Post on 26-Feb-2018
transcript
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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Chapter 4
Organic Chemistry , 8th Edition
L. G. Wade, Jr.
The Study of
Chemical Reactions
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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• “ We cannot remember thousands
of specific organic reactions but wecan organize the reactions into
logical groups based on how the
reactions work and whatintermediates are involved!
• “ The most interesting and useful
aspect of organic chemistry is the
study of reactions!
"# Wade$ % th &d$ p '()
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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• “The amounts of reactants and
products present at e*uilibrium
depend on their relative stabilities!
• “ +to understand a reaction$ we
must+ know the mechanism$ the
step,by,step pathway from
reactants to products!
"# Wade$ % th &d$ p '()
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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To -etermine a Reaction.s
/echanism$ We Will "ook 0t+
1 Thermodynamic -ata
0 &*uilibrium constants
2 3ree energy change
#56
C &nthalpy 756- &ntropy S56
& 2ond -issociation &nthalpy 2-&6
11 8inetic -ata
0 Reaction order
2 0ctivation &nergy
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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3or 9ur :ery 3irst Reaction$
We Will Study+• The 3ree Radical 7alogenation of
0lkanes$ starting with+
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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The Chlorination of /ethane
• Re*uires heat or light for initiation• The most effective wavelength is blue$ which is
absorbed by chlorine gas
• /any molecules of product are formed from absorptionof only one photon of light ∴
a chain reaction 6
C
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7alogenation 9f 0lkanes 1s 0+• 3ree Radical Chain Reaction$ and a
• Substitution Reaction ; where one group takes the place of
another group in a molecule
7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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Three Steps 1n a 3R Chain Reaction
1 1nitiation ; heat or light hν
6 generatesa free radical 3R6 intermediate
11 <ropagation ; 3R intermediate =
reactant > new 3R intermediate = product
111 Termination ; side r?ns consume @
destroy intermediates so r?n stops
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1 1nitiationA 3ormation of a
Chlorine Radical
0 chlorine molecule splits
homolytically into ) chlorine atoms
free radicals6
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"ewis Structures of 3ree
Radicals
• 3ree radicals have unpaired electrons
• 7alogens have B valence electrons so one electronwill be unpaired thus$ making halogen atom a freeradical6
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11 3irst <ropagation StepA
3ormation of /ethyl Radical @ 7Cl
The chlorine atom 3R6 collides with a
methane molecule and abstracts removes6
a proton$ forming a methyl free radical and
one of the products 7Cl 6
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11 Second <ropagation StepA
<roduct 3ormation
The methyl free radical collides with
another chlorine molecule$ producing theorganic product chloromethane 6 and
generating a new chlorine radical
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1mportantA #eneration of a
ew Chlorine Radical /eansthe Reaction Will ContinueD
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1n a free radical chain reaction$ the
reaction will continue until one of
the reactants is consumed
o more heat or light is necessary once
the reaction has startedD
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The 9verall Reaction 1s The Sum
9f The Two <ropagation Steps
+
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7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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7/25/2019 Wade Ch 4 Wade Powerpoint/Lecture for Chapter 4
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<rob 4,' -raw the structures for the
following free radicals
• &thyl radical radical
• tert,butyl radical
• 1sopropyl radical ),propyl6
• 1odine atom
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<rob 4,) a6 Write the propagation
steps leading to the formation of
dichloromethane C7 ) Cl ) 6
b6 &?plain why free radical halogenation
usually gives a mi?ture of products
c6 7ow could an industrial plant controlthe proportions of methane and
chlorine to favor production of CCl 4F
C7 (ClF
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<rob 4,( &ach of the following proposed
mechanisms for the chlorination of
methane is wrong &?plain how thee?perimental evidence disproves each
mechanism
a6 Cl ) = hν > Cl
) G G H an activated form of Cl
)
Cl ) G = C7 4 > C7 (Cl = 7Cl
b6 C7 4 = h
ν
> IC7 ( = 7I
IC7 ( = Cl ) > C7 (Cl = ClI ClI = 7I > 7Cl
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<rob 4,4 3ree,radical chlorination of
he?ane gives very poor yields of
',chlorohe?ane$ while cyclohe?ane can be
converted to chlorocyclohe?ane in good
yield
a6 7ow do you account for this differenceF
b6 What ratio of reactants cyclohe?ane and
chlorine6 would you use for the synthesis of
chlorocyclohe?aneF
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Lsing 8 e* values$ we can compare
stabilities of reactants and products
by comparing their energies
• We use the #ibbs 3ree &nergy$
#5
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#eneral RuleA 1f # 5 is more negative
than P ,') kENmol$ the reaction “goes
to completion$! ie O KKQ products
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3actors -etermining # °
3ree energy change depends onA &nthalpy
75 H enthalpy of products6 , enthalpy of
reactants6
&ntropy
S5 H entropy of products6 , entropy of
reactants6
# ° H 7 ° , T
S °
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&nthalpy
• 7 5 H heat released or absorbed
during a chemical reaction at standard
conditions
• &?othermicA , 76 heat is released
• &ndothermicA =
76 heat is absorbed
• Reactions favor products with lowest
enthalpy strongest bonds6
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&ntropy
• S 5 H change in randomness$
disorder$ or freedom of movement
• 1ncreasing heat$ volume$ or number
of particles increases entropy• Spontaneous reactions ma?imize
disorder and minimize enthalpy
• 1n the e*uation # 5 H 75 , T S 5 theentropy value is often small
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<rob 4,B When ethene is mi?ed with
hydrogen in the presence of a platinum
catalyst$ hydrogen adds across the doublebond to form ethane 0t room temperature$
the reaction goes to completion <redict the
signs of 7 ° and S ° for this reaction
&?plain the signs in terms of bonding andfreedom of motion
H
H
H
H+ H2
PtH
H
H
HH
H
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2ond,-issociation &nthalpies 2-&6
• 2ond,dissociation re*uires energy =2-&6
• 2ond formation releases energy , 2-&6
• 2-& can be used to estimate7 for a reaction
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2ond,-issociation &nthalpies 2-&6
• 2-& for homolytic cleavage of bonds in agaseous molecule
7omolytic cleavageA When the bond
breaks$ each atom gets one electron$eg 02 > 0I = 2I
7eterolytic cleavageA When the bond
breaks$ the most electronegative atomgets both electrons$ eg 02 > 0= = :2,
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7omolytic and 7eterolytic Cleavage
We use homolytic cleavage 2-&.s for 75 calculations
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&nthalpy Changes in Chlorination
C7 (,7 = Cl,Cl → C7 (,Cl = 7,Cl2onds 2roken 2-& per mole6 2onds 3ormed 2-& per /ole6
Cl,Cl )4) kM 7,Cl 4(' kM
C7 (,7 4( kM C7 (,Cl (' kM
T9T0"S BB kM T9T0" B%) kM
7 ° H Σ 2-& break , Σ 2-& make
7 ° H BB kM ; B%) kM H ,'J kM
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<rob 4,K a6 <ropose a mechanism for
the free,radical chlorination of ethane
C7 (,C7 ( = Cl ) > C7 (,C7 ) Cl = 7Cl hν
1nitiationA Cl ) > )ClI
<ropagation 'A ClI = C7 (
,C7 (
> C7 (
,C7 )
I = 7Cl
<ropagation )A C7 (,C7 ) I = Cl ) > C7 (,C7 ) Cl = ClI
9verallA C7 (,C7 ( = Cl ) > C7 (,C7 ) Cl = 7Cl
hν
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<rob 4,K b6 Calculate 7 ° for each
step in this reaction
1) CH3-CH3 + Cl• → CH3CH2• + HCl
98 kcal 103 kcal ∆ H 1 = 98 – 103 = -5
2)
CH3CH2• + Cl2 → CH3-CH2Cl + Cl• 58 kcal 81 kcal ∆ H 2 = 58 – 81 = -23
∆ H rxn = - 28 kcal/mol
c6 Calculate the overall value of 7 ° forthis reaction
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8inetics
• 8inetics is the study of reactionrates
• Rate of the reaction is a measure
of how the concentration of the products increase while theconcentration of the reactants
decrease
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8inetics
• 0 rate e*uation is also calledthe rate law and it gives therelationship between theconcentration of the reactantsand the reaction rate observed
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Th R t "
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The Rate "aw
• 3or the reaction 0 = 2 → C = -$
rate H k r 0U a2U b
a is the order with respect to 0
b is the order with respect to 2 a = b is the overall order k r is a temp,dependent rate constant
9rder is the number of molecules of that
reactant which are present in the rate, determining step of the mechanism
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<rob 4,'' The reaction of tert,butyl
chloride with methanolA
is found to follow this rate e*uationA
C7 ( 6(C,Cl = C7 (,97 > C7 ( 6(C,9C7 ( = 7Cl
Rate H k r [C7 ( 6(C,Cl ]
a6 What is the kinetic order with respect to
t,butyl chlorideF
b6 What is the kinetic order with respect to
methanolF
c6 What is the kinetic order overallF
0 i i &
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0ctivation &nergy • The value of k r depends on temperature as given by
the 0rrhenius e*uationA
RT E a Aek /
r
−=
where 0 H constant fre*uency factor6 & a H activation energy
R H gas constant$ %('4 MNkelvin,mole
T H absolute temperature& a is the minimum kinetic energy
needed to react
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The 0rrhenius e*n implies rate depends on the
fraction of molecules with kinetic energy V &a
• 0t higher temperatures$ more molecules have there*uired energy to react
• 0t higher temperatures$ all reactions go
faster$ even unwanted side reactionsD
&nergy -iagram of an
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&nergy -iagram of an
&?othermic Reaction
• The transition state is the highest point on the graph
• The activation energy is the energy difference betweenthe reactants and the transition state
∀ 75 is the energy difference between reactants and products
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The Transition State
• The highest,energy state in a molecular
collision that leads to a reaction
• LnstableD Cannot be isolatedD
transition statereactant
intermediate intermediate product
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The Transition State
• The “double dagger!6 symbol and
s*uare brackets are used to indicate a
transition state
• Transition state can go either direction$
forward to products$ or back to reactants
shows old bond
breaking
shows new bond
forming
shows charge or
radical e,
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1ntermediates• either reactants nor products$
they e?ist for a short time
• Can sometimes be isolated
because they have some stability• 3ree radicals are Eust one of four
common types of intermediates in
organic chemistry
Th R t "i iti St
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The Rate,"imiting Step
• Reaction intermediates are stable as long
as they don.t collide with another moleculeor atom$ but they are very reactive
• Transition states are at energy ma?imums
• 1ntermediates are at energy minimums
The reaction step with highest & a will be the
slowest$ therefore rate,determining for the
entire reaction
R ti & -i
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Reaction &nergy -iagrams
This one,stepreaction has no
intermediates$ only a
transition state
R ti & -i f
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Reaction &nergy -iagrams of
/ulti,Step Reactions+
e n e r g y
>
reaction coordinate >
The highest & a hill is the
rate,determining step
This two,step r?n has a
“valley! indicating that an
intermediate is involved
Reaction &nerg -iagram for
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Reaction &nergy -iagram for
the Chlorination of /ethane
'st
propagation
step
) nd
propagation
step
Rate & and Temperature
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Rate$ & a$ and Temperature
X + CH4 HX + CH3
X Ea(per Mole) Rate at 27 ° Rate at 227 °
F ! "#$,$$$ %$$,$$$
Cl "7 "%$$ "8,$$$
Br 7! & ' "$8 $.$"!
I "#$ 2 ' "$"&
2 ' "$&
Conclusions 0bout /ethane
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Conclusions 0bout /ethane
7alogenation Reactions
• With increasing & a$ rate decreases
• With increasing temperature$ rate
increases
• 3luorine reacts e?plosively
• Chlorine reacts at a moderate rate
• 2romine must be heated to react• 1odine does not react detectably6
0 t l t l th & b t d t ff t
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0 catalyst lowers the & a$ but does not affect
the energies of reactants or products
& awith catalyst
S l d < b 4 4 Th i
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Solved <rob 4,4 The reaction
C7 4 = ClI > IC7 ( = 7Cl
has an activation energy of ='B kMNmoland a 75 of =4 kMNmol -raw a reaction
energy diagram of this reaction
<rob 4 '4 -raw the reaction energy
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<rob 4,'4 -raw the reaction energy
diagram for the reverse reactionA
IC7 ( = 7Cl > C7 (Cl = ClI
P b 4 1 "# b % t% & t# d
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Prob 4-1! "#e bro$%nat%on o& $et#ane proceeds
t#rou'# t#e &ollo(%n' steps:
Br2
2Br
light
CH4 + Br
CH3 + HBr
CH3 + Br2 CH3Br + Br
HEa+ 46 kcal
46 kcal
+16 kcal
18 kcal
-24 kcal1 kcal
a) Draw the complete reaction-energy
diagram or thi! r"n#$) %a$el the rate-limiting !tep#
c) Draw the !tr&ct&re o each tran!ition!tate#d) Comp&te the o'erall 'al&e o H° or the$romination#
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Chlorination of <ropane /echanism
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Chlorination of <ropane /echanism
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Conclusion
• Some hydrogens are morereactive than others
2-&.s indicate that C,7 bondstrengths vary from '5 to )5 to
(5 carbons
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<rimary '56$ Secondary )56$
and Tertiary (56 7ydrogens
2-&.s for the 3ormation of 3ree
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2-& s for the 3ormation of 3ree
Radicals
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Chlorination &nergy -iagram
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Chlorination &nergy -iagram
• "ower & aH faster rate$ so most stable
intermediate is formed faster
2 i ti f <
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2romination of <ropane
statistical yieldA BQ )Q
actual yieldA (Q KBQ
• 2romination of propane is much
more selective than chlorination
• WhyF
&nergy -iagramsA Chlorination
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&nergy -iagramsA Chlorination
vs 2romination of <ropane
• Chlorination is e?othermic because bond energy
of 7,Cl bond is high 4(' kM6 while that of 7,2r is
( kM lower
&nergy -iagramsA Chlorination
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&nergy -iagramsA Chlorination
:s 2romination of <ropane
• 7igher & a for bromination e?plains why
bromination is slower$ but not why more selective
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Th l th & diff th
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The larger the & a difference$ the
more selective the reaction will beD
The 7ammond <ostulate
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The 7ammond <ostulate
•Related species that are similar inenergy are also similar in structure
• The structure of the transition stateresembles the structure of the closest
stable species• &ndothermic reactionA Transition state
is product,like
• &?othermic reactionA Transition state isreactant,like
1 & th i 3i t St
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1n &?othermic 3irst Stepshorter line shows bond mostly still intact
longer line shows bond
mostly yet to form
transition state looks
mostly like reactantsA
C7 4 = ClI
1n &ndothermic 3irst Step
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1n &ndothermic 3irst Step
longer line shows bond mostly broken
shorter line shows bondmostly formed
transition statelooks mostly like
productsA
7 (CI = 72r
Radical 1nhibitors
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Radical 1nhibitors
• 9ften added to food to retard spoilageby radical chain reactions
• Without an inhibitor$ each initiationstep will cause a chain reaction so that
many molecules will react• 0n inhibitor combines with the free
radical to form a stable molecule
• :itamin & and vitamin C are thought to protect living cells from free radicals
3ree Radical 1nhibitors
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3ree Radical 1nhibitors
• 0 radical chain reaction is fast and has manye?othermic steps that create more reactive radicals
• When an inhibitor reacts with the radical$ it creates
a stable intermediate$ and any further reactions willbe endothermic and slow
Reactive 1ntermediates
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Reactive 1ntermediates• Short,lived species that are never present in high
concentrations because they react as soon asthey are formed
• Commonly contain carbon with only ) or ( bonds
instead of 4
Trivalent intermediatesCarbocation ='6
3ree radical J6
Carbanion ,'6
-ivalent intermediateA Carbene J6
Th / t C 1 t di t
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The /ost Common 1ntermediates
Carbocation Structure
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Carbocation Structure
• Carbon has electronsX positively charged
• Carbon is sp)
hybridized$ ie trigonal planar geometry$ with vacant p orbital• Carbocations are STR9# &"&CTR9<71"&S
C b ti St bilit
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Carbocation Stability
Same as free radical stability orderD
Carbocation Stability
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Carbocation Stability
• Stabilized by alkyl
substituents in two waysA
' 1nductive effectA -onation
of electron density alongthe sigma bonds
) 7yperconEugationA
9verlap of sigma bonding
orbitals with empty porbital
<rob 4,)K Rank the following
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<rob 4 )K Rank the following
carbocations in decreasing order of
stability Classify each as '5$ )5 or (5
• 1sopentyl cation$ C7 ( 6) C7C7 ) C7 ) =
• (,methyl,),butyl cation$ C7 (,C7 =,C7C7 ( 6)
• ),methyl,),butyl cation$ C7 (,C =,C7 ( 6C7 ) C7 (
+
3ree Radicals
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• 0lso electron,deficient sp) species with ' unpaired e, $but zero chargeD
• Stabilized by alkyl substituents same as carbocations
• 9rder of stabilityA (° O ) ° O '° O methyl
Stability of 3ree Radicals
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Stability of 3ree Radicals
Same as carbocation stability orderD
"ater We Will See 7ow Resonance
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&nhances The Stability of Radicals$
Carbocations and Carbanions
+
H%
H+
H%
H2
H
H2
H
H2
H
H2
H% H2
H% H2
: : *
Carbanions
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Carbanions
• &ight electrons on carbonA bonding plus one lone pair $∴
carbon is tetrahedral sp(
• Carbon has a negative
charge• -estabilized by alkyl
substituents
• Stability orderA
/ethyl O'° O ) ° O (°
Carbanions
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Carbanions
• /ost common carbanions are stabilizedby resonance as in this e?ample from
ne?t semesterA
• 2ecause of their formal negative charge$carbanions are STR9# LC"&9<71"&SD
Carbenes
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Ca be es
• Carbon has only ) bonds$ but has a lone pair
• sp) $∴
trigonal planar
• :acant p orbital$ so can be electrophilic
• "one pair of electrons$ so can be nucleophilic
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The &nd