Lab 4 Thin Layer and Column Chromatography

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What was the point of performing part A?

Thin Layer Chromatography...

Explain relative Rf values of fluorene and fluorenone.

Fluorenone is more polar than fluorene because of its C=O bond. Fluorenone's spot will be lower than fluorene because of its "stickness."

For this experiment, the TLC of flask 3 shows two spots with Rf 0.4 and 0.6. Identify the spots and comment on the separation procedure.

0.4 = Fluorenone, 0.6 = Fluorene. Flask three was only supposed to have Fluorenone but this shows contamination of the other compound.

For a mixture of fluorene, fluorenone and -fluorenol is examined by TLC and gives the following Rf values: 0.3, 0.5, 0.8.| Identify each spot and rationalize the relative Rf values.

0.3 - Fluorenol, 0.5 - Fluorenone, 0.8 - Fluorene. Most polar to least. Alcohol > Ketone > Ene.

What are the hazards associated with the column solvents?

Ligroin: Volatile! Flammable! Defats skin!
TBME: skin irritant, don't consume, extremely flammable.

What was the adsorbent in this reaction?

Alumina.

(Used for separation of compounds of low to medium polarity and is More Polar than silica gel. BUT silica gel has advantage of being less likely to cause chemical reaction with substances being separated.)

What order of elution might you expect for a mixture of anthracene, 2-napthol, benzaldehyde and benzoic acid.

1. Anthracene (three benzene rings!)
2. Benzaldehye (one benzene plus aldehyde)
3. 2-naphthol (two benzenes plus OH stickin' out)
4. Benzoic acid (one benzene plus carboxylic acid)
(In order of elution) Because from more to less polar, acid > OH > aldehyde.

Consider naphthalene and beta-naphthol, a) Give the structures of each, b) Draw two TLC plates: Plate 1: 100% hexane; Plate 2: 50/50, hexane/ethanol

a) Fused, two benzenes attached.
b) Plate 1: two spots at the same level at the bottom of the plate. Plate 2: two spots at the same level higher up on the place. DOES THIS SOUND RIGHT?

The Rf of starting material A is 0.2 in 100% pentane, a) Draw the TLC plate, b) What change would you make to the solvent and why? c) Draw the TLC plate that would result from your changes.

a) I can't draw.
b) Add ethyl acetate or another polar solvent for better separation
c) Confused by this question. If this A is a mixture then components will separate, if not this lone spot will simply climb because the new solvent is more polar!

The Rfs of starting material A and product B are both 0.8 in 100% ethyl acetate, a) Draw the TLC plate, b) Based on TLC are A and B the same compound? Explain., c) Would you make a change to the solvent? If yes, what change? If not, why not?

a) I can't draw.
b) No! This is likely to be a case of poor separation because ethyl acetate is a polar solvent and polarity of a solvent increases sample migration. Try another solvent system
c) Yes! Include an ethyl acetate:hexane mixture to observe potentially better separation of compounds.

Benzoic acid, methyl benzoate and benzene are spotted on the same TLC plate, in lanes 1, 2 and 3 respectively. Draw that TLC plate explain the position the relative Rf values.

Bottom(s) Up: Benzoic acid, methyl benzoate, benzene. BA - alcohol, MB - ketone, B - nothin'

polarity

A separation of electric charge - electric dipole/multipole moment. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and asymmetry of the compound's structure. Presence of oxygen is more polar than nitrogen (more EN). Polarity Ranking: (most-least) Amide > Acid > Alcohol > Amines > Ketone ~ Aldehyde, esters > Ethers, halocarbons > Aromatic hydrocarbons > Alkenes > Alkanes.

soluble

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution of the solute in the solvent

solvent front

Total distance traveled by the solvent

spotting solvent

For the solids (sol), transfer a spatula tip of each material to a vial (one vial per sample!). For the liquid (liq), transfer 1 or 2 drops to a push-top vial. Add solvent to dissolve the sample - the height of solvent in the vial should be about 1 cm. A good spotting solvent is ethyl acetate: it dissolves most substances that we use in lab, evaporates readily and does not react with the substances under study.

developing solvent

The more polar the solvent, the faster the compounds will travel.

adsorbent

...? ie chromatography is a sorption process where certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column

Rf

The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent. The compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a low polarity compound will have a larger Rf value than a polar compound.

eluent

The solvent that will carry the analyte while the eluate is the mobile phase leaving the column

mobile phase

The phase which moves in a definite direction and consists of the sample being separated/analyzed and the solvent that moves the sample through the column. It moves through the chromatography column (the stationary phase) where the sample interacts with the stationary phase and is separated. The polarities of both mixture component and solvent used as mobile phase are determining factors in how fast compound travels

stationary phase

The substance which is fixed in place.. In Column and Thin Layer chromatographies, the stationary phase (the adsorbent: silica gel or alumina) is polar.

TLC

Separates components in a mixture based on differences in polarities and intermolecular interactions which is used to assess progress of reaction (as product forms, new spots appear), purity of compound (one v. two spots)

What do you do if the sample runs as a smear or a upward crescent?

Compounds which possess strongly acidic or basic groups (amines or carboxylic acids) sometimes show up on a TLC plate with this behavior. Add a few drops of ammonium hydroxide (amines) or acetic acid (carboxylic acids) to the eluting solvent to obtain clearer plates.

Chromatography

In these (and all types of) chromatographies, a mixture is separated by distributing the components between a stationary phase and a mobile phase.

Choosing solvents...

Mix polar and nonpolar solvent. Because of toxicity, cost, and flammability concerns, the common solvents are hexanes (or petroleum ethers, ligroin) and ethyl acetate (an ester). Diethyl ether can be used, but it is very flammable and volatile. Alcohols (methanol, ethanol) can be used. Acetic acid (a carboxylic acid) can be used, usually as a small percentage component of the system, since it is corrosive, non-volatile, very polar, and has irritating vapors. Acetone (a ketone) can be used. Methylene chloride (halogenated hydrocarbon) is a good solvent, but it is toxic and should be AVOIDED. If two solvents are equal in performance and toxicity, the more volatile solvent is preferred in column chromatography because it will be easier to remove from the desired compound after isolation from a column chromatography procedure.

TLC (further defined)

The stationary phase is a powdered adsorbent which is fixed to a aluminum, glass, or plastic plate. The mixture to be analyzed is loaded near the bottom of the plate. The plate is placed in a reservoir of solvent so that only the bottom of the plate is submerged. This solvent is the mobile phase; it moves up the plate causing the components of the mixture to distribute between the adsorbent on the plate and the moving solvent, thus separating the components of the mixture so that the components are separated into separate "spots" appearing from the bottom to the top of the plate.

Column Chromatography

The stationary phase is a powdered adsorbent which is placed in a vertical glass column. The mixture to be analysed is loaded on top of this column. The mobile phase is a solvent poured on top of the loaded column. The solvent flows down the column, causing the components of the mixture to distribute between the powdered adsorbent and the solvent, thus (hopefully) separating the components of the mixture so that as the solvent flows out of the bottom of the column, some components elute with early collections and other components elute with late fractions.

Acids, ethers, alcohols, esters. Order of polarity and spot order on a TLC plate?

Acids and alcohols travel shorter distance than less polar/nonpolar compounds like ethers and esters.

Why do you need alumina AND ligroin in column chromatog.? In TLC it makes sense because the solid phase is the plate itself and we need the spots to appear... And what's the sand for in CC?

WHY?

UV Visualization

Gel contains fluorescing zinc sulfide so plate shines brightly but UV absorbing compounds (ie containing pi bonds) show up as dark spots.

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