Ch05 Reactions of Alkenes and Alkynes

7/25/2019 Ch05 Reactions of Alkenes and Alkynes

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WILLIAM H. BROWN

THOMAS POON

www.wiley.com/college/brown

C H A P T E R F I V E

Reactions of Alkenes

and Alknes

Copyright2014 John Wiley & Sons, Inc. All rights reserved.


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n

CH3CH CH2

X2CH3 CH CH2

X

X

(anti) or transHalogenation

(X2=Cl2,Br2)

HXCH3 CH CH2

X H

Hydrohalogenation

H2O

H+ CH3 CH CH2

OH H

CH3OH

H+ CH3 CH CH2

OCH3 H

alkyl halide

alcohol

ether

Hydration


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Pd!C o" Pt!C

CH#CH CH$

H$CH# CH CH$

H HHd"o%enation

CH# CH CH$

OH OH

Hd"o&lation o"Mild O&idation

H' CH# C

O

OH

CH# CH CH$

H BH$

%lcol

(MnO)

NaOH

(MnO)' CO$ O&idati*e clea*a%e

o" St"on% O&idation

BH#Hd"o+o"ation

alkane

ca"+o&lic acid


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Energy Diagram

, Energy diagram-A

%"a/ of t/e ene"%c/an%es t/at occ0"

d0"in% a c/e1ical

"eaction2 ene"% is

lotted on t/e 3a&is.

Reaction "o%"ess on

t/e &3a&is.

, Fi%0"e 4.5 An ene"%

dia%"a1 fo" a one3ste

e&ot/e"1ic "eaction of Cand A3B to %i*e C3A and

B.

Copyright 20! "ohn #iley $ %ons, &nc' ll rights resered'


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Energy Diagram

, Transition state:An 0nsta+le secies of 1a&i101 ene"%

fo"1ed d0"in% t/e co0"se of a "eaction2 a 1a&i101 on anene"% dia%"a1.

, Activation energy Ea:T/e diffe"ence in ene"% +et6een t/e

"eactants and t/e t"ansition state.

7 Eadete"1ines t/e "ate of "eaction.

7 If the Eais large8 *e" fe6 1olec0la" collisions occ0" 6it/

s0fficient ene"% to "eac/ t/e t"ansition state8 and t/e

"eaction is slo6.

7 If the Eais small8 1an collisions %ene"ate s0fficient

ene"% to "eac/ t/e t"ansition state8 and t/e "eaction is

fast.



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Energy Diagram

, Also s/o6n on an ene"% dia%"a1 a"e-

7 Heat of reaction H- T/e diffe"ence in ene"% +et6een

"eactants and "od0cts.

7 Exothermic reaction-A "eaction in 6/ic/ t/e ene"% of

t/e "od0cts is lo6e" t/an t/e ene"% of t/e "eactants2 a

"eaction in 6/ic/ /eat is li+e"ated.7 Endothermic-A "eaction in 6/ic/ t/e ene"% of t/e

"od0cts is /i%/e" t/an t/e ene"% of t/e "eactants- a

"eaction in 6/ic/ /eat is a+so"+ed.



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Energy Diagram

, Fi%0"e 4.$ Ene"% dia%"a1 fo" a t6o3ste e&ot/e"1ic

"eaction in*ol*in% fo"1ation of a "eaction inte"1ediate.



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Energy Diagram

7 Reaction intermediate:An ene"% 1ini101 +et6een

t6o t"ansition stes. Inte"1ediates a"e /i%/l "eacti*e and"a"el8 if e*e"8 can one +e isolated.

7 Rate-determining step:T/e ste in a "eaction se90ence

t/at c"osses t/e /i%/est ene"% +a""ie"2 t/e slo6est ste

in a 10ltiste "eaction.



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Developing a Reaction Mechanism

,


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hy Mechanisms!

Mechanisms provide:

, A t/eo"etical f"a1e6o"k 6it/in 6/ic/ to o"%ani>e desc"iti*e

c/e1ist".

, An intellect0al satisfaction de"i*ed f"o1 const"0ctin% 1odels

t/at acc0"atel "eflect t/e +e/a*io" of c/e1ical sste1s.

, A tool 6it/ 6/ic/ to sea"c/ fo" ne6 info"1ation and ne60nde"standin%.



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Mechanism "atterns

, Befo"e 6e disc0ss an a"tic0la" "eactions and t/ei"

1ec/anis1s8 let 0s fi"st anal>e se*e"al of t/e co11onatte"ns to +e seen in t/e 1ec/anis1 6e 6ill enco0nte".

, "attern #: Add a "roton$In Section $.58 6e sa6 /o6

c0"*ed a""o6s can +e 0sed to s/o6 /o6 a "oton3t"ansfe"

"eaction takes lace. In t/is e&a1le8 c0"*ed a""o6s s/o6t/e "edist"i+0tion of *alence elect"ons and t/e fo"1ation of a

ne6 co*alent +ond 6/en a "oton is t"ansfe""ed f"o1 acetic

acid to a11onia.



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Mechanism "atterns

, "attern #: Add a proton$In t/is e&a1le8 a "oton is

added ac"oss t/e i +ond of a C?C do0+le +ond to fo"1a ne6 C?H +ond. Addin% a "oton is tical of all

"eactions t/at a"e catal>ed + an acid.



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Mechanism "atterns

, "attern %: Take a proton a&ay$Re*e"sin% Patte"n 5

co""esonds to takin% a "oton a6a. T/e 1ec/anis1 fo"takin% a "oton a6a is si1ila" to t/at fo" addin% a "oton8

e&cet t/at 6e foc0s o0" attention on t/e co1o0nd t/at

loses t/e "oton instead of t/e co1o0nd t/at adds a "oton.



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Mechanism "atterns

, "attern ': Reaction of an electrophile and a n(cleophile

to form a ne& covalent )ond$, Electrophile:an elect"on3oo" secies t/at can accet a

ai" of elect"ons to fo"1 a ne6 co*alent +ond2 a Le6is acid.

, *(cleophile:an elect"on3"ic/ secies t/at can donate a ai"

of elect"ons to fo"1 a ne6 co*alent +ond8 a Le6is +ase.



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Mechanism "atterns

, "attern +: Rearrangement of a )ond.A "ea""an%e1ent

occ0"s 6/en t/e elect"ons of a si%1a +ond +"eak t/e +ondf"o1 one ato1 and fo"1 a ne6 co*alent +ond to an ad@acent

ato1.



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Electrophilic Additions to Alkenes

, Addition of hydrogen halides .H/l0 Hr0 HI1

, Addition of &ater .H%23H%42+1Hydration

7

, Addition of halogens ./l%0 r%1 Halogenation



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Addition of Hydrogen Halides

, Ca""ied o0t 6it/ t/e 0"e "ea%ents o" in a ola" sol*ent s0c/ as acetic acid.

, Addition is "e%ioselecti*e.

7 Regioselective reaction:A "eaction in 6/ic/ one di"ection of +ondfo"1in% o" +ond +"eakin% occ0"s in "efe"ence to all ot/e" di"ections.

7 Markovnikov5s r(le:In additions of H to a do0+le +ond8 H adds to t/e

ca"+on 6it/ t/e %"eate" n01+e" of /d"o%ens al"ead +onded to it.



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Regioselectivity

, Ma"ko*niko*=s "0le is +0t one e&a1le of "e%ioselecti*it. We

6ill see 1o"e e&a1les in t/is and late" c/ate"s.



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Markovnikov5s R(le

, "ro)lem- Co1lete t/ese "eactions + "edictin% t/e 1a@o"

"od0ct fo"1ed in eac/ "eaction.



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, A t6o3ste 1ec/anis1

7 4tep #: Add a proton$Fo"1ation of a sec3+0tl cation8 a $ca"+ocation inte"1ediate.

7 4tep %: Reaction of an electrophile and a n(cleophile to

form a ne& covalent )ond$Reaction of t/e sec3+0tl cation

:an elect"o/ile; 6it/ c/lo"ide ion :a n0cleo/ile; co1letes t/e"eaction.

Addition of H/l to %-(tene



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Addition of H/l to %-(tene

, Fi%0"e 4. Ene"% dia%"a1 fo" t/e t6o3ste e&ot/e"1ic

addition of HCl to $3+0tene.



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/ar)ocations

, Fi%0"e 4.# T/e st"0ct0"e of t/e 1et/l and te"t3+0tl cations.



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/ar)ocations

, /ar)ocation-A secies containin% a ca"+on ato1 t/at /as

onl t/"ee +onds to it8 si& elect"ons in its *alence s/ell8 and+ea"s a ositi*e c/a"%e.

7 Bond an%les a+o0t t/e ositi*el c/a"%ed ca"+on a"e

a"o&i1atel 5$D.

7 Ca"+on 0ses sp$/+"id o"+itals to fo"1 si%1a +onds tot/e t/"ee attac/ed %"o0s.

7 T/e 0n/+"idi>ed $po"+ital lies e"endic0la" to t/e

si%1a +ond f"a1e6o"k and contains no elect"ons

, Ca"+ocations a"e-7 Electrophiles:t/at is8 t/e a"e elect"on3lo*in%.

7 6e&is acids:t/at is8 t/e a"e elect"on3ai" acceto"s.



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/ar)ocations

7A # ca"+ocation is 1o"e sta+le t/an a $ ca"+ocation8

and "e90i"es a lo6e" acti*ation ene"% fo" its fo"1ation.7A $ ca"+ocation is8 in t0"n8 1o"e sta+le t/an a 5

ca"+ocation8 and "e90i"es a lo6e" acti*ation ene"% fo" its

fo"1ation.

7 Met/l and 5 ca"+ocations a"e so 0nsta+le t/at t/e a"ene*e" o+se"*ed in sol0tion.



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Relative 4ta)ility of /ar)ocations

, Ind(ctive effect- T/e ola"i>ation of t/e elect"on densit of

a co*alent +ond as a "es0lt of t/e elect"one%ati*it of anea"+ ato1.

7 T/e elect"one%ati*it of a ca"+on ato1 +ea"in% a ositi*e

c/a"%e e&e"ts an elect"on36it/d"a6in% ind0cti*e effect

t/at ola"i>es elect"ons of ad@acent si%1a +onds to6a"dit.

7 T/0s8 t/e ositi*e c/a"%e of a ca"+ocation is not locali>ed

on t/e t"i*alent ca"+on8 +0t "at/e" is delocali>ed o*e"

nea"+ ato1s as 6ell.7 T/e la"%e" t/e a"ea o*e" 6/ic/ t/e ositi*e c/a"%e is

delocali>ed8 t/e %"eate" t/e sta+ilit of t/e cation.



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Markovnikov5s R(le

, T/e c/e1ical +asis fo" t/e "e%ioselecti*it e1+odied in

Ma"ko*niko*=s "0le lies in t/e "elati*e sta+ilities ofca"+ocation inte"1ediates. T/e "eason 6/ t/e "oton

of H? adds to t/e less s0+stit0ted ca"+on of t/e

do0+le +ond is t/at t/is 1ode of addition leads to t/e

1o"e sta+le ca"+ocation inte"1ediate.



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Addition of H%2 to an Alkene

, Addition of H$O to an alkene is called /d"ation.

7Acid3catal>ed /d"ation of an alkene is "e%ioselecti*e-

/d"o%en adds "efe"entiall to t/e less s0+stit0ted

ca"+on of t/e do0+le +ond. T/0s H7OH adds to alkenes

in acco"dance 6it/ Ma"ko*niko*s "0le.



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, 4tep #: Add a proton$ P"oton t"ansfe" f"o1 t/e acid catalst

:H#O'; to "oene %i*es a $ ca"+ocation inte"1ediate.



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Additi f H 2 t Alk


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, "ro)lem:


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Addition of /l%and r%

, "ro)lem: T"eat1ent of $31et/l"oene 6it/ 1et/anol in

t/e "esence of an acid catalst %i*es tert3+0tl1et/l et/e".At one ti1e t/is co1o0nd 6as added to %asoline to

inc"ease octane "atin%.


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, Ca""ied o0t 6it/ eit/e" t/e 0"e "ea%ents o" in an ine"t

sol*ent s0c/ as CH$Cl$.


Additi f /l% d %


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Addition of /l% and r%

, Addition is ste"eoselecti*e.

, 4tereoselective reaction:A "eaction in 6/ic/ oneste"eoiso1e" is fo"1ed o" dest"oed in "efe"ence to all

ot/e"s t/at 1i%/t +e fo"1ed o" dest"oed.

, Addition to a ccloalkene8 fo" e&a1le8 %i*es onl a t"ans

"od0ct. T/e "eaction occ0"s 6it/anti stereoselectivity$


Additi f /l d


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, 4tep #: Reaction of a n(cleophile and an electrophile to

form a ne& covalent )ond$ Reaction of t/e i +ond :an0cleo/ile; 6it/ +"o1ine :an elect"o/ile; %i*es a +"id%ed

+"o1oni01 ion inte"1ediate


Addition of /l and r


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, 4tep %: Reaction of a n(cleophile0 and an electrophile to

form a ne& covalent )ond$Attack of +"o1ide ion f"o1 t/eside oosite t/e +"id%ed +"o1oni01 ion oens t/e t/"ee3

1e1+e"ed "in%.



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/ ) ti R t


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/ar)ocation Rearrangements

, In t/e "ea""an%e1ents 6e e&a1ine8 ticall eit/e" an alkl

%"o0 o" a /d"o%en ato1 1i%"ates 6it/ its +ondin%elect"ons f"o1 an ato1 to an ad@acent elect"on3deficient

ato1 as ill0st"ated in t/e follo6in% 1ec/anis1. T/e ke ste

in t/is te of "ea""an%e1ent is called a #0%-shift$




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, 4tep #: Add a proton$ P"oton t"ansfe" f"o1 HCl to t/e

alkene to %i*e a $ ca"+ocation inte"1ediate.

, 4tep %: Rearrangement of a )ond$Mi%"ation of a 1et/l

%"o0 6it/ its +ondin% elect"ons f"o1 t/e ad@acent ca"+on

%i*es a 1o"e sta+le # ca"+ocation.



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/ar)ocations 4(mmary


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/ar)ocations74(mmary

, T/e ca"+on +ea"in% a ositi*e c/a"%e is sp$/+"idi>ed 6it/

+ond an%les of 5$D a+o0t it., T/e o"de" of ca"+ocation sta+ilit is #$5.

, Ca"+ocations a"e sta+ili>ed + t/e elect"on36it/d"a6in%

ind0cti*e effect of t/e ositi*el c/a"%ed ca"+on.

, Met/l and 5 ca"+ocations a"e so 0nsta+le t/at t/e a"ene*e" fo"1ed in sol0tion.

, Ca"+ocations 1a 0nde"%o "ea""an%e1ent + a 58$3s/ift8

6/en t/e "ea""an%ed ca"+ocation is 1o"e sta+le t/an t/e

o"i%inal ca"+ocation. T/e 1ost co11onl o+se"*ed atte"nof "ea""an%e1ent is f"o1 $ to #.


/ar)ocations 4(mmary


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/ar)ocations74(mmary

, Ca"+ocation inte"1ediates 0nde"%o t/"ee tes of "eactions-

7 5. Rea""an%e1ent + a 58$3s/ift to a 1o"e sta+leca"+ocation.

7 $. Addition of n0cleo/ile to fo"1 a ne6 co*alent +ond

:e.%.8 /alide ion8 H$O8 B"7;.

7 #. Loss of a "oton to %i*e an alkene :t/e "e*e"se of t/efi"st ste in +ot/ t/e addition of H and t/e acid3catal>ed

/d"ation of an alkene;.



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Hydro)oration 2xidation


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Hydro)oration-2xidation

, Bo"on8 ato1ic n01+e" 48 /as t/"ee elect"ons in its *alence

s/ell. To +ond 6it/ ot/e" ato1s8 +o"on 0ses sp$/+"ido"+itals. Beca0se of t/e *acant $po"+ital in t/e *alence s/ell

of +o"on8 BH#8 BF#8 and ot/e" t"i*alent co1o0nds of +o"on

a"e elect"o/iles.




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, Hd"o+o"ation is +ot/ "e%ioselecti*e and sn ste"eoselecti*e.

, Regioselectivity:7H adds to t/e 1o"e s0+stit0ted ca"+on and+o"on adds to t/e less s0+stit0ted ca"+on of a ca"+on7ca"+on

do0+le +ond.

, 4tereoselectivity:Bo"on and 7H add to t/e sa1e face of t/e

do0+le +ond :syn stereoselectivity;.



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Red(ction of Alkenes


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Red(ction of Alkenes

7 T/e 1ost co11on atte"n is sn addition of /d"o%ens2

+ot/ /d"o%ens add to t/e sa1e face of t/e do0+le +ond.7 Cataltic "ed0ction issyn stereoselectivity$



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Heats of Hydrogenation


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Heats of Hydrogenation



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Reactions of Alkynes


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Reactions of Alkynes

, As 6e sa6 in C/ate" )8 one of t/e 1a@o" diffe"ences

+et6een t/e c/e1ist" of alkanes8 alkenes8 and alknesis t/at te"1inal alknes a"e 6eak acids.



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Acetylide Anions in 4ynthesis


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, An acetlide anion is +ot/ a st"on% +ase and a n0cleo/ile. As

a n0cleo/ile8 it can donate a ai" of elect"ons to anelect"o/ilic ca"+on ato1 and fo"1 a ne6 ca"+on3ca"+on +ond.

, In t/is e&a1le8 t/e elect"o/ile is t/e a"tiall ositi*e ca"+on

of c/lo"o1et/ane. As t/e ne6 ca"+on3ca"+on +ond is fo"1ed8t/e ca"+on3/alo%en +ond is +"oken.

, Beca0se an alkl %"o0 is added to t/e o"i%inal alkne8 t/is

"eaction is called alkylation.




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, T/e i1o"tance of alklation of acetlide anions is t/at t/e

t6o3ca"+on 1olec0le acetlene can +e 0sed to c"eate la"%e"ca"+on skeletons.

, Fo" "easons 6e 6ill disc0ss f0ll in C/ate" G8 t/is te of

alklation is s0ccessf0l onl fo" 1et/l and "i1a" alkl/alides :CH# and RCH$;.


Red(ction of Alkynes


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Red(ction of Alkynes

, T"eat1ent of an alkne 6it/ H$in t/e "esence of a t"ansition

1etal catalst8 1ost co11onl Pd8 Pt8 o" Ni8 "es0lts inaddition of t6o 1oles of H$and con*e"sion of t/e alkne to

an alkane.



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Ch05 Reactions of Alkenes and Alkynes

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