2.1.2 Caffeic Acid
Synonym 3, 4-Dihydroxycinnamic acid.
Chemical Structure
Biological Source It occurs widely in more than twenty different species of plants as detailed below: Aconitum napellus L. (Ranunculacea)-Aconite, Monkshood, Blue Rocket; Arctium lappa L. (Asteraceae)-Edible Burdock, Great Burdock, Lappa; Arnica montana L. (Asteraceae)-Mountain Tobacco, Leopard’s-bane; Cinnamonum camphora (L.) J.S. Presl. (Lauraceae)-Camphor, Hon-Sho; Citrullus coloeynthis (L.) Sehrad. (Cucurbitaceae)-Colocynth, Bitter Apple, Wild Gourd Clematis vitalba L. (Ranunculaceae)-Traveler's Joy; Coniun maculatum L. (Apiaceae)-Hemlock; Convalaria majalis L. (Liliaceae)-Lily of the Valley; Crataegus oxycantha L. (Rosaceae)-Howthorn; Digitalis purpurea L. (Serophulariaceae)-Common Foxglove, Digitalis; Equisetum hyemale L. (Equisetaceae)-Shavegrass, Great Scouring Rush; Euphorbia pulcherrima Wild ex Klotsch (Euphorbiaceae)-Poinsettia; Euphrasia officinalis L. (Scrophulariaceae)-Eyebright; Gaultheria procumbens L. (Ericaceae)-Wintergreen, Teaberry, Boxberry; Leonurus cardiaca L. (Lamiaceae)-Motherwort, Santolina charnaecyparissus L. (Asteraceae)-Lavender-Cotton, Seopolia carniolica Jacq. (Solanaceae)-Seopolia; Solanum tuberosum L. (Solanaceae)-Potato; Solidago virgaurea L. (Asteraceae)-European Goldenrod, Woundwort; Stachys officinalis (L.) Trevisan (Lamiaceae)-Betony; Trifolium pratense L. (Fabaceae)-Red Clover, Pavine Clover, Cowgrass; Valeriana officinalis L. (Valerianaceae)-Valerian; and Viscum album L. (Loranthaceae)-European Mistletoe.
Preparation It occurs in plants only in conjugated forms e.g., chlorogenic acid. It has also been isolated from green coffee,* and from roasted coffee.**
It can also be prepared by the hydrolysis of chlorogenic acid in an acidic medium as shown below:***
Characteristic Features Caffeic acid has yellow crystals obtained from concentrated aqueous solutions and the corresponding monohydrate from dilute solutions. It gets softened at 194°C and decomposes at 223-225°C. It is sparingly soluble in cold water, but freely soluble in cold ethanol and hot water.
Chemical Tests
1. It changes colour from yellow to orange in an alkaline medium.
2. It readily forms the methyl ester (C10H10O4) which are obtained as colourless crystals from water (mp 152-153°C).
