Halogenacion de Alcanos

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



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



4.154.15

Chlorination of MethaneChlorination of Methane



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



4.164.16

Structure and Stability of FreeStructure and Stability of FreeRadicalsRadicals



contain unpaired electronscontain unpaired electrons

.... ....

.. ..$$ $$%xamples& $%xamples& $

22

'$'$ '' $$....

..

ClCl ClCl..

....

....

Free RadicalsFree Radicals



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



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



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




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."


thanthan


thanthan

CCHH##CC CHCH

##

CHCH##

..

tert tert But.l radicalBut.l radical

(tertiar."(tertiar."




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.




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



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



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



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



4.1!4.1!

Mechanis" of Chlorination ofMechanis" of Chlorination ofMethaneMethane



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



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




ClCl....


....

....HH##CC ..++ ++


++ ++

++ ++


ClCl....

......HH##CC ..

....ClCl....

ClCl....

....

HH##CC HH HH ClCl....

....

....ClCl....

ClCl....

....

HH##CC....

.... ClCl

HH##CC....

.... ClCl




,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 ..++ ++


++ ++

++ ++


ClCl....

......HH##CC ..

....ClCl....

ClCl....

....

HH##CC HH HH ClCl....

....

....ClCl....

ClCl....

....

HH##CC....

.... ClCl

HH##CC....

.... ClCl



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



4.1#4.1#

Halogenation of Higher AlkanesHalogenation of Higher Alkanes



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



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



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



@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$%1$%



@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#%



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



=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#>"

(#;>"(#;>"



@ercentage of product that results from@ercentage of product that results fromreplacement of indicated h.drogenreplacement of indicated h.drogen

!.$%!.$%

&!%&!%



#;#;;;

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



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



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



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

Halogenacion de Alcanos

Documents

Transcript of Halogenacion de Alcanos