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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.

REFERRENCE

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&ruice, aula H. Or"anic 3hemistry.

2thed. earson rentice 0all. 1992

&oyd, ;. N. Morrison, ;.T.

Or"anic 3hemistry. =th Ed. rentice

0all. !JJ1

3arey, (rancis A. Or"anic

3hemistry. 2th ed. McGra'$0ill.

1999

3han", ;eymond. 3hemistry. !9th

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McMury, Kohn.Or"anic.

3hemistry.?th ed. &rooksB3ole,

3en"a"e Learnin". 19!1

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