Colour and Chemical Constitution. 
2V6 
lias a value of 233, the ring 
H0-<->- 
has a value of 219 + 15 = 234, the ring 
H0-<->- 
X 
has a value of 241, and so on. In the j;-amino-rings the values are 4 greater 
in each case. To calculate ortho-tetrabromophenolphthalein therefore we 
split it into two rings 
--CD- 
X 
plus the tautonierism 
C - C - C, 
I 
CeH.CO.H 
which, as shown above, has the value of 103 + 18 = 121. 
Hence the substance should have (in alkali) X = 466 + 121 = 587. 
Observation gives X 584. Nearly the same observation is given if chlorine, 
iodine, or methyl be taken in place of bromine, hence I use X in the formulae. 
Consider now thymol-and carvacrolphthaleins. To calculate them they 
may be split into two 
H0-<->- 
Y 
rings, plus the 121 tautonierism. X is methyl and Y isopropyl in the one 
case, and vice versa in the other, but since all non-tautomeric groups have 
nearly the same value, the two substances have nearly the same colour. 
Taking the ortho X at 7 and the meta Y at 15 (as if it were meta-X), we 
have an addition of 22 to each ring ; hence both substances should have 
spectra which are 44 units above that of plienolplithalein. Observed : 
Phenolphthalein \ 554, thy molph thai ein X 597^ carvacrolplithalein X 601. If 
we allow the meta-isopropyl group 16 instead of 15, the calculated gain on 
phenolphthalein will be 46, and 554 + 46 = 600, which agrees with the 
observation on carvacrolphtlialein. The so-called sulphonephthaleins can 
also be calculated in the same way, giving the ortho-position in the tauto- 
meric rings the value 7 and the iiieta-position 15, and allowing 27 for the 
group 
SO3H 
/ \_ 
outside the tautonierism, e. g. " bromphenol-purple," which has 4 ortho- 
bromines, should be (4 X 7) + 27 or 55 above dioxybenzhydrol = X 596 ; 
observed X 594. 
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