Lab 13 a Diazotization-Coupling Reaction--The Preparation of Methyl Orange

8/12/2019 Lab 13 a Diazotization-Coupling Reaction--The Preparation of Methyl Orange

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A DiazotizationCoupling Reaction: The Preparation of Methyl

Orange

Formation of a diazonium ion

Azoteis an old word for nitrogen. Hence, the presence of azo in the nameof a chemical implies that nitrogen is present in the structure. Therefore, diazo

means two nitrogen atoms. When combined with onium, we have diazonium,which means two nitrogen atoms and a positive charge (i.e., N2

+. !iazonium ions

are produced when an ar"l amine reacts with cold nitrous acid. Nitrous acid is

unstable and is prepared #ust prior to its use b" a reaction between sodium nitrite

and h"drochloric acid. $igure % shows the conversion of aniline into the positive

benzene diazonium ion.

NH2+ HNO2

NN Cl-

aniline nitrous acidbenzene

diazoniu chloride

HCl+ ! "!O

#igure $%The diazotization of aniline.

Substitution Reactions of Diazonium Ions

& diazonium ion is the cation of a salt, and it is a reactive intermediate that

undergoes substitution or coupling reactions. Table % shows some groups that ma"

substitute for a diazo group bonded to an arene (&rN2+.

Table $%'roups that substitute for N2+in &rN2

+.

eagent 'roup )roduct

*ur (Hr

*u*l (H*l*u*N (*N

r

*l*N

&rr

&r*l&r*N

- - &r-

*u2, *u(N/2, H2 H &rH

H/)2 H &rH

Lab 13 %


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& generalized e0uation for a substitution reaction is shown below in which

the reagent can be an" reagent in Table % and 1 an" group.

N2+ reagent X

Coupling Reactions of Diazonium Ions

*ompounds such as aniline and phenol, which contain strong electron

donating groups (e.g., H and 3NH2 that activate the ortho and para positions on

a benzene ring, can undergo coupling reactions with a diazonium ion. & couplingreactionis one in which two ar"l rings are #oined b" an azo group. These

coupling reactions usuall" occur at the para position of the o,p director. $igure 2shows the coupling of benzene diazonium ion with phenol at the para position of

phenol.

+

benzenediazoniu chloride

+ "ClN N Cl- OHH N N OH

phenoldiazo product

#igure !%*oupling reaction with phenol.

&niline ma" serve as the substrate for the formation of a diazonium ion as

shown in $igure %, and it ma" serve as the substrate for a coupling reaction with

the diazonium ion as shown in $igure /.

+

benzenediazoniu chloride

+ "ClNN Cl- NH2H N N NH2

aniline diazo product

#igure &%*oupling reaction with aniline.

The mechanism for a coupling reaction is shown in $igure 4. The reaction is

an electrophilic aromatic substitution reaction5 therefore, the mechanism is similar

Lab 13 2


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to that for the nitration of benzene (i.e., it is a twostep reaction in which the first

step is rate determining.

+

benzene

diazoniu ion

N N OHH

N N OH

phenol

diazo product

N N OH

Hslow

fast

#igure '%6echanism of coupling.

The Experimental Reaction

-n this e7periment, sulfanilic acid is the primar" ar"l amine that will be

diazotized, and N,Ndimeth"laniline is the ringactivated benzene derivative to

which the diazonium ion will couple in the para position. $igure 8 shows the

structures of aniline, N,Ndimeth"laniline and sulfanilic acid. oth reactants are

derivatives of aniline.

SO3H

NH2N

H3C CH3NH2

aniline N(N)diethylaniline sulfanilic acid

#igure *%&niline derivatives.

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9ulfanilic acid contains a sulfonic acid group (9/H and an amino group

(NH25 therefore, it undergoes an internal acidbase reaction to form a diionic

species. 9odium carbonate acts as a base and deprotonates the 0uaternar"

ammonium ion. The ensuing sulfonate salt is diazotized with cold nitrous acid, as

shown in $igure :.

SO3H

NH2

sulfanilic acid

SO3-

NH3H+transfer Na2CO3

SO3-

NH2HNO2

cold

SO3-

N2+

diazotized

sulfanilic acid

#igur

e +%!iazotization of sulfanilic acid.

The diazotized sulfanilic acid couples with N,Ndimeth"lamine, as shown in

$igure ;. &d#ustment of the pH to about 4 b" the addition of NaH "ields an

orange product


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Procedure

Preparation of the Diazonium Ion of Sulfanilic Acid

%. Weigh >.:g anh"drous sodium carbonate and transfer it to a 28m? @rlenme"er

flas.2g sulfanilic acid and transfer it to the @rlenme"er.

4. $ill a %8>m? bea


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8. 9lowl" add %.8m? %>C sodium h"dro7ide to the @rlenme"er while it remains

in the ice bath.

:. emove the @rlenme"er from the ice bath, dr" it with a paper towel, place it on

the hot plate and heat the mi7ture to boiling.

;. emove the @rlenme"er from the hot plate and add >.8g sodium chloride to the

hot solution.

A. &llow the @rlenme"er to cool on "our bench for two min., and then place the

flas< in an icewater bath. &fter cr"stallization is complete, collect the d"e on a

preweighed filter paper in a Dchner funnel.

B. inse the @rlenme"er flas< with / m? of an icecold, saturated solution of

sodium chloride5 swirl the @rlenme"er and pour this solution into the Dchner

funnel.

%>. &llow the d"e to dr" on its filter paper, and then reweigh the filter paper to

determine the "ield of meth"l orange. *alculate the percent "ield.

If our next experiment is an un)no#n sugar" continue #ith thefollo#ing procedure" other#ise s)ip to Step * belo#$

Preparation of Solution for Specific Rotation

,To be conducted one .ee/ before obser0ing the rotation-

%. btain an un


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+ater #ill be added to the mar) at the next lab meeting$ The sugar #illundergo mutarotation during this time$

:. 9topper the volumetric flas


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Diazotization 1uestions?ast nameFFFFFFFFFFFFFFFFFFFFFFFFFFF, $irst nameFFFFFFFFFFFFFFFFFFFFFFFF

%. !raw the structure of mbromoN,Ndieth"laniline.

2. *onsider the structure of sulfanilic acid. -t has two functional groups, one is an

acid and the other is a base. Write the formula for the partial structure that

constitutes the acid part of sulfanilic acid. FFFFFFFFFFFFF Write the formula for

the partial structure that constitutes the base part of sulfanilic acid. FFFFFFFFFFFFF

$rom what inorganic acid and base are these partial structures derivedG

FFFFFFFFFFFFFFFFFFFFFF and FFFFFFFFFFFFFFFFFFF

-f "ou answer these 0uestions correctl", "ou should 0uic


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*omplete the following e0uations b" inserting the reagent or product.

:.

;.

A.

B.

%>.

N N

+

N N

+ -

N N

+

NH2

N N

+

H

N N

+

*N

Lab 13 B

Lab 13 a Diazotization-Coupling Reaction--The Preparation of Methyl Orange

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