[Sis290025]

a. Suppose you have a sample that is made up of carbon and hydrogen only and you want to do a TLC analysis on it. What will be a suitable solvent to develop the plate?

One suitable solvent might be hexane or a solvent mixture of hexane (nonpolar solvent) and acetone? Since it is a hydrocarbon, it is nonpolar, so it would travel more quickly in a polar solvent. If its R_f increases in a polar solvent, it might run too close to the TLC plate’s end?

b. What if you have a 50/50 mixture of carboxylic acid and a saturated hydrocarbon?

Carboxylic acid is very polar and moves the slowest. A saturated hydrocarbon is nonpolar and travels the most on the slide. Would a solvent of moderate polarity, such as acetone, work, or must a mixture of solvents be used? (How does one know when to use a mixture of solvents?)

Thank you.   

[russellm72]

In theory what you say is correct. You should choose a non-polar solvent like hexane to run your hydrocarbon and u might need some EtOAc in it to run your acid.

You could always start with 50-50 EtOAc-hexane and change your concentrations depending on whether you want a more or less polar system. I also use DCM a lot which is a nice solvent to use and you can mix with hexane to lower the polartity or useful for more polar compounds run mixture such as 10%, 20% etc MeOH in DCM. Also I like toluene-EtOAc which can give some nice spot shape. Also in addition for compounds such as acids you may need a developer such as AcOH in there to stop your product streaking so much. For amines you could use ammonia or triethylamine.

Also, you need to know how to visualise your spots. If it is aromatic usually a UV lamp will suffice but for saturated compounds etc you may need to use Ninhydrin, Iodine, permangenate, vanillin etc etc...

Like most things things become clearer and easier to decide with experience.

Have a play around and see how you get on.

R.