tabilized by hyperc onjug ationhyperc onjug ation

- Delocalization of electrons by the - Delocalization of electrons by the overlap of a overlap of a δδ bond bond orbital with an orbital with an empty empty p orbitalp orbital

S tabilized by S tabilized by res onanc eres onanc e

The more s table the radic a l, the les s energ y required to make it.The more s table the radic a l, the les s energ y required to make it.

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Hyperc onjug ationHyperc onjug ation- Delocalization of electrons by the overlap of a Delocalization of electrons by the overlap of a δδ bond bond orbital with orbital with

an an empty empty p orbitalp orbital- occurs only if the occurs only if the δδ bond orbital and the empty p orbital have bond orbital and the empty p orbital have the the

proper orientationproper orientationMovement of electrons from the Movement of electrons from the δδ bond orbitalbond orbital toward the toward the p orbital p orbital of of the ethyl radical the ethyl radical dec reas es the dec reas es the

charg e on the s pcharg e on the s p 22 c arbon c arbon

The unpaired electron is The unpaired electron is no long er no long er loc a lized loc a lized solely on one atom, but is solely on one atom, but is

s pread out over a g reater volume s pread out over a g reater volume of s pac eof s pac e

ee--ss

•An An a llylic radic a l a llylic radic a l has an unpaired electron on an allylic has an unpaired electron on an allylic carboncarbon

•The allyl radical is more stable than other radicals because The allyl radical is more stable than other radicals because two resonance forms can be drawn for it.two resonance forms can be drawn for it.

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The The spsp22 carbons carbons of an alkene are called of an alkene are called vinylic c arbonsvinylic c arbons . . An An spsp33 carbon carbon that that is adjacent to a vinylic carbon is called is adjacent to a vinylic carbon is called

an an a llylic c a rbon.a llylic c a rbon.

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• A A benzylic radic a l benzylic radic a l has an unpaired electron on a has an unpaired electron on a benzylic carbonbenzylic carbon

• Five contributing resonance structuresFive contributing resonance structures

• Because of their delocalized electrons, allyl and benzyl Because of their delocalized electrons, allyl and benzyl radicals are both more stable than other primary radicals. radicals are both more stable than other primary radicals.



• Radicals are formed from covalent bonds by adding energy in Radicals are formed from covalent bonds by adding energy in the form of the form of heat (heat (∆∆ )) or or light (hlight (hνν ).).

• S ome radical reactions are carried out in the presence of a S ome radical reactions are carried out in the presence of a radic a l initiatorradic a l initiator. .

– Radical initiators contain a especially Radical initiators contain a especially weak bond weak bond that that serves as a source of radicals.serves as a source of radicals.

– PeroxidesPeroxides , compounds having the general structure R, compounds having the general structure R O— OO— OR, R , are the most commonly used radical initiators.are the most commonly used radical initiators.

– Heating a peroxide readily causes homolysis of the weak Heating a peroxide readily causes homolysis of the weak O—O—OO bond, forming two RO• radicals. bond, forming two RO• radicals.

• Radicals undergo two main types of reactionsRadicals undergo two main types of reactions —they react —they react with with σσ bonds, and they add to bonds, and they add to ππ bonds bonds ..

G enera l Features of R adic a l Reac tionsG enera l Features of R adic a l Reac tions

Radical ReactionsRadical Reactions

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• A radical X• abstracts a hydrogen atom from a C— H A radical X• abstracts a hydrogen atom from a C— H σσ bond to bond to from H— X and a carbon radical.from H— X and a carbon radical.

Reac tion of a R adic a l X • w ith a C -H B ond.Reac tion of a R adic a l X • w ith a C -H B ond.

• A radical X• also adds to the A radical X• also adds to the ππ bond of a carbon— carbon double bond of a carbon— carbon double bond.bond.

Reac tion of a R adic a l X • w ith a C =C B ond.Reac tion of a R adic a l X • w ith a C =C B ond.


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1111

• A radical X•, once formed, A radical X•, once formed, rapidly reac ts w ith w hatever is rapidly reac ts w ith w hatever is ava ilableava ilable , usually a stable , usually a stable σσ or or ππ bond. bond.

• Occasionally, two radicals react to form a sigma bond.Occasionally, two radicals react to form a sigma bond.

Two R adica ls Reac ting w ith E ach Other.Two R adica ls Reac ting w ith E ach Other.

• The reaction of a radical with oxygen (a diradical in its ground state The reaction of a radical with oxygen (a diradical in its ground state electronic configuration) is another example of two radicals reacting electronic configuration) is another example of two radicals reacting with each other.with each other.

• Compounds that prevent radical reactions from occurring are called Compounds that prevent radical reactions from occurring are called radic a l inhibitorsradic a l inhibitors or or radic a l s c aveng ersradic a l s c aveng ers . Besides O. Besides O 22, vitamin , vitamin E and other related compounds are radical scavengers.E and other related compounds are radical scavengers.


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Halog enation Reac tionsHa log enation Reac tions• Alkanes undergo Alkanes undergo s ubs titution reac tions s ubs titution reac tions with halogens with halogens

(fluorine, bromine and chlorine) initiated by (fluorine, bromine and chlorine) initiated by heat or lightheat or light

• Radical halogenation can yield a Radical halogenation can yield a mixture of m ixture of ha log enated c ompounds ha log enated c ompounds because all hydrogen atoms because all hydrogen atoms in an alkane are capable of substitution.in an alkane are capable of substitution.

– For example, all degrees of methane halogenation will be For example, all degrees of methane halogenation will be seenseen

• Monosubstitution can be achieved by using a Monosubstitution can be achieved by using a la rg e larg e exc es s of the a lkaneexc es s of the a lkane

The Reaction of Alkanes with HalogensThe Reaction of Alkanes with Halogens

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• Three facts about halogenation suggest that the mechanism Three facts about halogenation suggest that the mechanism involves radical, not ionic, intermediates:involves radical, not ionic, intermediates:

Ha log enation of A lkanes — Reac tion M echanis mHa log enation of A lkanes — Reac tion M echanis m

The Reaction of Alkanes with HalogensThe Reaction of Alkanes with Halogens

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radic a l inhibitorradic a l inhibitor

U N LIK E c arboc ationsU N LIK E c arboc ations

Reag entsReag ents chlorine and methanechlorine and methane

C onditionsC onditions U V lig ht or s unlig ht - heat is an a lternative energ y U V lig ht or s unlig ht - heat is an a lternative energ y s ourc es ourc e

Equation(s )Equation(s ) C HC H 44(g ) + C l(g ) + C l22(g ) — — > HC l(g ) + C H(g ) — — > HC l(g ) + C H 33C l(g )C l(g )

chloromethanechloromethane

C HC H 33C l(g ) + C lC l(g ) + C l22(g ) — — > HC l(g ) + C H(g ) — — > HC l(g ) + C H 22C lC l22(l) (l)

dichloromethanedichloromethane C HC H 22C lC l22(l) + C l(l) + C l22(g ) — — > HC l(g ) + (g ) — — > HC l(g ) +

C HC lC HC l33(l) (l) trichloromethane trichloromethane C HC lC HC l33(l) + C l(l) + C l22(g ) — — > (g ) — — >

HC l(g ) + C C l HC l(g ) + C C l44(l)(l) tetrachloromethanetetrachloromethane

M ixturesM ixtures free radic a ls a re very reac tive - they are trying free radic a ls a re very reac tive - they are trying to pa ir their elec tronto pa ir their elec tron

w ith s uffic ient chlorine, every hydrog en w ill eventua lly w ith s uffic ient chlorine, every hydrog en w ill eventua lly be replac ed.be replac ed. M echanis mM echanis m C hlorination of methane proc eeds via C hlorination of methane proc eeds via FR E E R AD IC AL FR E E R AD IC AL S U B S TITU TION S U B S TITU TION bec aus e the methane is bec aus e the methane is attacked by attacked by free radica ls free radica ls res ulting in res ulting in hydrog en hydrog en atoms being s ubs titutedatoms being s ubs tituted by chlorine atoms .by chlorine atoms .

The proc es s is a The proc es s is a cha in reac tioncha in reac tion..In the propa g ation s tep, one radic a l is produc ed for In the propa g ation s tep, one radic a l is produc ed for

each one us edeach one us ed

Chlorination of MethaneChlorination of Methane

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Radical Reactions - MechanismRadical Reactions - Mechanism

mechanis m for the monochlorination mechanis m for the monochlorination of methaneof methane

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C lC l22 — — > 2C l — — > 2C l•• R AD IC ALS C R E ATE DR AD IC ALS C R E ATE D

The s ing le dots repres ent U N PA IR E D E LE C TRON SThe s ing le dots repres ent U N PA IR E D E LE C TRON S

During initiation, the During initiation, the WE AK E S T B ON D IS B ROK E NWE AK E S T B ON D IS B ROK E N as it a s it requires les s energ y.requires les s energ y.There are three pos s ible bonds in a m ixture of a lkanes and There are three pos s ible bonds in a m ixture of a lkanes and chlorine.chlorine.

412 412 348 348 242 242

Averag e bond entha lpy kJ molAverag e bond entha lpy kJ mol -1-1

The The C l-C l bond is broken C l-C l bond is broken in preferenc e to the others a s it is in preferenc e to the others a s it is the w eakes t and requires les s energ y to s eparate the the w eakes t and requires les s energ y to s eparate the atoms .atoms .


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InitiationInitiation

C lC l•• + C H + C H 44 — — > C H — — > C H 33•• + H C l + H C l

R AD IC ALS U S E DR AD IC ALS U S E D andand C lC l22 + C H + C H 33• • — — > C H — — > C H 33C l + C lC l + C l•• then then R E -R E -

G E N E R ATE DG E N E R ATE D

Free radic a ls a re very reac tiveFree radic a ls a re very reac tive bec aus e they bec aus e they w ant to pa ir up w ant to pa ir up their s ing le elec trontheir s ing le elec tron ..They do this by abs trac ting a hydrog en atom from methane; They do this by abs trac ting a hydrog en atom from methane; a methyl radica l is formed.a methyl radica l is formed.The methyl radic a l is a ls o very reac tive and attacks a The methyl radic a l is a ls o very reac tive and attacks a chlorine molec ule.chlorine molec ule.A chlorine radic a l is produc ed and the w hole proces s c an A chlorine radic a l is produc ed and the w hole proces s c an s tart over a g a in.s tart over a g a in.


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PropagationPropagation

C lC l•• + C l + C l•• — — > C l— — > C l22 R ADIC A LS R ADIC A LS R E M OV E DR E M OV E D

C lC l•• + C H + C H 33•• — — > C H — — > C H 33C lC l

C HC H 33•• + C H + C H 33•• — — > C— — > C 22HH 66

Removing the Removing the reac tive free reac tive free radic a ls bring s radic a ls bring s an end to the an end to the reac tion.reac tion.

This is not very This is not very likely at the likely at the s tart of the s tart of the reac tion reac tion bec aus e of bec aus e of their low their low c onc entration.c onc entration.


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TerminationTermination

InitiationInitiation C lC l22 — — > 2C l— — > 2C l•• radicals radicals c reatedc reated

Propa g ationPropa g ation C lC l•• + C H + C H 44 — — > C H — — > C H 33•• + HC l + HC l radicals radicals us edus ed and and

C lC l22 + C H + C H 33•• — — > C H— — > C H 33C l + C lC l + C l•• then then re-re-g eneratedg enerated

TerminationTermination C lC l•• + C l + C l•• — — > C l— — > C l22 radicals radicals removedremoved

C lC l•• + C H + C H 33•• — — > C H— — > C H 33C lC l

C HC H 33•• + C H + C H 33•• — — > C— — > C 22HH 66

OV E RV IE WOV E RV IE W

S ummaryS ummaryDue to lack of reac tivity, a lkanes need a very reac tive s pec ies to Due to lack of reac tivity, a lkanes need a very reac tive s pec ies to pers uade them to reac t.pers uade them to reac t.Free radic a ls need to be formed by homolytic fis s ion of c ova lent Free radic a ls need to be formed by homolytic fis s ion of c ova lent bonds .bonds .This is done by s hining U V lig ht on the mixture (heat c ould be This is done by s hining U V lig ht on the mixture (heat c ould be us ed).us ed).C hlorine radic a ls a re produc ed bec aus e the C l-C l bond is the C hlorine radic a ls a re produc ed bec aus e the C l-C l bond is the weakes t.weakes t.You only need one chlorine radic a l to s tart thing s off.You only need one chlorine radic a l to s tart thing s off.W ith exc es s chlorine you g et W ith exc es s chlorine you g et further substitution and a mixture of further substitution and a mixture of chlorinated productschlorinated products


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Radical Reactions - MechanismRadical Reactions - Mechanism

E lec tron Flow for the M onochlorination of E lec tron Flow for the M onochlorination of M ethaneM ethane

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FurtherFurtherpropag ationpropag ation I f I f exc es s chlorineexc es s chlorine is pres ent, is pres ent, further further s ubs titutions ubs titution takes plac e takes plac e

The equations s how the propag ation s teps for the The equations s how the propag ation s teps for the formation of...formation of...

dichloromethanedichloromethane C lC l•• + C H + C H 33C l C l — — > C H— — > C H 22C lC l•• + + H C l H C l

C lC l22 + C H + C H 22C lC l•• — — > C H— — > C H 22C lC l22 + C l + C l••

trichloromethanetrichloromethane C l C l•• + C H + C H 22C lC l22 — — > C HC l— — > C HC l22•• + + HC l HC l

C lC l22 + C HC l + C HC l22•• — — > C HC l— — > C HC l33 + C l + C l••

tetrachloromethanetetrachloromethane C l C l•• + C HC l + C HC l33 — — > C C l— — > C C l33•• + + HC lHC l

C lC l22 + C C l + C C l33•• — — > C C l— — > C C l44 + C l + C l••

M ixturesM ixtures B ec aus e of the B ec aus e of the many pos s ible reac tionsmany pos s ible reac tions there w ill there w ill be a be a m ixture of produc tsmixture of produc ts ..

Individua l ha loa lkanes c an be Individua l ha loa lkanes c an be s eparated by frac tiona l s eparated by frac tiona l dis tilla tiondis tilla tion..

Further Chlorination of MethaneFurther Chlorination of Methane

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Chlorination of EthaneChlorination of Ethane

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Halogenation of E thane— Reaction MechanismHalogenation of E thane— Reaction Mechanism


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Halogenation of E thane— EnergeticsHalogenation of E thane— Energetics

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Halogenation of Higher AlkanesHalogenation of Higher Alkanes

• Experimental: Experimental: Chlorination of CHChlorination of CH 33CHCH 22CHCH 33 yields a 1:1 mixture of yields a 1:1 mixture of CHCH 33CHCH 22CHCH 22Cl and (CHCl and (CH 33))22CHCl.CHCl.

• S ince the observed ratio between CHS ince the observed ratio between CH 33CHCH 22CHCH 22Cl and (CHCl and (CH 33))22CHCl CHCl is 1:1, the 2° C— H bonds must be more reactive than the 1° C— H is 1:1, the 2° C— H bonds must be more reactive than the 1° C— H bonds.bonds.

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• Thus, when alkanes react with C lThus, when alkanes react with C l22, a mixture of products results, with , a mixture of products results, with more product formed by cleavage of the weaker C— H bond than you more product formed by cleavage of the weaker C— H bond than you would expect on statistical grounds.would expect on statistical grounds.

Halogenation of Higher AlkanesHalogenation of Higher Alkanes

relative s tabilities of a lkyl radic a lsrelative s tabilities of a lkyl radic a ls

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Percent Yield of Monohalogenated ProductsPercent Yield of Monohalogenated Products

Relative R ates of A lkyl R adic a l Formation by a C hlorine Relative R ates of A lkyl R adic a l Formation by a C hlorine R adic a lR adic a l

Factors that Determine Product DistributionFactors that Determine Product Distribution

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Relative Amounts of Produc tsRelative Amounts of Produc ts•probabilityprobability - - the number of hydrogens that can be abstracted the number of hydrogens that can be abstracted that will lead to the formation of the particular productthat will lead to the formation of the particular product•reac tivityreac tivity - - the relative rate at which a particular hydrogen is the relative rate at which a particular hydrogen is abstractedabstracted

relative amount relative amount = = number of hydrogens × reactivitynumber of hydrogens × reactivity

perc ent yield perc ent yield = = relative amountrelative amount x 100 x 100

sum of the relative amounts of all sum of the relative amounts of all the alkyl halide productsthe alkyl halide products

________________________________________________________________


R adic a l monochlorination of butaneR adic a l monochlorination of butane

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R adic a l monochlorination of 2,2,5-R adic a l monochlorination of 2,2,5-trimethylhexanetrimethylhexane

Because radical chlorination of an alkane can yield several different Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one monosubstitution products as well as products that contain more than one chlorine atom, it is chlorine atom, it is not the bes t method not the bes t method for synthesizing an alkyl halide.for synthesizing an alkyl halide.

N OTE : N OTE :

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• Although alkanes undergo radical substitutions with both C lAlthough alkanes undergo radical substitutions with both C l22 and and BrBr22, chlorination and bromination exhibit two important , chlorination and bromination exhibit two important differences.differences.

1.1. Chlorination is Chlorination is fa s terfas ter than bromination. than bromination.

2.2. C hlorination is uns elec tiveC hlorination is uns elec tive , yielding a mixture of , yielding a mixture of products, but products, but bromination is often s elec tivebromination is often s elec tive , yielding , yielding one major product.one major product.

C hlorination vers us B rominationC hlorination vers us B romination


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The Reac tivity–S elec tivity Princ ipleThe Reac tivity–S elec tivity Princ iple• The more reac tive a s pec ies is , the les s s elec tive it The more reac tive a s pec ies is , the les s s elec tive it

w ill be. w ill be.

• A bromine radical is A bromine radical is les s reac tive les s reac tive and and more s elec tive more s elec tive than than a chlorine radical.a chlorine radical.

relative rates of radic a l formation by a bromine radic a l relative rates of radic a l formation by a bromine radic a l at 125 °Cat 125 °C


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Why are the relative rates of radical formation Why are the relative rates of radical formation s o different s o different when a when a bromine radical rather than a chlorine radical is used as the bromine radical rather than a chlorine radical is used as the hydrogen-abstracting reagent?hydrogen-abstracting reagent?

The Reactivity- Selectivity PrincipleThe Reactivity- Selectivity Principle

~ c ompare the ∆H° va lues ! ~ c ompare the ∆H° va lues ! (∆H°= ∆H°= bonds being broken - bonds being formed)

Bromination is a much slower reaction than Bromination is a much slower reaction than chlorination.chlorination.

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• The differences in chlorination and bromination can be explained The differences in chlorination and bromination can be explained by considering the energetics of each type of reaction.by considering the energetics of each type of reaction.

• Calculating the Calculating the ∆∆ HH00 using bond dissociation energies reveals that using bond dissociation energies reveals that abstraction of a 1abstraction of a 1°° or 2 or 2°° hydrogen by Br• is endothermic, hydrogen by Br• is endothermic, but it but it takes less energy to form the more stable 2takes less energy to form the more stable 2°° radical. radical.

B rominationB romination

Chlorination vs. BrominationChlorination vs. Bromination

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Remember Hammond pos tulateRemember Hammond pos tulate

C onclus ion:C onc lus ion: Because the rate-determining step is endothermic, Because the rate-determining step is endothermic, the the more s table radic a l is formed fas ter,more s table radic a l is formed fas ter, and often a single radical and often a single radical halogenation product predominates.halogenation product predominates.

BrominationBromination

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T.S . will T.S . will have a partially unpaired have a partially unpaired electron on carbonelectron on carbon

The The same factors same factors that stabilize the that stabilize the radical product stabilize the partially radical product stabilize the partially

unpaired transition state.unpaired transition state.

more stable the transition more stable the transition state, state, the smaller the Ea, the smaller the Ea, the the

faster the reactionfaster the reaction

• Calculating the Calculating the ∆∆ HH°° using bond dissociation energies for using bond dissociation energies for chlorination reveals that abstraction of a 1chlorination reveals that abstraction of a 1°° or 2 or 2°° hydrogen by C l• is hydrogen by C l• is exothermic.exothermic.

C hlorinationC hlorination

• S ince chlorination has an exothermic rate-determining step, the S ince chlorination has an exothermic rate-determining step, the transition state to form both radicals resembles the same starting transition state to form both radicals resembles the same starting material, CHmaterial, CH 33CHCH 22CHCH 33. Thus, . Thus, the relative stability of the two radicals the relative stability of the two radicals is much less importantis much less important, and both radicals are formed., and both radicals are formed.


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C onclus ion:C onc lus ion: Because the rate-determining step in chlorination is Because the rate-determining step in chlorination is exothermic, the exothermic, the transition state resembles the starting material, transition state resembles the starting material, both both radicals are formed, and a mixture of products results.radicals are formed, and a mixture of products results.

Energy diagram for the exothermic reaction:CH3CH2CH3 + C I• → CH3CH2CH2

• or (CH3)2CH• + HC I

ChlorinationChlorination

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the relative stability of the two radicals the relative stability of the two radicals is much less importantis much less important

The Reactivity- Selectivity PrincipleThe Reactivity- Selectivity Principle

chlorine radic a l abs trac tionchlorine radic a l abs trac tion bromine radic a l abs trac tionbromine radic a l abs trac tion

E nerg y D ia g ramE nerg y D ia g ram

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Reaction of Methane with Other HalogensReaction of Methane with Other Halogens

The order of reactivity of methane substitution with halogens The order of reactivity of methane substitution with halogens is:is:

fluorinefluorine >> chlorine chlorine > > brominebromine > > iodineiodine

The order of reactivity is based on the values of The order of reactivity is based on the values of Ea for the Ea for the first step first step of chain propagation and of chain propagation and ΔHΔHoo for the entire chain for the entire chain propagationpropagation

The The energy values of the initiation step are unimportant energy values of the initiation step are unimportant since since they occur so rarelythey occur so rarely– On the basis of ΔOn the basis of ΔHHoo values for X values for X 22, the initiation step , the initiation step

iodination should be most rapidiodination should be most rapid38CHEM31 adr


C hlorinationC hlorination is also highly exothermic overall, but more controllable with is also highly exothermic overall, but more controllable with a higher value of a higher value of Eact and lower overall ΔHEact and lower overall ΔH o o valuesvalues

B romination. B romination. The bromine atom has a significant Eact in the first step of The bromine atom has a significant Eact in the first step of propagation so the reaction is much propagation so the reaction is much more controllable and selectivemore controllable and selective..S till exothermic overallS till exothermic overall

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– Fluorine shows almost Fluorine shows almost no discrimination no discrimination because it is so because it is so reactivereactive

– S o reactive that only per fluoro compounds S o reactive that only per fluoro compounds (all H replaced by (all H replaced by F) F) are made via direct fluorinationare made via direct fluorination

Would Fluorination be s elec tive?Would Fluorination be s elec tive?

FluorinationFluorination

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Fluorination and IodinationFluorination and Iodination

Would Fluorination be s elec tive?Would Fluorination be s elec tive?

D irec t iodination is not a us eful reac tionD irec t iodination is not a us eful reac tion

1. High Ea in first propagation step means very few successful collis ions1. High Ea in first propagation step means very few successful collis ions2. Overall reaction is endothermic2. Overall reaction is endothermic 41CHEM31 adr

E xerc is e:E xerc is e:1. Predict the major product:1. Predict the major product:


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E xerc is e:E xerc is e:


2. Calculate the percent yield of the products in the (a) 2. Calculate the percent yield of the products in the (a) monochlorination (b) monobromination of the following alkanes:monochlorination (b) monobromination of the following alkanes:

chlorine chlorine radic a lradic a l

bromine bromine radic a lradic a l

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• An An a llylic c arbon a llylic c arbon is a carbon adjacent to a double bond.is a carbon adjacent to a double bond.• Homolysis of the allylic C— H bond in propene generates an Homolysis of the allylic C— H bond in propene generates an a llylic a llylic

radic a l radic a l which has an unpaired electron on the carbon adjacent to the which has an unpaired electron on the carbon adjacent to the double bond.double bond.

R adic a l H a log enation at an A llylic C arbonR adic a l H a log enation at an A llylic C arbon

• The bond dissociation energy for this process is even less than that The bond dissociation energy for this process is even less than that for a 3for a 300 C— H bond (91 kcal/mol). C— H bond (91 kcal/mol).

• This means that an This means that an a llyl radic a l a llyl radic a l is more stable than a 3is more stable than a 300 radical. radical.


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•The allyl radical is more stable than other radicals because The allyl radical is more stable than other radicals because two resonance forms can be drawn for it.two resonance forms can be drawn for it.



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• Because allylic C— H bonds are weaker than other Because allylic C— H bonds are weaker than other spsp33 hybridized C— hybridized C—H bonds, the allylic carbon can be selectively halogenated using NBS H bonds, the allylic carbon can be selectively halogenated using NBS in the presence of light or peroxides.in the presence of light or peroxides.

• NBS contains a weak N— Br bond that is homolytically cleaved with NBS contains a weak N— Br bond that is homolytically cleaved with light to generate a bromine radical, initiating an allylic halogenation light to generate a bromine radical, initiating an allylic halogenation reaction.reaction.

• Propagation then consists of the usual two steps of radical Propagation then consists of the usual two steps of radical halogenation.halogenation.



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R adic a l H a log enation at an A llylic C arbonR adic a l H a log enation at an A llylic C arbon• NBS also generates a low concentration of BrNBS also generates a low concentration of Br22 needed in the second needed in the second

chain propagation step (S tep [3] of the mechanism).chain propagation step (S tep [3] of the mechanism).

• The HBr formed in S tep [2] reacts with NBS to form BrThe HBr formed in S tep [2] reacts with NBS to form Br22, which is then , which is then used for halogenation in S tep [3] of the mechanism.used for halogenation in S tep [3] of the mechanism.

• Thus, an alkene with allylic C— H bonds undergoes two different reactions Thus, an alkene with allylic C— H bonds undergoes two different reactions depending on the reaction conditions.depending on the reaction conditions.

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R adic a l H a log enation at an A llylic C arbonR adic a l H a log enation at an A llylic C arbonQues tion: Ques tion:

Why does a low concentration of BrWhy does a low concentration of Br22 (from NBS ) favor allylic substitution (from NBS ) favor allylic substitution (over ionic addition to form the dibromide)?(over ionic addition to form the dibromide)?Ans w er: Ans w er: • The key to getting substitution is to have a low concentration of The key to getting substitution is to have a low concentration of

bromine (Brbromine (Br22).).

• The BrThe Br22 produced from NBS is present in very low concentrations. produced from NBS is present in very low concentrations.

• A low concentration of BrA low concentration of Br22 would first react with the double bond to would first react with the double bond to form a low concentration of the bridged bromonium ion.form a low concentration of the bridged bromonium ion.

• The bridged bromonium ion must then react with more bromine (in the The bridged bromonium ion must then react with more bromine (in the form of Brform of Br¯̄) in a second step to form the dibromide.) in a second step to form the dibromide.

• If concentrations of both intermediates— the bromonium ion and BrIf concentrations of both intermediates— the bromonium ion and Br¯̄ are low (as is the case here), the overall rate of addition is very slow, are low (as is the case here), the overall rate of addition is very slow, and the products of the very fast and facile radical chain reaction and the products of the very fast and facile radical chain reaction predominate.predominate.

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• Halogenation at an allylic carbon often results in a mixture of products. Halogenation at an allylic carbon often results in a mixture of products. Consider the following example:Consider the following example:

• A mixture results because the reaction proceeds by way of a A mixture results because the reaction proceeds by way of a res onanc e s tabilized radic a lres onanc e s tabilized radic a l ..



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• Ozone is vital to life, and acts as a shield, protecting the earth’s Ozone is vital to life, and acts as a shield, protecting the earth’s surface from harmful UV radiation.surface from harmful UV radiation.

The Ozone Layer and C FC sThe Ozone Layer and C FC s

• Current research suggests that Current research suggests that chlorofluoroc arbons (C FC s ) chlorofluoroc arbons (C FC s ) are are responsible for destroying ozone in the upper atmosphere.responsible for destroying ozone in the upper atmosphere.


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• CFCs are inert, odorless, and nontoxic, and have been used as CFCs are inert, odorless, and nontoxic, and have been used as refrigerants, solvents, and aerosol propellants.refrigerants, solvents, and aerosol propellants.

• They are water insoluble and volatile, and readily escape into the They are water insoluble and volatile, and readily escape into the upper atmosphere, where they are decomposed by high-energy upper atmosphere, where they are decomposed by high-energy sunlight to form radicals that destroy ozone by a radical chain sunlight to form radicals that destroy ozone by a radical chain mechanism.mechanism.


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•The overall result is that OThe overall result is that O 33 is consumed as a reactant and O is consumed as a reactant and O 22 is is formed.formed.

• In this way, a small amount of CFC can destroy a large amount In this way, a small amount of CFC can destroy a large amount of Oof O33..

•New alternatives to CFCs are New alternatives to CFCs are hydrochlorofluoroc arbons hydrochlorofluoroc arbons (HC FC s ) (HC FC s ) and and hydrofluoroc arbons (H FC shydrofluoroc arbons (H FC s )) such as such as CHCH 22FCFFCF 33..

•These compounds are decomposed by HO• before they reach These compounds are decomposed by HO• before they reach the stratosphere and therefore they do not take part in the the stratosphere and therefore they do not take part in the radical reactions resulting in Oradical reactions resulting in O 33 destruction. destruction.


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1. free radical substitution

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