The Diels-Alder Reaction of Anthracene with Maleic Anhydride

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    publisher: H.A. Neidig

    12 SYNT717. laboratory. program in chemistry

    organic editor: Joe Jeffers

    PURPOSE OF THEEXPERIMENT

    BACKGROUND REQUIRED

    EXPERIMENTAL OPTIONS

    BACKGROUNDINFORMATION

    The Diels-Alder Reactionof Anthracene withMaleic Anhydrideprepared by L. G. Wade, Jr., Whitman College

    Use the Diels-Alder reaction to form a bridged polycyclic anhydride.Recrystallize the product and characterize it by using melting point andinfrared spectroscopy.

    Youshould be familiar with reflux techniques, vacuum filtration, recrys-tallization, melting point measurement, and infrared spectroscopy.

    Semi-Microscale Diels-Alder ReactionMicroscale Diels-Alder Reaction

    Otto Diels, Professor of Chemistry at the University of Kiel, Germany,and his student Kurt Alder published a paper in 1928 on additions ofelectron-poor alkenes and alkynes to electron-rich dienes to form cyclo-hexenes and cyclohexadienes. These [4 + 2] cycloadditions came to beknown as Diels-Alder reactions. Diels and Alder received the 1950 No-

    bel Prize in chemistry for this work.The Diels-Alder reaction is one of the most useful synthetic reac-

    tions in organic chemistry. In one step the reaction forms a six-membered ring with one or two double bonds from an open-chain com-pound, as shown in Equations 1 and 2 on the next page. A diene is the4-1t-electroncomponent. It is electron-rich, like a nucleophile in a Lewisacid-base reaction. Simple dienes like 1,3-butadiene are sufficientlyelectron-rich to react, but electron-releasing groups such as alkyl groups(-R) or alkoxy groups (-OR) enhance a diene's reactivity.

    The 2-1t-electroncomponent is called a dienophile ("lover of die-nes"). Good dienophiles contain relatively electron-poor double bondsor triple bonds; at least one strongly electron-withdrawing group (W) isneeded. Therefore, ethylene and acetylene are not good dienophiles.

    Because the reaction is concerted-that is, bond breaking and bondforming take place in the same step-the stereochemistry of the

    Copyright @ 1998 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America

  • 156SYNT 717: The Diels-Alder ReaCtion of Anthracene with Maleic Anhydride

    (diene

    (diene

    (Eq.l)

    adduct

    (Eq. 2)

    adduct

    reactants and the symmetry of their molecular orbitals control thestereochemistry of the products. Using well-chosen reactants, a chemistcan control the stereochemistry of a Diels-Alder product at up to fourcarbon atoms, as shown in Figure 1.

    oII H

    ( (C-OCH3 a~ COOCH3+ I -+- I~ :C-OCH3 = COOCH3II Ho

    (cis, but not trans)

    (endo)

    OCH3 CH30. H

    ( + (=N_ a~N(cis-1,2)

    (trans, but not cis)

    but not

    (exo)

    and not

    (trans -1,2) (1,3)

    Figure 1 Stereochemical patterns of Diels-Alder reactions

    In this experiment, maleic anhydride is used as the dienophile.Maleic anhydride is an excellent dienophile because two stronglyelectron-withdrawing groups are attached to the double bond.

    The diene is anthracene, which is commonly thought of as an aro-matic compound, and not as a diene. However, in polynuclear aromaticcompounds like anthracene, each individual ring may not be as well sta-bilized as an isolated benzene ring. Anthracene has only 141t electrons,compared with 18 needed for three fully independent aromatic rings.

    @ 1998 Chemical Education Resources

    + (W

    dienophile(alkene)

    WIC

    + IIICI

    Hdienophile

    (alkyne)

  • 156SYNT 717: The Diels-Alder ReaCtion of Anthracene with Maleic Anhydride

    (diene

    (diene

    (Eq.1)

    adduct

    (Eq. 2)

    adduct

    reactants and the symmetry of their molecular orbitals control thestereochemistry of the products. Using well-chosen reactants, a chemistcan control the stereochemistry of a Diels-Alder product at up to fourcarbon atoms, as shown in Figure 1.

    oII H

    ( (C-OCH3 a~ COOCH3+ I -+- I~ :C-OCH3 = COOCH3II Ho

    (cis, but not trans)

    (endo)

    OCH3 CH30. H

    ( + (=N_ 6

  • ..

    When anthracene's center ring reacts as a diene, the product has twofully aromatic rings, each with six pi electrons, as shown in Equation 3.

    o\\

    o C\

    O=C

    SYNT 717: The Diels-Alder ReaCtion of Anthracene with Maleic Anhydride

    00Jg +19

    (p-C\\o

    anthracene

    (diene)maleic anhydride

    (dienophile)

    157

    (Eq. 3)

    (adduct)

    The reaction is carried out in xylene, which is actually a mixture ofthe three dimethylbenzenes,for three reasons.First, the 1400 C boilingpoint provides a good reaction temperature. Second, the xylene mixturedoes not freeze when it is cooled in ice water. Third, the reactants aremore soluble in xylene than is the product, which crystallizes.

    The product is a relatively stable anhydride. Although anhydrides re-act with water in air, this one reacts slowly and is easily isolated and char-acterized before much hydrolysis occurs. There are several possible waysto name the product, but 9,10-dihydroanthracene-9,10-a,j3-succinicanhy-dride is probably the simplest.

    Semi-Microscale Diels-Alder Reaction

    Equipment

    250-mL beaker*

    boiling chipBuchner funnel, with adaptercondenser, with tubingelectric flask heater

    50-mL Erlenmeyer flask125-mL filter flask,

    with vacuum tubingfilter paperglass stirring rod'for ice bath

    Reagents and Properties

    substance

    anthracene

    9,10-dihydroanthracene-9,10-a,~-succinic anhydride*

    maleic anhydride

    potassium bromide

    xylene'product

    @ 1998 Chemical Education Resources

    10-mL graduated cylinderhot plate2 melting point capillary tubesmicrospatula25-mL round-bottom flask

    support stand13 x 100-mm test tube

    2 utility clamps3-mL product vialwatch glass

    quantity molarmass(g/mol)

    0.5 g 178.23

    0.25g

    100mg27mL

    276.29

    98.06 200

    262-264

    54-56

    106.17 137-144

  • 158SYNT 717: The Diels-Alder Reaction of Anthracene with Maleic Anhydride

    Preview

    Weigh anthracene and maleic anhydride Place weighed reagents in flask; add xylene Assemble a reflux apparatus Heat anthracene and maleic anhydride at reflux for 30 min Cool the reaction mixture in an ice bath Use vacuum filtration to collect crude product Weigh the crude product Recrystallize the product from xylene Use vacuum filtration to collect purified product Dry and weigh the product on a watch glass Measure the melting points of both crude and purified product Take an IR spectrum of the product

    PROCEDURE Chemical Alert

    1. Reacting Anthracene withMaleic Anhydride

    anthracene-irritant

    maleic anhydride-toxic and corrosivepotassium bromide-irritant and hygroscopicxylene-flammable and irritant

    Caution: Wear departmentally approved safety goggles at all timeswhile in the chemistry laboratory.

    Caution: Anthracene is irritating. Maleic anhydnde is toxic and cor-rosive. Xylene is flammable and irritating. Keep away from.flamesor other heat sources. Use afume hood. Prevent eye, skin, and cloth-ing contact. Avoid inhaling and ingesting these compounds.

    Weigh 0.5g of anthracene and 0.25g of maleic anhydride. Place them in a25-mLround-bottom flask.Add 6mL of xylene. Add aboiling chip. Use theround-bottom flask to assemble the reflux apparatus shown in Figure 2.

    -waterout

    -water in

    electricflask heater

    Figure 2 Refluxapparatus for semi-microscale glassware

    @ 1998 Chemical Education Resources

  • SYNT 717: The Diels-Alder Reaction of Amhracene with Maleic Anhydride 159

    NOTE 1: The reactants do not dissolve

    completely at first. They gradually dis-solve as the reaction takes place.

    2. Purifying the Product

    3. Characterizing the Product

    Turn on the water to the condenser. Heat the mixture at reflux for 30

    min, boiling vigorously for good mixing. [NOTE1]Cool the mixture to room temperature. Prepare an ice bath, using a

    25Q-mLbeaker. Then cool the mixture in the icebath for 5 min. At the same

    time, pour 6 mL of xylene into a test tube and chill the xylene in the icebath.Collect the crystallized solid by vacuum filtration, using a Buchner

    funnel. Rinse the crystals with 3 mL of ice-cold xylene.Weigh the crude product. Set aside a small sample to dry for a melt-

    ing point measurement.

    For recrystallization, place the product in a 50-mL Erlenmeyer flask.Add 5-10 mL of xylene. Heat the mixture gently on a hot plate until thexylene boils. Gradually add more xylene until all the product dissolvesor until no more appears to be dissolving. Do not exceed 15mL total vol-ume of xylene to dissolve the crystals.

    Allow the solution to cool to room temperature. Then cool the solu-tion in an ice bath for 5 min. If necessary, scratch the bottom of the flaskwith a glass rod to induce crystallization.

    Collect the crystallized solid by vacuum filtration, using a Buchnerfunnel. Rinse the crystals with 3 mL of ice-cold xylene.

    Spread the product crystals thinly over a clean watch glass. Allowthe crystals to dry for about half an hour, with occasional stirring.

    Weigh the product and record the mass. Place the product in a la-beled product vial.

    Caution: Potassium bromide (KBr) is irrltating. Prevent eye, skin,and clothing contact. Avoid inhaling dust and ingesting KBr.

    Measure the melting points for both the crude product and therecry