HO | | OH HOOC | | OH
V V
OH

A drawback to the acceptance of this formula is the absence of an
asymmetrical C-atom; the formula, therefore, does not explain the
optical activity exhibited by tannin. Schiff attempted to overcome this
difficulty by adopting a diagonal structural formula, but even when
adopting Clauss' diagonal formula for benzene the optical activity of a
number of other compounds depends upon the existence of the asymmetrical
C-atom. Biginelli [Footnote 2: _Gazz chim. Ital_., 1909, 39, 268.] also
opposed the digallic acid formula, and supported his view by referring
to the arsenic compounds obtained by him on heating arsenic acid and
gallic acid, instead of obtaining digallic acid. Walden, [Footnote 3:
_Ber_., 1898, 31, 3168.] on the other hand, found, on analysing the
digallic acid thus prepared, only slight traces of arsenic and, by the
elementary analysis, obtained figures closely corresponding to those of
digallic acid.

Bottinger [Footnote 4: _Ibid_., 1884, 17, 1476.] prepared the so-called
_[Greek: b]_-digallic acid by heating ethyl gallate with pyroracemic
acid and sulphuric acid and proposed the so-called ketone-tannin
formula:--

HO_____OH ______OH
HO{_____}--------CO--------{______}OH
COOH OH

Schiff completed this formula by a diagonal, so as to explain the
optical activity observed--