Lab 5 Questions

Rank the following organic solvents in order of polarity: -hexane, ethyl acetate, acetone, methanol -diethyl ether, ethanol, acetic acid, toluene

-hexane < ethyl acetate < acetone < methanol -toluene < diethyl ether < ethanol < acetic acid

1. What was the adsorbent for the TLC portion? 2. What was the absorbent for the column chromatography portion?

1. TLC: silica gel (SiO2) 2. CC: alumina (Al2O3)

Rf of starting material A is 0.2 in 100% pentane 1. Draw TLC plate 2. What change would you make to the solvent and why? 3. Draw the TLC plate that would result from your changes

1. The TLC plate has a spot is a fifth of the way up the solvent front 2. Make solvent more polar to help pull the starting material up the plate. 3. The spot is higher up the plate, and the solvent is lower

Rfs of starting material A and product B are both 0.8 in 100% ethyl acetate 1. Draw TLC plate 2. Based on TLC are A and B the same compound? Explain 3. Would you make a change to the solvent? If yes, what change? If not, why not?

1. Two spots traveled same distance (and too far) 2. Can't know if same compound because the spots went too far up the plate to yield information about the products -TLC Rfs should preferably be between 0.2-0.3 and 0.75 3. change should be made to the solvent to make it less polar and reduce the Rf values -The addition of an alkane like hexane would be perfect for reducing the polarity and allow us to read the plates effectively.

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

1. structure? 2. -Plate 1: beta-napthol spot doesn't move, napthalene spot hardly moves -Plate 2: both spots have higher Rf, napthalene spot moves further

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

-0.8 is fluorene, 0.5 is fluorenone, 0.3 is fluorenol. -Fluorenol is an alcohol, so it is more polar than the ketone fluorenone, giving it a lower Rf value.

What order of elution from a column chromatography procedure might you expect for a mixture of anthracene, 2-napthol, benzaldehyde and benzoic acid?

-Anthracene (least polar), benzaldehyde, 2-napthol, benzoic acid (most polar)

-Imagine that the TLC of flask 3, following column chromatography to separate fluorene and fluorenone, shows two spots with Rf 0.4 and 0.6. -Identify the spots and comment on the success of separation procedure.

-Fluorenone is 0.4, fluorene is 0.6 -Unsuccessful separation

Explain relative Rf values of fluorene and fluorenone.

-Fluorenone is more polar (ketone), has lower Rf value than fluorene (alkene/aromatic) -More attracted to polar stationary phase (silica gel)

In general, how will Rf values change with increasing solvent polarity?

-Incr. solvent polarity = incr. Rf -Rf = (distance traveled by substance/ distance traveled by solvent) -if solvent is more polar, it will interact with the polar Silica gel, and not travel as far. -So incr. solvent polarity = decr. solvent distance = incr. Rf

Define: polarity, soluble, solvent front, TLC chamber, spotting solvent, developing solvent, adsorbent, Rf, mobile phase

-Polarity = tendency of a molecule to be attracted to electric charges as a result of uneven electron sharing between atoms of different electronegativities -Soluble = Whether a substance can be dissolved in a liquid. -Solvent front = distance up a TLC plate that the developing solvent travels. --used in determining the Rf value. -TLC chamber = medium in which the TLC is performed. --Capped jar with developing solvent and TLC plate standing straight up to prevent curved travel -Spotting solvent = solvent used to dissolve the compound to be analyzed. --Often ethyl acetate or methylene chloride -Developing solvent = solvent that the TLC is placed into inside the chamber, creates the solvent front -Adsorbent = what holds onto the analyte, usually a solid --a thin, uniform layer of a dry, finely powdered material applied to a suitable support -Rf = Retention factor - The ratio of the distance traveled by the spot over the distance traveled by the solvent front. -Mobile phase = Phase that moves up the plate via capillary action; contains developing solvent with samples

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

-The benzoic acid would have the lowest Rf value, followed by methyl benzoate, followed by benzene -Due to decreasing polarity as you go from benzoic acid to benzene.

Rank the following types of solutes in order of Rf value: -carboxylic acid, aldehyde, alcohol, aromatic ether -Long chain conjugated alkene, ketone, ester, phenol

-carboxylic acid (most polar) < alcohol < aldehyde < aromatic ether -phenol (most polar) < ketone < ester < alkene

Define the terms eluent, mobile phase, stationary phase, adsorbent

1. Eluent = The solvent that the TLC is placed into inside the chamber. -AKA developing solvent; creates the solvent front, is the mobile phase that rises via cap. action. -In column chromatography, the eluent does the same thing, but moves because of gravity 2. Mobile phase (column) = solvent systems that passes through the stationary phase -Phase that moves up the plate via capillary action; contains developing solvent with samples 3. Stationary phase = contains analyte/absorbent system, remains in column/on TLC plate -Phase that doesn't move during TLC -In column chromatography, its the tightly packed dry component of the column. -normally consists of a finely divided adsorbent used in the form of a thin layer on a supporting material. -The mobile phase (a solution containing the sample) flows over or through the stationary phase to effect the separation. 4. Adsorbent = what holds onto the analyte, usually a solid -a thin, uniform layer of a dry, finely powdered material applied to a suitable support

1. How does TLC reveal purity of a given compound? 2. How does TLC provide information about the identity of a given compound? 3. How can TLC be used to judge completeness of reaction?

1. If you see multiple spots in a lane then it's not pure! 2. TLC provides Rf values so that the Rf values of the crude compound can be compared with the Rf of a known compound. it does NOT tell us polarity but uses polarity to separate compounds. 3. If the reaction did not complete then you'll be able to see a spot/spots from the starting materials (multiple spots)

What hazards do most organic solvents share in common?

flammable