Spectroscopic Analysis of Anthraquinones

Study Notes

Anthraquinones are naturally occurring compounds with a characteristic fused tricyclic ring structure.
They are known for their diverse biological activities and are often found in plants as glycosides.

Provides information about functional groups present in the anthraquinone molecule.
Characteristic peaks for aromatic C=C stretches, carbonyl (C=O) stretches, and C-O stretches (from glycosidic linkage) are helpful in identifying anthraquinones.
Barbaloin's IR spectrum shows prominent peaks for aromatic C=C stretches, a strong carbonyl (C=O) peak, and C-O stretching vibrations associated with the glycosidic linkage.
Aloin exhibits a similar IR spectrum to barbaloin, with characteristic peaks for aromatic rings, carbonyl groups, and the glycosidic linkage.

Anthraquinones exhibit strong UV absorption due to their extended conjugated π-electron system.
The position and intensity of absorption bands provide information about the substitution pattern on the aromatic rings.
Barbaloin shows characteristic UV absorption maxima around 220 nm, 275 nm, and 345 nm due to its conjugated aromatic rings.
Aloin exhibits similar UV absorption behavior to barbaloin, with specific peaks depending on the substitution pattern on its aromatic rings.

Provides detailed information about the chemical environment of individual protons and carbons within the anthraquinone molecule.
Analysis of ¹H NMR and ¹³C NMR spectra helps identify the number and type of protons and carbons present, their connectivity, and the substitution pattern on the aromatic rings.
¹H NMR of barbaloin reveals distinct peaks for aromatic protons, methyl groups, and the sugar protons associated with the glycosidic linkage.
Aloin's ¹H and ¹³C NMR spectra provide similar information, allowing for the identification of specific proton and carbon environments and the substitution pattern on the aromatic rings.

Provides the molecular weight of the intact anthraquinone molecule and fragmentation patterns, aiding in structural analysis.
Techniques like Electrospray Ionization (ESI-MS) are often used due to the polarity of anthraquinones.
MS can help differentiate between closely related anthraquinones with similar structures.
Barbaloin exhibits a characteristic molecular ion peak in its ESI mass spectrum, which helps confirm its identity.
Aloin also shows a distinct molecular ion peak in its mass spectrum, allowing for its differentiation from barbaloin or other anthraquinones.