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ELIMINATION
Traya, Levie Grace M.
INTRODUCTION
The dehydration of alcohol is an
important elimination reaction that
takes place under acidic rather
than asic condition. It involves an
E! mechanism. The function of the
acidic rea"ent is to convert the
hydro#yl "roup to a etter leavin"
"roup y protonation.This elimination reaction is the
reverse of acid$cataly%ed
hydration. &ecause a carocation
or closely related species is the
intermediate, the elimination step
'ould e e#pected to favor the
more sustituted alkene. The E!
mechanism also e#plains the
"eneral trend in relative reactivity.Tertiary alcohols are the most
reactive, and reactivity decreases
"oin" to secondary and primary
alcohol. Also in accord 'ith E!
mechanism is the fact that
rearran"ed products are found in
cases 'here a carocation
intermediate 'ould e e#pected to
rearran"e.
(or many alcohols, e#chan"e of
the hydro#yl "roup 'ith solvent
competes 'ith dehydration. This
e#chan"e indicates that the
carocation can under"o )N!
capture in competition 'ith
elimination. *nder conditions
'here proton removal is rate$
determinin", it 'ould e e#pected
that si"ni+cant isotope eect 'ould
e seen. This is, in fact, oserved.
In connection to the facts stated
this e#periment aims to understand
the concept of elimination andsynthesi%e an alkene from an
alcohol and test it usin" potassium
perman"anate test and romine
test.
METHODOLOGY
a. Materials
This e#periment 'ill use the
follo'in"- a/ tert$utyl alcohol, /
con. 01)O2, c/ 3a3l1, d/ acetone,
e/ !4 a5ueous 6MnO2, f/ caron
tetrachloride, "/ 14 romine, h/
separatory funnel, i/ "lass
dropper, 7/ 189$mL round$
ottomed :ask, k/ pipet, l/
aspirator, m/ thermometer, n/;e:u# condenser ;elfu# set$up/,
o/ Iron rin", p/ thermometer
holder, 5/ alcohol lamp, and r/
'ire "au%e.
b. Procedure
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In this e#periment you 'ill e
usin" tert$utyl alcohol 'hich melts
at 18$18.8 . It is more
convenient to handle t$utyl
alcohol as li5uid< if your laoratoryis not 'arm enou"h to melt the
alcohol naturally< it is
recommended that it e li5ue+ed
y placin" the container in a pan of
'arm 'ater.
In a 189$mL round$ottom :ask,
place !2 mL 'ater and add
cautiously, 'ith s'irlin", !2 mL
concentrated sulfuric acid. 3ool the
diluted acid to aout 89 and
slo'ly add !1.= mL of t$utyl
alcohol. Attach once a re:u#
condenser and oil the material
"ently for 28 mins.
3ool the reaction mi#ture to room
temperature, transfer it to a
separatory funnel, and carefullydra' o the a5ueous acid layer.
>ash the hydrocaron layer 'ith
'ater to remove traces of acid and
dry it 'ith aout ! " anhydrous
calcium chloride. Transfer the dry
li5uid into a :ask arran"ed for
distillation, collectin" the fraction
at !99 $!9? .
3arry out the romine and
potassium perman"anate test for
unsaturation. )umit the remainin"
product to the instructor in a
laeled vial.
Potassium Permanganate Test
In a small test tue dissolve @
drops of the alkene product in ! mL
of alcohol$free acetone. Add
drop'ise, 'ith vi"orous shakin", a!4 a5ueous solution of potassium
perman"anate. A positive test is
the loss 'ithin ! minute of the
purple perman"anate ion color and
formation of the insolule ro'n
hydrated o#ides of man"anese.
Bromine Test
This test should e carried out ina hood. In a small test tue
dissolve @ drops of the alkene
product in ! mL caron
tetrachloride, and ddd drop'ise,
'ith shakin", 14 solution of
romine in caron tetrachloride. A
positive test is the loss of ro'n
color.
DISCUSSION
Many secondary and tertiary
halides under"o E! elimination in
competition 'ith the )N! reaction
in neutral or acidic solutions. (or
this e#periment, 'hen tert$utyl
alcohol solvoly%es in sulfuric acid it
'ill produce diisoutylene yelimination.
Alcohols and ethers rarely
under"o sustitution or elimination
unless stron" acid is present. The
acid is necessary to convert a
relatively poor leavin" "roup, O0
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for this particular e#periment, into
a relatively "ood one, 01O. Thus
the dehydration of alcohols to
alkenes is an acid$cataly%ed
reaction re5uirin" stron" acids. In
this e#periment, the startin"
material is an alcohol, tert$utyl
alcohol, 'as reacted to an acid,
sulfuric acid, to produce
diisoutylene.
In the reaction, the startin"
material 'ill react 'ith sulfuric acid
'here the acid converted the poor
leavin" "roup O0 into 01O 'hich is
a etter leavin" "roup. The leavin"
"roup 'ill cleave out producin" the
carocation 'here it 'ill e
attacked y the ase. The ase
removes a eta$proton, leavin"
ehind its ondin" electrons to
form the ne' doule ond. (urther
reaction "ives a product
diisoutylene at a rate independent
of the 'ater concentration.iisoutylene 'as 'ill e produced
ecause isoutylene is an unstale
product and thus react furthermore
'ith the carocation ein" created.
The alcohol is the product of an
)N! reaction and the alkene is the
product of the E! reaction. The
characteristics of these t'o
reaction mechanisms are similar,as e#pected. They oth sho' +rst
order kinetics< neither is much
in:uenced y a chan"e in the
nucleophileBase< and oth are
relatively non$stereospeci+c.
To summari%e, 'hen carocation
intermediates are formed, one can
e#pect them to react further y
one or more of the follo'in"
modes-
!. The cation may ond to
a nucleophile to "ive a
sustitution product.1. The cation may
transfer a eta$proton to a
ase, "ivin" an alkene
product.@. The cation may
rearran"e to a more stale
carocation, and then react
y mode C! or C1.
)ince the )N! and E! reactions
precede via the same carocation
intermediate, the product ratios are
diDcult to control and oth
sustitution and elimination usually
take place.
The hydrocaron 'ill e su7ectedto t'o con+rmatory tests namely
potassium perman"anate test and
romine test.
(or romide test, the ro'n
solution ecome colorless that
mi"ht indicate a positive test
aDrmin" the presence of alkene.
>hen the romine test for the
alkene crude product is positive, itis ecause romine and the alkene
forms a doule ond that are open
for an attack since romine is
electron de+cient and the elkene
have pi electrons. ue to their 33
doule onds 'hich can e roken,
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alkenes react readily 'ith romine
to produce saturated
diromoalkanes. In the presence of
&r1 or &r$&r 'hich is redBro'n/,
the &r$&r ond attacks the doule
ond formin" 3$&r ne' onds. Thus
the redBro'n color disappears.
In the potassium test, the violet
solution 'ill turn into a ro'n
insolule solution. *nder acidic
conditions, the man"anateFII/ ions
'ill e reduced to man"aneseII/
ions. *nder alkaline conditions, the
man"anateFII/ ions 'ill +rst
reduce to "reen man"anateFI/
ions and then further to dark ro'n
solid man"aneseIF/ o#ide
man"anese dio#ide/. The last
reaction is also the one that 'ould
e otain if the reaction 'as done
under neutral conditions. It can e
oserve in the reaction there are
neither hydro"en ions nor
hydro#ide ions on the left$handside of the e5uation.
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&oyd, ;. N. Morrison, ;.T.
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1999
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