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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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(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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7lossar&

8hat9s the ke&word?

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)ultiple#choice :ui$

2.11 Mechanism

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