420 
Artificial  Formation  of  Indigo. 
(  Am.  Jour.  Pliarm^ 
t      Aug.,  1881. 
ARTIFICIAL  FORMATION  OF  INDIGO. 
delation  of  Cinnamic  Acid  to  the  Indigo  Group. 
By  a.  Baeyer. 
After  giving  a  sliort  historical  sls:etcli  of  tlie  different  steps  wliicli  led  to 
the  discovery  of  the  artificial  formation  of  indigo,  the  author  describes  the 
preparation  of  indigo-blue  from  cinnamic  acid.  Orthonitrocinnamic  acid, 
CgH^NO^  (m.  p.  !240°),  is  best  prepared  by  the  process  described  by  Beilstein 
and  Kuhlberg  ("Ann.,"  163,  p.  125),  which  converts  about  60  per  cent,  of 
the  cinnamic  acid  into  the  orthonitro  acid.  The  ortho-  and  para  acids  are- 
separated  by  means  of  the  difference  of  their  solubility  in  ether.  Ethyl 
orthonitrocinnamate  forms  rhombic  crystals  (a  :  6  :  c  =0*9265  : 1  :  0'5174) 
which  melts  at  44°.  This  ortho  acid  can  easily  be  identified  by  the  blue 
color  which  is  produced  when  it  is  dissolved  in  warm  sulj^huric  acid. 
Dibi  omonitrocinnamic  acid,  formed  by  the  direct  addition  of  bromine  to 
the  nitro  acid,  cr^^stallizes  in  colorless  needles  or  plates,  which  melts  at 
180°  with  decomposition.  The  crystals  are  soluble  in  hot  water  and  in 
benzin.  The  ethyl  and  methyl  salts  of  this  acid  melt  respectively  at  110° 
and  !)8°.  When  a  solution  of  the  dibromo  acid  in  caustic  alkalies  is  gently 
warmed  it  changes  into  orthonitropropiolic  acid  and  isatin.  Indigo-blue 
is  also  formed  if  the  solution  is  boiled,  and  if  zinc-dust  is  added  to  the 
mixture  indole  is  produced. 
If  a  solution  of  dibromorthonitrocinnamic  acid  in  excess  of  caustic  alkali 
is  allowed  to  stand  for  some  time,  and  an  acid  is  then  added,  a  white  pre- 
cipitate of  orthonitrophenylpropiolic  acid  is  thrown  down.  The  acid  may 
be  obtained  in  colorless  needles  by  careful  recrystallization  from  hot  water; 
it  melts  at  155°  with  decomposition.  The  potassium  and  sodium  salts  of 
this  acid  are  freely  soluble  in  water,  but  much  less  soluble  in  the  presence- 
of  free  alkali.  The  silver  salt  explodes  when  heated.  The  ethyl  salt  is. 
deposited  from  an  ethereal  solution  in  large  tables  (ni.  p.  60°). 
Orthonitroacetylene,  C8H5NO.2,  is  prei)ared  by  the  action  of  water  at  150° 
on  nitropbenylproi:)i()lic  acid,  or  by  boiling  an  aqueous  solution  of  the  acid 
in  a  fiask  provided  with  an  upright  condenser.  The  product  is  distilled  in 
a  current  of  steam,  and  tln^  nitroacetylene  purified  by  recrystallization 
from  alcohol.  It  forms  colorless  needles  (m.  p.  81°)  which  acquire  a  yellow 
color  on  exposure  to  light.  The  crystals  dissolve  freely  in  hot  water  and 
in  most  solvents.  An  ammoniacal  solution  of  silver  nitrate  yields  a  yel- 
lowish-white, and  ammoniacal  cuprous  chloride  solution  forms  a  red  pre- 
cipitate with  nitroacetylene.  On  reduction  with  zinc-dust  and  ammonia 
an  oily  base  is  produced,  which  has  the  peculiar  odor  characteristic  of  the 
indigo  vat.  The  decomposition  of  orthonitropropiolic  acid  by  the  action 
of  a  hot  solution  of  potash  may  be  advantageously  employed  as  a  method 
for  preparing  isatin,  86  per  cent,  of  the  theoretical  yield  being  obtained. 
^  C  i  C.COOH  ^  CO— CO 
CfiH^  —  CO2  +  CgH^ 
^NO^  ^NH 
