Upload
juan-david
View
247
Download
0
Embed Size (px)
Citation preview
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 1/34
4.144.14
Halogenation of AlkanesHalogenation of Alkanes
RH + XRH + X22 →→ RX + HX RX + HX
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 2/34
RH + XRH + X22 →→ RX + HX RX + HX
explosive for Fexplosive for F22
exothermic for Clexothermic for Cl22 and Br and Br 22
endothermic for Iendothermic for I22
EnergeticsEnergetics
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 3/34
4.154.15
Chlorination of MethaneChlorination of Methane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 4/34
carried out at high temperature (4 !C"carried out at high temperature (4 !C"
CHCH44 + Cl + Cl22 →→ CH CH##Cl + HClCl + HCl
CHCH##Cl + ClCl + Cl22 →→ CH CH22ClCl22 + HCl + HCl
CHCH22ClCl22 + Cl + Cl22 →→ CHCl CHCl## + HCl + HCl
CHClCHCl## + Cl + Cl22 →→ CCl CCl44 + HCl + HCl
Chlorination of MethaneChlorination of Methane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 5/34
4.164.16
Structure and Stability of FreeStructure and Stability of FreeRadicalsRadicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 6/34
contain unpaired electronscontain unpaired electrons
.... ....
.. ..$$ $$%xamples& $%xamples& $
22
'$'$ '' $$....
..
ClCl ClCl..
....
....
Free RadicalsFree Radicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 7/34
ost free radicals in )hich car*on *earsost free radicals in )hich car*on *ears
the unpaired electron are too unsta*le to *ethe unpaired electron are too unsta*le to *e
isolatedisolated
,l-.l radicals are classified as primar./ ,l-.l radicals are classified as primar./
secondar./ or tertiar. in the same )a. thatsecondar./ or tertiar. in the same )a. that
car*ocations arecar*ocations are
CCRR RR
RR
..
Alkyl Radicals Alkyl Radicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 8/34
Figure 4.19 Structure of methyl radical.Figure 4.19 Structure of methyl radical.
eth.l radical is planar/ )hich suggests that car*on iseth.l radical is planar/ )hich suggests that car*on is spsp22
h.*ridi0ed and that the unpairedh.*ridi0ed and that the unpaired
electron is in aelectron is in a p p or*italor*ital
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 9/34
CCRR RR
RR
..
1he order of sta*ilit. of free radicals is the1he order of sta*ilit. of free radicals is the
same as for car*ocationssame as for car*ocations
Alkyl Radicals Alkyl Radicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 10/34
CCHH HH
HH
..
eth.l radicaleth.l radical
less sta*leless sta*le
thanthan
CC
HH##CC HH
HH
..
%th.l radical%th.l radical(primar."(primar."
CCHH##CC CHCH
##
HH
..
Isoprop.l radicalIsoprop.l radical
(secondar."(secondar."
less sta*leless sta*le
thanthan
less sta*leless sta*le
thanthan
CCHH##CC CHCH
##
CHCH##
..
tert tert But.l radicalBut.l radical
(tertiar."(tertiar."
Alkyl Radicals Alkyl Radicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 11/34
1he order of sta*ilit. of free radicals can1he order of sta*ilit. of free radicals can*e determined *. measuring *ond strengths*e determined *. measuring *ond strengths
B. 3*ond strength3 )e mean the energ.B. 3*ond strength3 )e mean the energ.
reuired to *rea- a covalent *ondreuired to *rea- a covalent *ond
, chemical *ond can *e *ro-en in t)o , chemical *ond can *e *ro-en in t)o
different )a.s5heterol.ticall. ordifferent )a.s5heterol.ticall. orhomol.ticall.homol.ticall.
Alkyl Radicals Alkyl Radicals
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 12/34
In a homol.tic *ond cleavage/ the t)o electrons inIn a homol.tic *ond cleavage/ the t)o electrons in
the *ond are divided euall. *et)een the t)o atomsthe *ond are divided euall. *et)een the t)o atoms
$ne electron goes )ith one atom/ the second )ith$ne electron goes )ith one atom/ the second )ith
the other atomthe other atom
In a heterol.tic cleavage/ one atom retains *othIn a heterol.tic cleavage/ one atom retains *oth
electronselectrons
6 6 ++
Homol.ticHomol.tic
Heterol.ticHeterol.tic
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 13/34
1he species formed *. a homol.tic *ond cleavage1he species formed *. a homol.tic *ond cleavage
of a neutral molecule are free radicals 1herefore/of a neutral molecule are free radicals 1herefore/
measure energ. cost of homol.tic *ond cleavage tomeasure energ. cost of homol.tic *ond cleavage to
gain information a*out sta*ilit. of free radicalsgain information a*out sta*ilit. of free radicals
1he more sta*le the freeradical products/ the )ea-er 1he more sta*le the freeradical products/ the )ea-er
the *ond/ and the lo)er the *onddissociation energ.the *ond/ and the lo)er the *onddissociation energ.
Homol.ticHomol.tic
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 14/34
Bonddissociation energ. measurements tellBonddissociation energ. measurements tellus that isoprop.l radical is 7# -89mol moreus that isoprop.l radical is 7# -89mol more
sta*le than prop.lsta*le than prop.l
4747 #:;#:;
CHCH##CHCH22CHCH##
CHCH##CHCH22CHCH22
++
HH
..
..CHCH
##CHCHCHCH##
++
HH
..
..
Measures of Free Radical Staility Measures of Free Radical Staility
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 15/34
Bonddissociation energ. measurements tellBonddissociation energ. measurements tellus thatus that tert tert *ut.l radical is # -89mol more*ut.l radical is # -89mol more
sta*le than iso*ut.lsta*le than iso*ut.l
4747
(CH(CH##""##CHCH
(CH(CH##""22CHCHCHCH22
++
HH
..
..(CH(CH
##""##CC
++
HH
..
..#<#<
Measures of Free Radical Staility Measures of Free Radical Staility
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 16/34
4.1!4.1!
Mechanis" of Chlorination ofMechanis" of Chlorination ofMethaneMethane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 17/34
1he initiation step 3gets the reaction going31he initiation step 3gets the reaction going3
*. producing free radicals5chlorine atoms*. producing free radicals5chlorine atoms
from chlorine molecules in this casefrom chlorine molecules in this case
Initiation step is follo)ed *. propagationInitiation step is follo)ed *. propagation
steps %ach propagation step consumes onesteps %ach propagation step consumes one
free radical *ut generates another onefree radical *ut generates another one
....ClCl.... ClCl....
.... ....ClCl.... .. ClCl....
......++
Freeradical chain mechanismFreeradical chain mechanism
Initiation step&Initiation step&
Mechanism of Chlorination of MethaneMechanism of Chlorination of Methane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 18/34
ClCl....
......HH##CC HH HH ClCl
....
....HH##CC ..++ ++
First propagation step&First propagation step&
=econd propagation step&=econd propagation step&
++ ++ ClCl....
......HH##CC ..
....ClCl....
ClCl....
....
HH##CC....
.... ClCl
Mechanism of Chlorination of MethaneMechanism of Chlorination of Methane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 19/34
ClCl....
......HH##CC HH HH ClCl
....
....HH##CC ..++ ++
=econd propagation step&=econd propagation step&
++ ++
++ ++
First propagation step&First propagation step&
ClCl....
......HH##CC ..
....ClCl....
ClCl....
....
HH##CC HH HH ClCl....
....
....ClCl....
ClCl....
....
HH##CC....
.... ClCl
HH##CC....
.... ClCl
Mechanism of Chlorination of MethaneMechanism of Chlorination of Methane
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 20/34
,lmost all of the product is formed *. repetitive ,lmost all of the product is formed *. repetitive
c.cles of the t)o propagation stepsc.cles of the t)o propagation steps
ClCl....
......HH##CC HH HH ClCl
....
....HH##CC ..++ ++
=econd propagation step&=econd propagation step&
++ ++
++ ++
First propagation step&First propagation step&
ClCl....
......HH##CC ..
....ClCl....
ClCl....
....
HH##CC HH HH ClCl....
....
....ClCl....
ClCl....
....
HH##CC....
.... ClCl
HH##CC....
.... ClCl
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 21/34
stop chain reaction *. consuming free radicalsstop chain reaction *. consuming free radicals
hardl. an. product is formed *. termination stephardl. an. product is formed *. termination step
*ecause concentration of free radicals at an.*ecause concentration of free radicals at an.
instant is extremel. lo)instant is extremel. lo)
++HH##CC ..
ClCl
....
...... HH##CC
....
....ClCl
!ermination Steps!ermination Steps
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 22/34
4.1#4.1#
Halogenation of Higher AlkanesHalogenation of Higher Alkanes
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 23/34
CHCH##CHCH
## + Cl + Cl22 CHCH
##CHCH22Cl + HClCl + HCl42!C42!C
(;<>"(;<>"
ClCl+ Cl+ Cl22 + HCl+ HCl
hh ν ν
(;#>"(;#>"
can *e used to prepare al-.l chloridescan *e used to prepare al-.l chlorides
from al-anes in )hich all of the h.drogensfrom al-anes in )hich all of the h.drogensare euivalent to one anotherare euivalent to one another
Chlorination of AlkanesChlorination of Alkanes
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 24/34
a?or limitation&a?or limitation&
Chlorination gives ever. possi*leChlorination gives ever. possi*lemonochloride derived from original car*onmonochloride derived from original car*on
s-eletons-eleton
'ot much difference in reactivit. of 'ot much difference in reactivit. of different h.drogens in moleculedifferent h.drogens in molecule
Chlorination of AlkanesChlorination of Alkanes
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 25/34
CHCH##CHCH
22CHCH22CHCH
## ClCl
22
hh ν ν
Chlorination of *utane gives a mixture of Chlorination of *utane gives a mixture of 7chloro*utane and 2chloro*utane7chloro*utane and 2chloro*utane
CHCH##CHCH
22CHCH22CHCH
22ClCl
CHCH##CHCHCHCH22CHCH##
ClCl
(2<>"(2<>"
(;2>"(;2>"
E"ampleE"ample
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 26/34
@ercentage of product that results from@ercentage of product that results fromsu*stitution of indicated h.drogen ifsu*stitution of indicated h.drogen if
ever. collision )ith chlorine atoms isever. collision )ith chlorine atoms isproductiveproductive
1$%1$%
1$%1$%1$%1$%
1$%1$%
1$%1$%
1$%1$%
1$%1$%
1$%1$%1$%1$%
1$%1$%
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 27/34
@ercentage of product that actuall. results@ercentage of product that actuall. resultsfrom replacement of indicated h.drogenfrom replacement of indicated h.drogen
4.6%4.6%
4.6%4.6%4.6%4.6%
1#%1#%
1#%1#%
1#%1#%
4.6%4.6%
4.6%4.6%4.6%4.6%
1#%1#%
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 28/34
divide *. 4Adivide *. 4A
4A4A
4A4A
7 7
7<7<
4A4A #: #:
A A secondarysecondar y hydrogen is astracted #.9 timeshydrogen is astracted #.9 timesfaster than afaster than a primary primary hydrogen y a chlorinehydrogen y a chlorine
atom.atom.
4.6%4.6%1#%1#%
Relati$e rates of hydrogen atom astractionRelati$e rates of hydrogen atom astraction
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 29/34
=imilarl./ chlorination of 2meth.l*utane=imilarl./ chlorination of 2meth.l*utanegives a mixture of iso*ut.l chloride andgives a mixture of iso*ut.l chloride andtert*ut.l chloridetert*ut.l chloride
HH
CHCH##CCHCCH
##
CHCH##
CHCH##CCHCCH
22ClCl
CHCH##
HH
ClCl
CHCH##CCHCCH
##
CHCH##
hh ν ν
ClCl22
(A#>"(A#>"
(#;>"(#;>"
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 30/34
@ercentage of product that results from@ercentage of product that results fromreplacement of indicated h.drogenreplacement of indicated h.drogen
!.$%!.$%
&!%&!%
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 31/34
#;#;;;
divide *. ;divide *. ;
;; 7 7
;; # #
A A tertiarytertiar y hydrogen is astracted %.# times fasterhydrogen is astracted %.# times fasterthan athan a primary primary hydrogen y a chlorine atom.hydrogen y a chlorine atom.
Relati$e rates of hydrogen atom astractionRelati$e rates of hydrogen atom astraction
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 32/34
RR##CHCH DD RR22CHCH22 DD RCHRCH##
chlorination&chlorination& 4 4 77
*romination&*romination& 7A47A4 <2<2 77
Chlorination of an al-ane gives a mixture ofChlorination of an al-ane gives a mixture ofever. possi*le isomer having the same s-eletonever. possi*le isomer having the same s-eletonas the starting al-ane Eseful for s.nthesis onl.as the starting al-ane Eseful for s.nthesis onl.)hen all h.drogens in a molecule are euivalent)hen all h.drogens in a molecule are euivalent
Bromination is highl. regioselective forBromination is highl. regioselective forsu*stitution of tertiar. h.drogens a?or s.ntheticsu*stitution of tertiar. h.drogens a?or s.ntheticapplication is in s.nthesis of tertiar. al-.lapplication is in s.nthesis of tertiar. al-.l
*romides*romides
Selecti$ity of free&radical halogenationSelecti$ity of free&radical halogenation
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 33/34
Chlorination is useful for s.nthesis onl. )henChlorination is useful for s.nthesis onl. )hen
all of the h.drogens in a molecule are euivalentall of the h.drogens in a molecule are euivalent
(A4>"(A4>"
ClCl22
hh ν ν
Synthetic application of chlorination ofSynthetic application of chlorination of
an alkanean alkane
ClCl
7/26/2019 Halogenacion de Alcanos
http://slidepdf.com/reader/full/halogenacion-de-alcanos 34/34
Bromination is highl. selective for su*stitutionBromination is highl. selective for su*stitution
of tertiar. h.drogensof tertiar. h.drogens
a?or s.nthetic application is in s.nthesis ofa?or s.nthetic application is in s.nthesis of
tertiar. al-.l *romidestertiar. al-.l *romides
(;A>"(;A>"
Br Br 22
hh ν ν
HH
CHCH##CCHCCH
22CHCH22CHCH
##
CHCH##
Br Br
CHCH##CCHCCH
22CHCH22CHCH
##
CHCH##
Synthetic application of romination of an alkaneSynthetic application of romination of an alkane