[Sis290025]

Consider the following errors that could be made when running TLC. Indicate what should be done to correct the error.

1. A 2 component mixture containing 1-octene and 1,4-dimethylbenzene gave only one spot with an R_f value of 0.95. The used solvent was hexane.

Should a new solvent be used, so one spot does not result? However, it cannot be a polar solvent, since the mobility of the components would increase and they might run off the plate?

2. When a TLC plate was developed, the solvent ran off the plate's top.

Should the experimenter prepare a new TLC plate and develop all over again?

Thank you.

[mike]

1. Yes, your elution solvent is too polar. Try a new elution solvent mixture which is less polar and the single spot should separate more efficiently.

2. Yes, a new TLC plate must be prepared and run. (May depend on the type of plate thoug as sometimes sample can be recovered and reloaded, generally though you would do it again)

[Sis290025]

I'm a bit confused on the first part when you state that the "elution solvent is too polar." Isn't hexane the elution solvent, so how can hexane, an alkane, be too polar? 

Thank you again.

[mike]

Yes good point, I misread you question, sorry. I would try a larger alkane maybe, heptane, octane etc or a mixture. I am not sure why the Rf is so high though.

[movies]

If hexane gives too high an R_f, then your best bet is to switch the solid phase part of the equation.  Alumina or Fluorosil coated plates are generally less polar than silica gel plates and therefore will retain less polar compounds a little better.

You can also try pentane, which has slightly less eluting power than hexane, but not so much so that you'd notice the difference.  Higher alkanes are technically more polar (think London dispersion forces).

[GSK]

Try petroleum ether

[chiralic]

Hello Sis290025:

I´ll give you some tips to solve your questions that you post about Thin-Layer Chromatography
and Developing Solvent Selection...

Here some handouts about Solvent Selection:

The secret to success in TLC often depends on choosing the developing solvent correctly. Optimizing a separation usually centres on the selection of the best developing solvent by adjusting solvent strength and selectivity to maximize separation. The Rf values are considered “optimum” between 0.2-0.6 (according to several authors).  Normally you can get the best developing solvent using the classical technique “trial-and-error” or empirical approaches based on experience and intuition.

With polar adsorbents , non-polar solutes exhibit little affinity for the adsorbent surface and are not retained. In contrast, polar or polarizable (e.g. aromatic compounds) solutes are strong retained because of relatively strong dipole-dipole and dipole-induced dipole forces.

The solute elution on a polar adsorbent is, predictably, in the order of the POLARITY/POLARIZABILITY of the Solute Functional Groups, that is:

Alkanes <alkenes <aromatics,halides <ethers, polynuclear aromatics, nitriles, nitro compounds <esters <ketones <aldehydes <amines <alcohols, phenols <amides <carboxylic acids

You can see better this point on a drawing that I put on this link:
http://tinypic.com/view/?pic=48fpjy0

If you have access to Journal of Chromatography A (Science Direct), please read this reference:
Snyder, J. Chromatogr. (1974) 92:223-230 (Talk about the "Chromatographic Polarity")

Regards,

Chiralic

[firassh2000]

1. u should increase the polarity of the solvents by using the mixture (benzene:methanol-9.5:0.5)