1) Alizarin is an important red dye that is found naturally in the madder root. It is synthesized commercially by sulphonating anthraquinone with oleum at 140°C and then fusing the resulting sodium salt with NaOH and NaClO3 at 200°C under pressure. 2) Analysis shows that alizarin has a molecular formula of C14H8O4 and is a dihydroxy derivative of anthraquinone, as treatment with bromine and then KOH produces alizarin from anthraquinone. Alizarin also forms a diacetyl derivative upon acetylation, confirming the presence of two hydroxyl groups. 3) The structure of alizarin is

1. 1 | P a g e
2. ALIZARIN
Synthesis of alizarin (1, 2 - Dihydroxy anthraquinone)
a) It is an important dye occurring in madder root in the form of its glycoside
ruberthric acid.
b) It is manufactured by sulphonating anthraquinone with oleum at 140oC . The
sodium salt of the resulting -sulphonic acid (also known as “Silver salt”) on fusion
with NaOH and NaClO3 at 200oC under pressure gives alizarin.
Structure of Alizarin
1) The molecular formula of alizarin is C14H8O4.
2) On distillation with Zn dust it forms anthracene. This shows that alizarin is an
anthracene derivative.
C14H8O4
 
/Zn
C14H10
Alizarin Anthracene
3) Anthraquinone on treatment with calculated quantity of bromine gives dibromo
anthraquinone. This on fusion with KOH gives alizarin. Thus alizarin must be a
dihydroxy derivative of anthraquinone.

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C14H8O2
 
2Br2
C14H6O2Br2  
KOH
C14H6O2(OH)2
Anthraquinone Dibromo Alizarin
anthraquinone
4) Alizarin on acetylation gives a diacetyl derivative. This also proves the presence of
two hydroxy groups in alizarin.
Thus the structure of alizarin must be written as
5) Alizarin can be synthesised by condensing phthalic anhydride and catechol. This
proves the presence of two hydroxy groups in the ortho position to each other.
(or)
6) On oxidation with MnO2 and H2SO4 , alizarin forms purpurin (1, 2, 4 - trihydroxy
anthraquinone).

3. 3 | P a g e
7) Only structure I can explain the formation of purpurin. Therefore the structure of
alizarin is,
8) Finally the above structure of alizarin is confirmed by its synthesis from
anthraquinone.
Prepared by
Dr. A. Syed Mohamed
HoD and Senior Assistant Professor, Research Dept. of Chemistry
Sadakathullah Appa College (Autonomous)
Tirunelveli, Tamilnadu, India. asm2032@gmail.com
The author ackowledges Prof. H. Kassali Rahmathullah, Dr. T. Syed Ismail, Dr. M.
Kamalutheen, Former Professor, Dept. of Chemistry, Sadakathullah Appa College for their
valuable contribution.