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How are halogenoalkanes made?
There are several ways by which halogenoalkanes can
be made, inclding!
free radical substitution of an alkane!
electrophilic addition of "# or #2 to an alkene!
CH4 + Cl2 CH3Cl + HCl
C2H4 + HBr C2H5Br
C2H4 + Br 2 C2H4Br 2
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Free radical substitution Cl2 + CH4
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!ther products of chain reactions
%f an alkane is more than two carbons in length then any of
the hydrogen atoms may be sbstitted, leading to a mi&tre
of different isomers' (or e&am)le!
The mi&tre of )rodcts is difficlt to se)arate, and this is
one reason why chain reactions are not a good method of
)re)aring halogenoalkanes'
"#chloropropane 2#chloropropane
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Further substitution in chain reactions
+ome chloromethane molecles formed dring free radical
sbstittion between methane and chlorine will ndergo
frther sbstittion to form dichloromethane' (rther
sbstittion can occr ntil all hydrogens are sbstitted'
The frther sbstitted chloroalkanes are im)rities thatmst be removed' The amont of these molecles can be
decreased by redcing the )ro)ortion of chlorine in the
reaction mi&tre'
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Chain reactions and o$one
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Free radical reactions true or false?
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Bonding in alkenes
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'ouble bonds and electrophiles
The doble bond of an alkene is an area of high electron
density, and therefore an area of high negative charge'
The negative charge of the doble bond may be attacked
by electron/deficient s)ecies, which will acce)t a lone )air
of electrons'
lkenes ndergo addition reactions when attacked by
electro)hiles' This is called electrophilic addition'
These s)ecies have either a fll )ositive charge or slight
)ositive charge on one or more of their atoms' They are
called electrophiles, meaning electron loving'
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(lectrophilic addition mechanism "
%n the first stage of electro)hilic addition,
the )ositive charge on the electro)hile is
attracted to the electron density in the
doble bond'
s the electro)hile a))roaches the
doble bond, electrons in the Bbond are re)elled towards B'
The )i bond breaks, and bonds to the
carbon, forming a carbocation an ionwith a )ositively/charge carbon atom'
The two electrons in the B bond
move to B forming a B/ ion'
δ+ δ-
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(lectrophilic addition mechanism 2
%n the second stage of electro)hilic
addition, the B/ ion acts as a ncleo)hile
and attacks the carbocation'
The lone )air of electrons on the B
/
ionare attracted towards the )ositively/
charged carbon in the carbocation,
casing B to bond to it'
Becase both electrons in the bond that 4oins B / to the
carbocation ion come from B/, the bond is a co/ordinate bond'
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)ore on the bromine water test
sim)le e5ation for the bromine water test with ethene is!
"owever, becase water is )resent in sch a large
amont, a water molecle 6which has a lone )air7 adds toone of the carbon atoms, followed by the loss of a "8 ion'
CH2*CH2 + Br 2 + H2! CH2BrCH2Br + H2!
CH2*CH2 + Br 2 + H2! CH2BrCH2!H + HBr
The ma4or )rodct of the test is not 1,2/dibromoethane
6"2Br"2Br7 bt 2/bromoethan/1/ol 6"2Br"2:"7'
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(lectrophilic addition reactions
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ddition to uns&mmetrical alkenes
;hen an electro)hile
6e'g' "Br7 attacks an
alkene with three or
more carbon atoms
6e'g' )ro)ene7, a mi&
of )rodcts is
formed' This isbecase these
alkenes are
uns&mmetrical'
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chain of carbon atoms can be re)resented by = when
drawing organic strctres' This is an alk&l group 6general
formla n"2n817'
-rimar& ."/0 carbocations have
one alkyl gro) attached to the
)ositively/charged carbon'
%econdar& .2/0 carbocations
have two alkyl gro)s attached
to the )ositively/charged carbon'
1ertiar& .3/0 carbocations have
three alkyl gro)s attached to
the )ositively/charged carbon'
%tructure of carbocations
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The stability of carbocations increases as the nmber of alkyl
gro)s on the )ositively/charged carbon atom increases'
The stability increases becase alkyl gro)s contain agreater electron density than hydrogen atoms' This
density is attracted towards, and redces, the )ositive
charge on the carbon atom, which has a stabili>ing effect'
%tabilit& of carbocations
increasing stabilit&
tertiar&primar& secondar&
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%tructure of carbocations
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(lectrophiles true or false?
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-olar bonds and nucleophiles
The carbonhalogen bond in halogenoalkanes is )olar
becase all halogens are more electronegative than carbon'
The )olar bond means that the carbon atom has a small)ositive charge 6δ+7, which attracts sbstances with a lone
)air of electrons' These are nucleophiles, meaning
ncles 6)ositive charge7 loving' ?&am)les inclde!
δ+ δ- δ+ δ- δ+ δ- δ+ δ-
ammonia c&anide h&droide
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@cleo)hiles 6@/7 attack the carbon
of a carbonhalogen 6#7 bond,
becase the electron )air on the
ncleo)hile is attracted towards the
small )ositive charge on the carbon'
eaction with nucleophiles
The electrons in the # bond are
re)elled as the @/ a))roaches the
carbon atom'
δ+ δ-
The @/ bonds to the carbon and the #
bond breaks' The two electrons move tothe halogen, forming a halide ion'
The halide is sbstitted, so this is a
nucleophilic substitution reaction'
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ucleophilic substitution reactions
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ate of nucleophilic substitution
The rate of a ncleo)hilic sbstittion reaction de)ends on
the strength of the carbonhalogen bond rather than the
degree of )olari>ation in the bond'
The % bond is the weakest and so most readily ndergoes
ncleo)hilic sbstittion' The rate of reactions involving
iodoalkanes is the highest'
23.%
2-Br
33.l
$.$(
%trength .k mol#"0Bond
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ucleophilic substitution
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(li i ti i h l lk
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(limination in halogenoalkanes
%n the reaction with a strong base, halogenoalkanes will
ndergo not only ncleo)hilic sbstittion bt also
elimination reactions, forming alkenes and water'
The :"/ acts as both a base and a ncleo)hile' ;hen acting
as a base, the :"/ removes "8 from the halogenoalkane,
which also reslts in the formation of a halide ion'The reaction between a halogenoalkane and a strong base
sally reslts in the formation of a mi&tre of sbstittion
and elimination )rodcts'
(li i ti h i
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(limination mechanism
)i t f li i ti d t
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)iture of elimination products
%f the carbon chain is for or more carbons in length and
the halogen is not attached to a terminal carbon, a
mi&tre of )ositional isomers may be formed'
attack at attack at B B
but#2#ene but#"#ene
C diti i t t
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Conditions are important
The conditions for the reaction that favor sbstittion or
elimination are different'
Base strength! the stronger the base sed, the more
elimination is favored' +odim hydro&ide in a5eos
soltion contains :"/, bt when dissolved in ethanol,
"3
"2
:/ is also )resent, which is a stronger base'
Therefore elimination is favored by @a:" in ethanolic
soltion, and sbstittion is favored by @a:" in
a5eos soltion'
1emperature! elimination is favored at hotter
tem)eratres whereas sbstittion is favored by warm
conditions'
- i d t ti ?
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-rimar&6 secondar& or tertiar&?
Arimary halogenoalkanes favor sbstittion whereas
tertiary halogenoalkanes favor elimination'
primar& tertiar&secondar&
elimination more likel&
substitution more likel&
(li i ti b tit ti ?
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(limination or substitution?
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7lossar&
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7lossar&
8hat9s the ke&word?
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8hat9s the ke&word?
)ultiple choice :ui$
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)ultiple#choice :ui$