Hi,

it depends on the type pf metabolites you are planning to study. For example, if your aim is to analyze phenolics and flavonoids you should use polar solvents (methanol, ethanol, water) if you use a non-polar solvent such as n-hexane you will not fully extract this kind of compounds.

Like Dr. Spinola said, it depends on what you wish to isolate. Carotenoids are best extracted with non-polar solvents such as ethyl acetate, capsaicins extract well with toluene. however most alkaloids extract best with slightly acidic methanol or water. antioxidant compounds such as flavanoids are best extracted with polar solvents such as methanol. Tocopherols are antioxidants but are best extracted with non-polar solvents such as hexanes or ethyl acetate.

In reply to your question(Is there any need to employ polar solvents for plant extractions?), yes there may be a need. Look at the famous extracting solvents & you will find among them: strongly polar, moderately polar & slightly polar (e.g. diethyl ether, DCM, chloroform, methanol, THF...etc.). All this depends on the specific compound you want to extract. If water can do the job, then it is the solvent of choice.

Thanks to all..

When we select a plant  to study a particular biological activity, the selection of the plant is basically based on the knowledge of  traditional use by various ethnic community. Generally, water is used  as solvent in such traditional use (in the form of juice, decoction or infusion) as medicine. Since, the ethnic community get therapeutic efficacy with aqueous extract,therefore, the therapeutically active constituents are expected to be present in the aqueous extract. So, if we want to further determine the therapeutic efficacy of the extract of plant part (s) then, we should use polar solvent first. For specificity, precision, protection of the phyto-constituents from degradation and   convenience, solvents in the order of non-polar to polar  are used for total isolation.  But when we try to extract phytoconstituents, selection of solvent depends on the physico-chemical property of the constituents to be extracted. Polar solvent for polar substances and non-polar solvents for non-polar substances.

Thank you sir, but when we are studying antioxidant activity, are polar solvents affect there?

You must first perform an extraction fractions of hydrophilic and lipophilic, then you must perform the detection and quantification:

Acidified methanol (1% HCl v/v) extract: hydrophilic phenolic compounds and antioxidant activity.

Extract with ethyl acetate: lipophilic antioxidant activity.

Because your focus is on antioxidants, you should use polar solvents, these solvents also could be extracted some phenolic glycosides, which are good antioxidants.

Thank you sir,  My further work is such as...After extraction, I will go through Thin layer chromatoghraphy for the separation of different compounds then I will move for IR, NMR and Mass spectroscopy. After that I will study quantitative analysis of phenollic content, antioxidant activity and enzyme inhibition assay. Kindly help me if it is right or wrong..

For separation of phytochemicals, TLC is not enough. Although you can do prep-TLC but your yield would be very less which is not enough for all the things you want to do. In my suggestion you should use column chromatography with silica or sephadex.

Good luck

Deepak

Dear Shital,

You should use polar solvents for extraction. As extract obtained from polar solvents of many medicinal plants have shown promising antioxxidant activity. 

You are trying to characterize the components responsible for antioxidant activity using TLC, IR, MS, etc. This is good way to about your research. I would like to suggest to use column chromatography or thin layer chromatography for separation of some components. And do test those components for their biological activity for eg antioxidant acttivity. 

Thanks to all for your valuable guidance..

Yes, polar solvents are required. Drug discovery  process  mostly based on the traditional information and you will notice that  people generally use devotion or fresh juice of the plant Parts. Therefore it is expected  that the constituents having biological activity would be present in polar solvents only. However, for ease of evaporation of solvent from the extract hydroalcoholic solvent is used in stead of water. In addition to  explore  other constituents  solvents  of different  polarity are used and for this successive solvent extraction is followed. Constituents showing  antioxidant activity (phenolic, flavonoids etc.) would be present in polar  solvents. So use of polar  solvents is important for your  extraction.

Thank you sir for your valuable suggestions...It will very helpful while working.