Chapter10ReactionSummary
RadicalReactions
RadicalStabilityTrend
vinyl
CH 3
H 3C CH2
H 3C C CH 3
H
H 3C C CH 3
CH 3
methyl
1o
2o
3o
allylic
benzylic
Increasing Stability
RadicalHalogenation
R H
•
•
•
Δ
or
hν
+ X2
+ HX
R X
Radicalhalogenationcanbeinitiatedbyheatorlight.
Cl2andBr2canbeused.F2istooreactivewhileradicaliodinationisendothermic.
Radicalchlorinationisnotselectiveandthereisthepotentialtogetachlorineatevery
carbonthatcontainingahydrogen.
Cl
Cl2
+
Cl
hν
•
Cl
+
+
Cl
Radicalbrominationismuchmoreselective.Themajorproductfromabrominationwill
betheonethatresultsfromthemoststableradicalintermediate.3°>2°>1°>methyl.
Br 2
Major Product
Br
hν
•
X = Cl or Br
Radicalchainmechanismshavethreesteps:1.Initiation,2:propagation,3:termination.
Initiation
Br
Δ
or
hν
Br
2 Br
Propagation
a. Br
H
+
HBr +
Br
b.
+
Br
Br
+ Br
continues propagation
cycle in step a.
Termination
Br
+
Br
+
Br
Br
Br
Br
+
a termination is any step that
quenches two radicals.
CommonMistake–Anextremelycommonmisstateistoconfusethepropagationstepin
themechanismwiththeterminationstep.Althoughoneoftheterminationstepsdoes
produceamoleculeofthedesiredproduct,thisterminationdoesnothappenoftenand
doesnotproduceasignificantamountofproduct.Thepropagationcycleiswhere
significantproductformationoccurs.
AllylicBromination
• TheallylicpositionofanalkenecanbebrominatedusingNBSinthepresenceoftrace
hydrobromicacidandaninitiatorsuchasheatorlight.
Br
NBS
trace HBr
Δ or hν
•
Br
NBS =
N
O
O
Allylicradicalshave2ormoreresonancestructures.Thebrominationproductcanresult
fromeachresonancestructure.Ifoneproducthasamorehighlysubstituteddoublebond,
thatproductwillbethemajorproduct.
NBS
trace HBr
Δ or hν
Br
Br
Major
•
NBSprovidesaverylowequilibriumconcentrationofBr2,whichistheactivebrominating
agent.
RadicalAdditionofHBrtoAlkenes
H
HBr
HOOR
•
•
•
•
Br
Anti-Markovnikov addition - H goes to the
more substituted carbon and Br goes to the
less substituted carbon.
AdditionofHBrtoanalkeneinthepresenceofperoxideschangesthereactionfroman
ionicmechanismtoaradicalmechanism.
Bychangingthemechanism,theregioselectivityofthereactionischanged.Inthe
presenceofperoxides,HBrisaddedinananti-Markovnikovfashion.
Theperoxideactsasaradicalinitiator.TheO-Obondissoweakthatitundergoes
homolysistogiveaperoxyradicalwhichinitiatestheradicalprocess.
Threestepmechanism:1.Initiation,2:propagation,3:termination.
Initiation
2 RO
RO OR
RO
+
H Br
ROH
+ Br
Propagation
Br
Br
+
Br
+
H Br
Br
Not:
Br
+
The 2o radical
is favored over
this 1 o radical.
Br
Termination
-any step where two of the radicals from above come together