O conc H2SO4 SnCl2 reduction Zn dust O COOH 2-benzoylbenzoic acid by csgirla



                      O                                        O

                                      conc.                                      SnCl2
                                      H2SO4                                     reduction
           2-benzoylbenzoic acid                        anthraquinone
               MW 226.22                                MW 208.20


                                        Zn dust

                  anthrone                                  anthracene
                 MW 194.22                                 MW 178.22

       Treatment of 2-benzoylbenzoic acid with concentrated sulfuric acid effects cyclodehydration

to anthraquinone, a pale-yellow, high-melting compound of great stability. Because anthraquinone

can be sulfonated only under forcing conditions, a high temperature can be used to shorten the

reaction time without loss in yield of product; the conditions are so adjusted that anthraquinone

separates from the hot solution in crystalline form, which favours rapid drying.

       Reduction of anthraquinone to anthrone can be accomplished rapidly with tin(II) chloride

in acetic acid solution. A second method, which involves refluxing anthraquinone with an aqueous

solution of sodium hydroxide and sodium hydrosulfite, is interesting to observe because of the

sequence of colour changes: anthraquinone is reduced first to a deep-red liquid containing

anthrahydroquinone diradical dianion; the red colour then gives place to a yellow colour

characteristic of an anthranol radical anion; as the alkali is neutralized by the conversion of Na2S2O4

to 2NaHSO3, anthranol ketonizes to the more stable anthrone. The second method is preferred in

industry because sodium hydrosulfite costs less than half as much as tin(II) chloride and because

water is cheaper than acetic acid and no solvent recovery problem is involved.

       Reduction of anthrone to anthracene is accomplished by refluxing in aqueous sodium

hydroxide solution with activated zinc dust. The method has the merit of affording pure, beautifully

fluorescent anthracene.

Fieser's Solution, an Oxygen Scavenger

       A solution of 2 g of sodium anthraquinone-2-sulfonate and 15 g of sodium hydrosulfite in

100 mL of a 20% aqueous solution of potassium hydroxide affords a blood-red solution of the

diradical dianion:

                                                                          -   +
                O                                                       O Na
                                 -   +                                                     -   +
                             SO 3 Na                                                   SO 3 Na
                                         Na2S2O 4, NaOH
                                                                          -   +
                O                                                       O Na

This solution has a remarkable affinity for oxygen. It is used to remove traces of oxygen from gases

such as nitrogen or argon when it is desirable to render them absolutely oxygen-free. This solution

has a capacity of about 800 mL of oxygen. The colour fades and the solution turns brown when it

is exhausted.


I. Anthraquinone

       In a reaction tube dissolve 200 mg of o-benzoylbenzoic acid in 1.0 mL of concentrated

sulfuric acid by gently heating and stirring. Immerse a thermometer in the reaction mixture and heat

it at 150-155° for 45 minutes. Allow the tube to cool to below 100°C, and then, using extreme

caution, add a very small drop of water to the mixture. Mix the contents of the tube and continue

adding water in minute drops to the mixture. This will cause the product to crystallize and, if done

slowly enough, the crystals will be large enough to collect easily by filtration. Fill the tube with

water, and collect the product by filtration on the Hirsch funnel. Return the damp product to the

reaction tube and boil the product with 1.0 mL of concentrated ammonium hydroxide to remove

unreacted starting material. Filter, then wash the product well with water and then with acetone, dry,

determine the weight, and then calculate the percentage yield. Continue with Part II. Determine the

melting point when the product is completely dry.

II. Anthrone

       In a reaction tube put 75 mg of crude anthraquinone, 0.6 mL of acetic acid, and a solution

made by warming 0.2 g of tin(II) chloride dihydrate with 0.2 mL of concentrated hydrochloric acid.

Add a boiling chip, note the time, and reflux gently until crystals of anthraquinone have completely

disappeared (8-10 minutes); reflux 15 minutes longer and record the total time. Then add water

(about 0.18 mL) dropwise until the solution is saturated. Let the solution stand for crystallization.

Collect and dry the product and take the melting point (156°C reported).

III. Fluorescent Anthracene

       Put 100 mg of zinc dust into a 5 mL round bottomed flask and activate the dust by adding

0.6 mL of water and 0.1 mL of a 5% aqueous copper(II) sulfate solution and swirling for a minute

or two. Add 40 mg of anthrone, 100 mg of sodium hydroxide, and 1 mL of water, heat to boiling

on the sand bath, note the time, and start refluxing the mixture. Anthrone at first dissolves as the

yellow anion of anthranol but anthracene soon begins to separate as a white precipitate.

       In about 15 minutes the yellow colour initially observed on the walls disappears, but

refluxing should be continued for a full 30 minutes. Then remove the heat, rinse down any

anthracene in the condenser with a few drops of water, and cool the flask in ice. Remove the solvent

with a Pasteur pipet and add very carefully and dropwise 0.2 mL of concentrated hydrochloric acid

to dissolve the remaining zinc. More HCl may be needed. Again, cool the mixture and add water

to wash down any unreacted zinc; after all has reacted, remove the acid and wash the product

liberally with water. Collect the product on the Hirsch funnel and then wash once with ice-cold

methanol to remove as much of the water as possible.

       Transfer product to a test tube and recrystallize from about 0.5 mL of boiling toluene. Slow

cooling of the toluene solution will give thin, colourless, beautifully fluorescent plates. Collect the

product on the Hirsch funnel. In washing the equipment with acetone, you should be able to observe

the striking fluorescence of very dilute solutions. The fluorescence is quenched by a slight trace of



1.     Show in mechanistic steps how the diradical dianion shown previously is converted finally

       into anthrone in Part II of the experiment.

2.     If sulfonation of anthraquinone (Part I) were to occur at what position would you expect


3.     Why is Fieser's solution such a good trap for molecular oxygen?


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