A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUNDS. 239
It is readily obtained by heating citric acid rapidly until tlte distillate
begins to assume a dark colour. The liquid thus obtained is
heated to 120°, and the crystals which form on cooling are recrystallized
from water. It is also formed by heating an aqueous solution of
citric acid in closed tubes to 160°. It forms large rhombic prisms
possessing a sour taste. It melts at 161°, and is decomposed by distillation
into water and the anhydride of citraconic add.
GUracmic Add is isomeiic with itaconic acid, and is obtained by
repeatedly distilling the latter acid; citraconic anhydride C6H4OS
being formed, an oily liquid, which when exposed to the air absorbs
moisture, and is converted into citraconic acid, forming rhombic
octahedrons melting at 120°. On heating it with water to 120° it is
again converted into itaconic acid.
Mesaemio Acid,—When itaconic acid is boiled with weak nitric
acid, a new isomeric modification is formed, which crystallizes from
alcohol in sltiuiug prisms, which are sparingly soluble in water, melt
at 208°, and sublime at a higher temperature.
Tltese three isomeric acids are non-saturated compounds which
combine with nascent hydrogen, all tliree yielding one and the same
product, viz. pyrotartaric add C3H0-J prj'Xxj- To explain the iso-
merisra of itaconic, citraconic, and mesaconic acids O3H4| po"/vrr
we must assume that in the propene group C3Ha of pyrotartaric acid,
two atoms of hydrogen are wanting, occupying different positions.
This is proved by the fact tlmt the three acids, ou coinbiniug with
bromine or with hydrochloric acid, yield isomeric substitutiou-products
of pyrotartaric acid.
Bibromopyrotartaric Add CsH4Br2(CO.OH)? is obtained by adding
bromine to an aqueous solution of itaconic acid. On heating it with
water and silver oxide it is converted into an acid which, being
homologous with tartaric acid, has been called itatartaric acid
G,H4(OH)jj(CO.OH)jj. By boiling an. aqueous solution of sodium
bibromopyrotartarate it is resolved into sodium bromide and aemic
and O^O, = C8H8(CO.OH)2.
This acid forms large crystals melting at 154°; it is a monobasic
ocid, On boiliiig it with baryta-water it is resolved into formic acid
and succinic acid. It is not acted upon by heating it with acetic
anhydride, showing that it contains no hydroxyl, and has probably
the following constitution;—
)H2—CO.OH
CHraWfromypyrotartarie Arid CsH4Br.(C0«0H)a is obtained by
combining bromine with citraconic acid. It is readily soluble in
water, and yields, on boiling its solution with potash, potassium bromide,
potassium carbonate, and monobromo-tsocrotonie add C4H6Br08.