Question

These questions are from an experiment where we had a mixture of Ferrocene, acetylferrocene and diacetyl ferrocene and we separated each by using column chromatography. Before doing the column chromatography we did TLC analysis to ensure that we did, in fact, have a mixture of these 3 compounds.

During the column chromatography we first eluted the ferrocene using pure hexane. Then we eluted the acetylferrocene using a 1:1 mixture of hexane and methylene chloride. Then we eluted the diacetylferrocene using a 9:1 mixture of hexane and methylene chloride.

1. Rank ferrocene, acetylferrocene, and diacetylferrocene in order of increasing polarity. Do the TLC results from your fractions support this ranking? Explain.

2. Rank the solvents used in the experiment in order of increasing polarity.

3. Why do you start with the least polar solvent/solvent mixture and progress to increasing polar solvent/solvent mixtures when eluting the ferrocene compounds from the column rather than starting with more polar solvent system and progressing to less polar solvents?

4. How do the thin layer and column chromatography for this experiment compare in regard to stationary and mobile phases?

5. What key feature in the IR spectrum of acetylferrocene distinguishes it from the spectrum of ferrocene?

Answer 1

As you know that column chromatography is a technique used to separate a mixture of compounds based on the differences in their polarity.

For any standard column chromatography, the compound which is collected first is considered to be the least polar and the others followed by.

1) Thus, polarity of the 3 compounds are in the order,

Ferrocene < Acetylferrocene < Diacetylferrocene

Your TLC data must also support this observation . You can cross check it by measuring the values of Rf ( Retention factor = distance travelled by solute/solvent ) .The Rf values should follow the order: -

Ferrocene > acetylferrocene > diacetylferrocene

2) Since, hexane is a completwly non-polar solvent ( as it contains just a hydrocarbon chain with molecular formula C₆H₁₄) , Polarity of hexane is increased by adding some polar solvents such as ethyl acetate, methylene chloride etc.

Therefore, the increasing order of polarity of solvents is: -  

hexane < 1:1 mixture of hexane: methylene chloride < 9:1 mixture of methylene chloride: hexane ( which your question wrongly mentions as 9:1 mixture of hexane: methylene chloride because it would be less polar than 1:1 mixture and we never use a less polar solvent after a more polar solvent, which will be more clear to you after after part 3.)

3) All chromatographic techniques have a stationary phase and a mobile phase. In column chromatography, the silica gel ( which is used in packing of column ) is the stationary phase and the solvent that we pour is the mobile phase.

Silica gel is highly polar in nature. When we begin with a non-polar solvent, the polar components of the mixture will bind strongly to the silica gel whereas, the non-polar component flows out with the solvent. With a particular polarity of solvent, a compound with polarity more than that will never be eluted. On the contrast, if we would havce started with a more polar solvent, the entire mixture would have eluted at once. Thus, in order to ensure a better separation , we begin with the least polar solvent and progressively increase the polarity as the desired coomponent is eluted.

4) both TLC and column chromatography have the same stationary phase i.e. silica gel. Moreover, we use the same solvent mixture in both the techniques. thus, the result of the 2 are nearly same. The only difference observed is that, TLC works against the gravity and column chromatography works in the direction of the gravity. Thus, the component which travels the least distance on TLC plate will be eluted the last from the column and vice versa.

5) The key feature in the IR spectra of the acetylferrocene that will be absent in the spectra of ferrocene is the presence of carbonyl stretching frequency ( CO stretching frequency) at nearly 1700 cm^-1. This peak can easily distinguish between acetyl ferrocene and ferrocene.

Feel free to ask your queries in the comments.