Polynuclear hydrocarbons module8
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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
Polynuclear hydrocarbons module8
- 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.
- 2. 2 | P a g e 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.