Alizarin NEW

Alizarin (1,2-dihydroxyanthraquinone) is a naturally occurring red anthraquinone dye derived historically from the roots of the madder plant (Rubia tinctorum), now produced synthetically. Here's a detailed chemical breakdown:

Chemical Formula: C₁₄H₈O₄

IUPAC Name: 1,2-Dihydroxy-9,10-anthraquinone

Structure: It consists of a planar, conjugated anthraquinone core (three fused 6-membered rings) with hydroxyl (-OH) groups attached to positions 1 and 2 (adjacent/ortho positions) on the central ring.

• Simplified: O=C₁C₂=C(C=CC₃=C₂C(=O)C₄=C(C=CC=C₄)C₃=O)O

  Key Properties:

• Appears as orange-red crystals or powder.

• Insoluble in cold water, sparingly soluble in boiling water, soluble in alkalis, ethanol, ether, and organic solvents.

• Its intense red color arises from its extended conjugated π-electron system.

  Function as a Dye (Mordant Dye):

• Alizarin itself is not a direct dye. It forms insoluble, colored complexes (lakes) when chelated with metal ions (mordants) like Al³⁺ (alum), Ca²⁺, or Sn²⁺.

• The ortho-dihydroxy groups on the anthraquinone core are essential for chelating metal ions. The specific metal determines the final shade (e.g., Al³⁺ gives bright red "Turkey red", Sn²⁺ gives pink, Fe³⁺ gives violet/brown).

  Historical & Modern Significance:

• One of the oldest known natural dyes, used for millennia.

• The first natural dye to be synthetically produced industrially (Carl Graebe & Carl Liebermann, 1869), revolutionizing the dye industry and leading to the decline of madder cultivation.

  Modern Uses:

• Histological stain: Used to stain calcium deposits in tissues (e.g., Alizarin Red S, the sulfonated derivative).

• Indicator: Acts as a pH indicator (yellow below pH 5.5, red above pH 6.8, violet above pH 12).

• Pigment: In artists' pigments (e.g., Alizarin Crimson).

• Teaching: A classic example of mordant dye chemistry.

  CAS Number: 72-48-0

  Toxicity: Low acute toxicity, but can be a skin/eye irritant. Suspected of causing genetic defects (EU H351).

In short: Alizarin (C₁₄H₈O₄) is a pivotal ortho-dihydroxy anthraquinone compound, famed as a natural red mordant dye requiring metal ions (like Al³⁺) to form its characteristic insoluble red lakes on fabrics. Its synthesis in 1869 marked a milestone in industrial chemistry. Today, its derivatives are primarily used as biological stains and pH indicators.