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of the acid, the discrepancies being clue to the fact that it loses
water on heating, the anhydride being formed. According t

1 SchciLlei, Her. Dcutsch. Chcm. Gcs. i, 125 ; Groth, Jahresbcr. Ctem. 1870, 5.
- Bourgoin, Hull. Hoc. Chim. xxix. 247.



4iJ6



AROMATIC COMPOUNDS.



Lessen, this takes place at 184, while Ador states that the
unbroken crystals melt at 213, but the powder at 203 . 1

It is completely burned by chromic acid solution on heating, 2
and on this account cannot be prepared by means of this reagent
from orthoxylene or other ortho-compounds which possess two
side chains. On distillation with lime it decomposes into carbon







dioxide and benzene, but if the temperature be not allowed to
rise above 330 350, it is converted into benzoic acid, which
was formerly manufactured in this way (p. 155).

2241 The normal phthalates of the alkali metals are readily
soluble in water and crystallize in small plates or scales.

Acid ammonium phthalate, C 8 H 5 O 4 (NH 4 ), separates on the
spontaneous evaporation of a solution of the normal salt in six-

1 Ann. Chcm. Pharm. clxiii. 230. " Fittig and Bieber, ibid. clvi. 242.



SALTS OF PHTHALIC ACID. 457

sided, rhombic tablets, prisms or pyramids, which dissolve very
readily in water but only slightly in alcohol.

Calcium phthalate C 8 H 4 O 4 Ca+H 2 O, is tolerably soluble in
water and crystallizes in lustrous, rhombic prisms. 1

Acid barium phthalate, (C 8 H 5 4 ) 2 Ba, is readily soluble in hot
water and crystallizes in small, rhombic prisms.

Normal barium phthalate, C 8 H 4 4 Ba, is obtained by preci-
pitating the normal ammonium salt with barium chloride, or by
mixing hot solutions of the acid and caustic baryta ; it forms
small, white scales or silky needles, which are only slightly
soluble in hot water. On evaporating the solution, a basic salt
separates out in crystalline crusts (Carius, Hermann).

Lead j^hthalate, C 8 H 4 O 4 Pb, is a precipitate, consisting of scaly
crystals.

Copper phthalate, C 8 H 4 4 Cu + H 2 0, crystallizes in lustrous
blue, rhombic prisms, which dissolve readily in hot water
(Hermann).

Silver phthalate, C 8 H 4 O 4 Ag 2 , forms a light, crystalline powder,
which is tolerably soluble in water and detonates when rapidly
heated.

Ethers of phthalic acid are formed by the action of hydrochloric
acid on the alcoholic solution of the acid and by that of an alco-
holic iodide upon the silver salt. The two following have been
prepared, and are both odourless liquids : 2

Boiling-point.

Methyl phthalate, C 6 H 4 (CO 2 .CH 3 ) 2 ..... 280

Ethyl phthalate, C H 4 (CO 2 .C 2 H 5 ) 2 ..... 294

Phthalyl oxide or Phthalic anhydride, (C H 4 .CO 2 ) 2 O, is formed
when the acid is heated alone or with acetyl chloride :



/CO.OH

PH 4 < +CH 3 .COC1 =

\CO.OH



/.
C 6 H 4 < >0 + CHs.CO.OH + HC1.



The anhydride remains behind in splendid prisms after the
acetic acid and excess of acetyl chloride have been removed. 3
It is manufactured by subliming phthalic acid in a current of

1 Hermann, Ann. Chcm. Pharin. cli. 77.

2 Grabc and Born, ibid, cxlii. 344 ; Grttbe, Bcr. Dcutsch. Clicm. Gcs. xvi. 860.

3 Anschiitz, ibid. x. 2J5 ; Ann. Chcm. Pharm. ccxxvi. 1.



458 AROMATIC COMPOUNDS.

air, or better, of carbon dioxide, the apparatus shown in Fig. C
bsing employed. 1

It is thus obtained in long, white, pliant, rhombic needles,
Avhile on rapid heating it disti's as a liquid and then solidifies
to a hard, crystalline mass. It melts at 128 and boils at 284'5
(Schultz, Grabe), is only very slightly soluble in cold, more
readily in hot water, and is reconverted into phthalic acid when
boiled with water for a long time, more rapidly in presence of
alkalis.

It is reduced by zinc dust and acetic acid to phthalide, di-
phthalyl (v. post) and zinc phthalate being also formed. 2 It is
employed in the manufacture of dyes and of many other com-
pounds, as it very readily enters into chemical reactions. Thus
it combines with the acid anhydrides, forming with acetic

X C=CH.C0. 2 H
anhydride, phthalylacetic acid, C 6 H 4 \ >O



It also combines with the aromatic hydrocarbons in presence of
aluminium chloride, forming acids ; thus benzene yields benzoyl-
benzoic acid :

,CO X /CO.OH



C 6 H 4



XX CO.C H IV



A specially characteristic property is that of combining wi
phenols on heating to form phthale'ins, water being eliminated.
As already mentioned, resorcinol yields fluorescein in this way,
the formation of this substance, which is employed as a test for
the presence of the phenol in question, taking place in the
following manner :

/OH
/C 6 H 4 /

co c\ c H /

C G H 4 /0 +2C 9 H 4 <ol = C 6 H 4 <(0 X OH+2H 2 0.

CO CO

2242 Phthalyl chloride, C 8 H 4 O 2 C1 2 , was first prepared by Hugo
Mitller from phthalic acid by the action of phosphorus penta-

1 Schultz, Stcinkohlcntheerc.

2 "VVislicenus, Ber. Dcutsch. Chem. Gcs. xvii. 2178.



y l -

ith



PHTIIALYL CHLORIDE.



459




460 AttOMATIC COMPOUNDS.

chloride. 1 From analogy with other acid chlorides it would be
formed according to the following equation :

/CO.OH /COC1

C 6 H 4 < + 2PC1 5 = C 6 H 4 < +2POCL + 2HC1.

NXXOH '\COC1

Careful investigation, however, has shown that it does not
possess this constitution. It is formed in two stages :

/CO.OH .CO.

+ POC1 3 +2HC1.



C 5 = 64

\CO.OH XX

x co x ecu

c H <oo>



It is also obtained, therefore, when equal molecules of phthalic
anhydride and phosphorus pentachloride are heated together for
12 hours at 170 . 2

In order to prepare it, phthalic acid is boiled with rather more
than two molecules of phosphorus chloride for 5-6 hours and
then distilled. 3 It is an oily liquid, which boils at 268 and
solidifies at about 0. It is only slowly decomposed by boiling
water and even boiling caustic soda solution. On heating with
finely-divided silver, diphthalyl, which will be subsequently
described, is obtained, its formation taking place according to
the following equation :

/ C0 \ / C0 \

C H <cc,> C H <c>

+ 4Ag= || +4AgCl.

2 \ / C \

C 6 H 4 / )0

MXK



The proof that it possesses this constitution rests not only on the
whole behaviour of the substance, which shows it to be a dilac-

1 Zeitschr. Chcm. 1863, 257.

- Clausjind Hoch, Ber. Dcutsch. Chtm. Gcs. xix. 1187.

3 Wischin, Ann. Chcm. P/iarm. cxliii. 259.



PHTHALYL CHLORIDE. 461

tone, but also on the fact that it is formed when phthalido is
heated with phthalic anhydride : l

x co \ x co \

! *\CH Z / 6 4 \C/

+ H 2 0.

X CO \ / G \

ri TT / \/~\ r^ TT / ^^r\

V./ A tl lX y\J Utl, v ;U



The following compounds have been prepared by the action
of sodium methylate and ethylate on phthalyl chloride :
/C(OCH 3 ) 2 Boiling-point.

C 6 H/>O 280

>C(OCW,

C 6 H 4 />0 294

These are liquids which boil at the same temperature as the
isomeric phthalic ethers, from which they differ only in possess-
ing a slightly lower specific gravity. That they are actually
distinct bodies is shown by the fact that tetrachlorophthalic acid
yields compounds which differ both in crystalline form and melt-
ing point from those derived from its chloride. 2

The corresponding phenyl ether, C 6 H 4 (CO)C(OC 6 H 5 ) 2 0, is
prepared by heating the chloride with phenol, and crystallizes
from alcohol in small prisms, melting at 70. 3

When phthalyl chloride is heated in a sealed tube with
phosphorus pentachloride to 210 220, two isomeric chlorides,
C 8 H 4 C1 4 O, are formed, both of which crystallize in monosyrn-
metric forms ; the one melts at 47, and the other, which is also
a product of the action of phosphorus pentachloride on phthalide,
at 88. Theory, indeed, allows the formation of two isomeric
tetrachlorides :

,Of!l,, xCOCl



1



e compounds in question, however, exhibit an identical
chemical behaviour. On heating with concentrated sulphuric
acid, or on boiling with alcoholic potash, they are converted into

1 Grabe and Guye, Ser. Deutsch. Chem. Ges. xvii. 2851.

2 Griibe, Ber. Dcutsch. Chem. Ges. xvi. 860.

3 Schi-cder, ibid. vii. 705 ; v. Gerichteii, ibid. xin. 419.



462 AROMATIC COMPOUNDS

phthalic acid, and phenol combines with them to form the phcuyl
ether mentioned above. These facts seem to indicate a case of
physical isomerism, but all endeavours to convert either of the
compounds into the other have proved unsuccessful (v. Gerichten).

According to Glaus and Hoch, the tetrachloride, which melts
at 88 and boils at about 274, is alone obtained and is best
prepared by heating for 15 hours at 245. Carbonyl chloride
and orthochlorobenzoyl chloride are also formed in the reaction. 1

A compound is formed by the action of zinc ethyl on phthalyl
chloride, which is looked upon by Wischin as phcnyleiudidhyl-
acetone, C 6 H 4 (CO.C 2 H 5 ) 2 . It separates from ethereal solution in
splendid crystals, which possess a pleasant, fruity smell, and
melt at 52. Wischin was, however, unable to obtain a com-
pound of it with acid sodium sulphite, and V. Meyer has shown
that it is not a ketone, since it does not form a hydroxylamine
compound. 2 The constitution of this compound can readily be
explained in accordance with the present views in regard to
phtbalyl chloride, and is as follows :






C H/ \ CO / C

Succinyl chloride (Part II., p. 213) has obviously an analogous
constitution to phthalyl chloride, and its reduction to the
anhydride or lactone of 7-hydroxy butyric acid (Part. II., p.
190) thus receives a simple explanation :



C 2 H 4 < >0 + 4H = C 2 H 4 < )0 -f 2HC1.

\CO / : \CO /

Phthalyl sulphide or Thiophthalic anhydride, C 8 H 4 2 S, is
formed by the action of potassium hydrosulphide on phthalyl
chloride 3 or its phenyl ether : 4



C 6 H/ 2 \0 4 2KSH = C 6 H/ \0 + 2KC1 + SH 2 .



CS

64
CO/ \CO

It crystallizes from alcohol in small plates or needles, melts at
114 and boils at 284.

1 Ber. Deutsch. Chem. Ges. xix. 1187. 2 Ibid. xvii. 817.

3 Schroder, ibid. vii. 705. * Grabe and Zschokke, ibid. xvii. 1175,



PHTHALAMIC ACID. 463

2243 Phthalimide, C 8 H 4 O 2 (NH), is obtained by heating acid
ammonium phthalate : 1

/CO.ONH 4 > C=NH

CflH/ = C C H 4 < >0+H 2 0.

\CO.OH \CO

It is also formed by the action of ammonia on the anhydride,
chloride, 2 or sulphide, 3 as well as by heating phthalic acid with
ammonium or potassium thiocyanates. 4 It is insoluble in cold
water, slightly soluble on boiling, and crystallizes from ether in
six-sided prisms, which melt at 228 5 and sublime in small plates.
It is decomposed on heating with slaked lime into carbon
dioxide and benzonitril, 6 and is reduced to phthalimidine (p. 444)
by tin and hydrochloric acid.

When phthalimide is heated with aniline or one of its
homologues, ammonia is evolved and the corresponding substi-
tuted phthalimide formed. If a phenol be substituted for the
amine, ammonia is also evolved, and a substance belonging to
the class of phthalems is produced.

These bodies are identical with those prepared from phthalic
anhydride. 7

Potassium phthalimide, C 8 H 4 2 (NK), is obtained in white
plates by adding alcoholic potash to an alcoholic solution of phthal-
imide (Cohn).

Silver phthalimide, C 8 H 4 2 (NAg), is formed as a heavy, white
precipitate when silver nitrate is added to an aqueous solution of
the potassium salt, or when an alcoholic solution of phthalimide
is treated with an ammoniacal silver solution (Laurent).

Phthalamic acid, C 6 H 4 (CO.NH 2 )CO 2 H. Marignac prepared
the ammonium salt of this acid by dissolving anhydrous naph-
thalic acid (phthalic acid) in ammonia, and named it naphthal-
amide. 8 It was also obtained by Laurent, who added ammonia to
a hot, alcoholic solution of the anhydride . 9

/COv X CO.NH 2

C 6 H 4 < >0 + 2NH 3 = C 6 H 4 <;

NXK \CO.ONH 4

Laurent, Ann. Chcm. Pharm. xli. 110.

Kuhara, Amer. Chcm. Journ. iii. 26.

Grabeand Zschokke, loc. tit.

O.ian Aschan, Bcr. Deutsch. Chcm. Gcs. xix. 1398.

Cohn, Ann. C/wm. Pharm. ccv. 301.

Laurent, Jahrcsb. Chem. 1868, 549.

Hall, Private Communication.

Ann. Chem Pharm. xlii. 219.

Ann. Chim. Phys. [3] xxiii. 117.



464 AROMATIC COMPOUNDS.

The barium salt is formed when phthalimide is boiled with
baryta water (Kuhara), and the potassium salt by the continued
boiling of potassium phthalimide with water. 1 It may be more
simply prepared by the action of 25 per cent, caustic potash
solution on phthalimide. 2 When concentrated hydrochloric acid
is added to the solution, the phthalamic acid is gradually deposited
in short, well developed, transparent prisms, which melt at 148
149 and decompose at a slightly higher temperature into water
and phthalimide. It has an acid taste, is tolerably soluble in cold
water and is gradually converted by it into acid ammonium
phthalate, the change taking place rapidly on boiling (Aschan).

Ethyl phthalimide, C 6 H 4 O 2 (NC 2 H 5 ), is prepared by the distill
tion of phthalic anhydride with aqueous ethylamine; 3 it crystal-
lizes in needles or prisms, melts at 78*5 and boils at 276 278 . 4
Acetpkthalimide, C 8 H 4 O 2 (NCO.CH 3 ), is formed by heating
phthalimide with acetic anhydride for some time. It crystallizes
in large octohedra and is decomposed by boiling water or by
alkalis in the cold into acetic acid and phthalimide. 5

Phthalurie acid is obtained by heating phthalic anhydride
with amido-acetic acid :

/ C0 \
C C H 4 < >0 + NH 2 .CH 2 .C0 2 H =

\CCK

/ C=N.CH,.C0 2 H
C 6 H 4 < >0 +H 2 0.

\co

It crystallizes from hot water in long, thin needles, which
melt at ]91 192, and are decomposed on boiling with con-
centrated hydrochloric acid into phthalic and amido-acetic acids. 6

Phenylphthalimide or Phthalanil, C 8 H 4 O 2 (NC 6 H 5 ), is prepared
by distilling phthalic acid with aniline; it crystallizes from
alcohol in needles, which melt at 205, but sublime at a lower
temperature. 7 On boiling with ammonia and alcohol, it is con-
verted into phthalawUic acid, or phenylphthalamic acid :

/C:=NC H 6 / CO.'NH.C H 6

C 6 H 4 < >0 + H 2 0=C 6 H /

\CO \CO.OH.

1 Landsberg, Ann. Chem. Pharm. ccxv. 198.

2 Ossian Aschan, Ber. Deutsch. Chem. Gcs. xix. 1401.

3 Michael, ibid. x. 1646. 4 Wallach and Kamenski, ibid. xiv. 171.

5 Ossian Aschan, ibid. xix. 1400.

6 Drechsel, Journ. PraTct. Chem. [2] ccxxvii. 418.

7 Gerhard t and Laurent, Ann. Chem. Phys. [2] xxiv. 189 ; Dobner, Ann.
Pharm. ccx. 267.






DIPHENYLPHTHALEIN.



465



This substance crystallizes from hot water or alcohol in small
plates, which melt at 192 and decompose into water and
phthalanil at a higher temperature.

Analogous compounds are obtained from the substitution
products of aniline l and its homologues, 2 from the amido-
phenols, 3 and from the amidobenzoic acids. 4

Diphenylphthalamic acid is formed when equal molecules of
phthalic anhydride and diphenylamine are heated together :



/ x

C 6 H/ >0 + NH(C 6 H 5 ) 2 = 64

N C(K \CO.OH.

It is insoluble in water and crystallizes from alcohol in warty
masses of small, lustrous prisms, which melt at 147 148 . 5

If two molecules of diphenylamine are employed, the so-called
diphenylpkthalein is formed ; it is also obtained by the action
of the base on phthalyl chloride, 6 and is tolerably soluble in
alcohol, more readily in benzene, from which it crystallizes in
large prisms, melting at 238. It has the following constitu-
tion :

(CWsNv /
/ C \
C 6 H 4 < >0

X CCK

The aromatic diamines form two series of compounds with
nhthalic anhydride : 7

/C=N.C C H 4 .NH 2 / C=H.C H 4 .N=C X

G H 4 < >0 C H 4 / >0 0< ;>C H 4 .

\CO \CO CCK

When orthamidothiophenol is heated with phthalic anhydride
or phthalyl chloride, the compound C 20 H 12 N 2 S 2 is formed, and
probably has the following constitution :



PIT/ \rrTT n^ \PH

^"AN, /vA^U^U^ /^o n 4-



1 Gabriel, Bcr. Dcutsch. Chem. Ges. xi. 2260 ; Frohlicb, ibid xvii. 1801 ami
2G79. - Michael, ibiil. x. 579.

" Ladenburg, ibid. ix. 1528. 4 Gabriel loc. tit.

s Piutti, Gaz. Chim. Ital. xiii. 542 ; xiv. 470.

6 Lellmann, Bcr. Dcutsch. Chem. Oat. xv. 830.

7 Biedermann, ibid. x. 1160.

VOL. III. PART IV. n n



466 AROMATIC COMPOUNDS.

It crystallizes from alcohol in thin needles or thick prisms,
which melt at 112; it is a weak base and forms a hydrochloride
which is only slightly soluble and crystallizes well, but is readily
decomposed by water. 1

2244 Phthalyldiamide. Phthalylmalonic ether is formed by
the action of phthalyl chloride on sodmalonic ether, and reacts
with ammonia to form malonamide, alcohol and phthalyldi-
amide : 2

/C=C(CO.OC 2 H,) 2
4 <(>0 +4NH 3 =

XNH,

/ c ^NH;

CH 2 (CO.NH 2 ) 2 + 2HO.C 2 H 5 + C 6 H 4 < >O .



This body may also be obtained in a similar manner from
phthalylaceto-acetic ether, 3 and by allowing phthalimide to
stand for some hours in contact with concentrated ammonia/
It forms a glittering powder, consisting of microscopic, very
refractive rhombohedra, only slightly soluble in water and
alcohol, and is decomposed into ammonia and phthalimide
when heated with either of these. It fuses at 219 220, when
carefully heated, forming a clear liquid, but ammonia h given
off and the residue finally consists of phthalimide.

Phthalylhydroxylamine, N(C 8 H 4 O 2 )OH,is formed when phthalyl
chloride and sodium carbonate are alternately added to a con-
centrated solution of hydroxylamine hydrochloride, so that the
liquid is always kept alkaline :

/CCU .CnrN.OH

co/ " 4N \co



_ >0 + H 2 N.OH = C 6 H 4 <; J>O + 2HC1.



It is insoluble in ether, but dissolves slightly in water, more
readily in boiling alcohol, from which it crystallizes in small
plates or needles, which, after drying, form a yellow powder,
melting at 230 with decomposition. It is an acid, decomposes
carbonates and forms a red solution in alkalis. Alcoholic potash

1 Hofmann, Scr. Deutsch. Chcm. Ges. xiii. 1233.

2 Wislicenus, ibid. xvii. Ref. 529.

3 Billow, Ann. Chcm. Pharm. ccxxxvi. 188.

4 Ossian Aschan, Ber. Deutsch. Chcm. Gcs. xix. 1398.



ETHYLPHTHALYLHYDROXYLAMINE. 467

added to its solution in alcohol precipitates the potassium salt,
N(C 8 H 4 O 2 )OK, which consists, as does the sodium compound, of
an amorphous red powder. The silver compound is obtained by
double decomposition as a dark-red precipitate, the barium and
lead salts being light red and yellowish red precipitates, while the
copper salt, which is only thrown down from a concentrated solu-
tion, is green, and the aluminium and mercury salts are yellow.

Ethylphthalylhydroxylamine, N(C 8 H 4 2 )OC 2 H 5 , is obtained by
treating the silver salt with ethyl iodide. It forms large crystals
which melt at 103 104 and boil at 270.

Phthalylhydroxylamine decomposes on dry distillation into
phthalic anhydride, ammonia and nitrogen ; a boiling solution
of caustic potash resolves it into phthalic acid and hydroxylamine,
while orthamidobenzoic acid is formed when it is heated in
alcoholic solution with one molecule of potash :

/CzrN.OH /NH 2

C.H 4 < >0 +KOH = C 6 H/ ^ +CO,



Hydroxyphthalamic acid is formed when an alcoholic solution
of phlhalylhydroxylamine is heated with potash for a short time :

C=N.OH



Potassium hydroxyphthalamate separates out on cooling ; it
is readily soluble in water, and is deposited on evaporation in
hard, yellow crystals.

The lead salt is thrown down when lead acetate is added to a
solution of the potassium salt ; it has the following constitution :






\CO



When this is suspended in water and treated with sulphuretted
hydrogen, a solution of the free acid is formed, which reddens lit-
mus and, like that of the potassium salt, is coloured violet by feme
chloride. The solution of the acid decomposes gradually on
standing, more rapidly when heated, with separation of phthalyl-
hydroxylamine.

Phthalylhydroxylamine shows great similarity to the n



II



468 AROMATIC COMPOUNDS.



acids, which, like the former, are colourless but are coloured red
by the slightest trace of an alkali, and have a similar constitu-
tion : 1

Phthalylhydroxylamine Ethylnitrolic acid.

/C=N.OH ^N.OH

C fl H/ >0
\CO

Benzenylazoximebenzenylcarboxylic acid, C 15 H 10 N 2 O 3 , is formed
when phthalic anhydride is fused with benzenylamidoxime :

X N.OH /CO.

cjL.cf +CLF/ Xn -



C.C 6 H 4 .C0 2 H f H 2 0.

It crystallizes from hot alcohol in lustrous needles melting at
15 1 . 2

Orthocyanobenzoic acid, C 6 H 4 (CN)CO 2 H, is obtained by adding
orthodiazobenzoyl chloride to a hot solution of copper sulphate
and cuprous cyanide ; it is a thick, viscous liquid, which has not
hitherto been prepared pare, since it readily changes into the
isomeric phthalimide, phthalamic acid being probably formed
as an intermediate product :

/ON /CO.NH,

C 6 H 4 < fH 2 = C c H 4 <

\CO.OH \CO.OH

This is then resolved into phthalimide and water. 3
The conversion may however be simply explained by an
intermolecular change : 4



C 6 H 4 =C 6 H 4 < >0 .

\CO-OH \CO

Ethyl orthocyanobenzoate, C 6 H 4 (CN)CO 2 .C 2 H 5 , is prepared by
diazotizing the ethyl ether of orthamidobenzoic acid and treating
the product in the manner just described; it crystallizes in
thick needles, melting at 70. 5 It is gradually attacked when

1 Cohn, Ann. Chcm. Pharm. ccv. 295.

2 Schulz, Ber. Dcutsch. Chcm. Ges. xviii. 2463.

3 Sandmeyer, ibid, xviii. 1496. 4 Liebermann, ibid. xix. 2283.
5 Miiller, ibid. xix. 1491.



DIHYDROPHTHALIC ACID. 469

heated with hydroxylamine in alcoholic solution, pkthalimidc-
oxime being formed :

/CEEN /C=NH

C 6 H 4 < 4H 2 N.OH = C 6 H 4 < >0 4 HO.C 2 H 6 .

\CO.OC 2 H 5 \C=:N.OH

The latter crystallizes from dilute alcohol in needles, which
melt at 250 and are converted into phthalimide by boiling with
ferric chloride and hydrochloric acid.



ADDITION PRODUCTS OF PHTHALIC ACID.

2245 Dihydrophthalic acid, C H 6 (CO 2 H) 2 , is formed when a
cold alkaline solution of phthalic acid is treated with sodium
amalgam, 1 or better when the operation is carried on at the
boiling point. 2 It crystallizes in hard, rhombic tablets, which
are only slightly soluble in cold, more readily in hot water, and
readily in alcohol. It is a strong dibasic acid ; on heating with
soda lime it decomposes into carbon dioxide, hydrogen and
benzene, while the action of phosphorus pentachloride upon it
yields hydrochloric acid, carbon dioxide, phosphorus oxychloride
and benzoyl chloride. Bromine acts upon its aqueous solution
in the following manner (Grabe and Born) :

C C H 6 (C0 2 H) 2 4 Br 2 = C 6 H 5 .C0 2 H 4 CO 2 4 HBr.

On heating with sulphuric or nitric acids, benzoic acid is also
formed, together with phthalic acid, while ethyl benzoate, pro-
bably accompanied by ethyl formate, is produced when hydro-
chloric acid is passed into its alcoholic solution.

Hydrophthalic acid can be heated to 200 without undergoing
any alteration ; at a higher temperature, however, it decomposes
with formation of phthalic anhydride.

All these reactions may be simply explained if we ascribe the
following constitution to the acid :

CH

JH C0 2 H

^JcH-coj-r



HC



CH



1 Grabe and Born, Ann. CTunn. Pharm. cxlii. 1,30.

2 Bacycr, Bcr. DnUsch. Chcm. Gcs. xix. 1807.



470 AROMATIC COMPOUNDS.

Tetrahydrophthalic acid, C 6 H 8 (CO 2 H) 2 . The anhydride of this
acid, C 8 H 8 O 3 , is formed by the distillation of hydropyromellitic
acid, C 6 H 6 (C0 2 H) 4 , and crystallizes from ether in hard, lustrous
plates, which melt at 68 and sublime readily. It is insoluble in
cold water, but dissolves in hot water with formation of the acid,
Avhich crystallizes in large plates, and is readily soluble in water,
being, however, converted into the anhydride when the solution
is heated to about 100 . 1

Dibromotetrahydrophthalic acid, C 6 H 6 Br 2 (CO 2 H) 2 , is obtained
by the combination of dry bromine with dihydrophthalic acid,
and crystallizes in rhombohedra (Baeyer).

Hcxhydrophthalic acid, C 6 H 10 (CO 2 H) 2 , is formed when tetra-
hydrophthalic acid or dihydrophthalic acid 2 is heated to 230 with
hydriodic acid, or when the former is treated with sodium amalgam
and water, while dihydrophthalic acid is not attacked by these
re-agents (Baeyer). It is slightly soluble in cold, somewhat more
readily in hot water, and crystallizes in small prisms or plates,
which melt at 203 205 without forming an anhydride.

Bromomalophthalic acid, C 6 H 8 Br(OH)(CO 2 H) 2 , is prepared by
adding bromine to an aqueous solution of tetrahydrophthalic
acid, a dibromide being first formed, which is then decomposed



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