CHEM 43A E4

purpose of chromatography

separates compounds based on intermolecular interactions between the compound, solvent, and stationary phase, and can be used to identify compounds or purification

mobile phase

solvent, liquid, eluent

stationary phase

adborbant, si gel, solid

liquid chromotragraphy

solid stationary phase and liquid mobile phase

the surface of silica gel is

highly POLAR

polar molecules will ____ interact with the stationary phase and move ____

strongly, slowly

nonpolar molecules will ____ interact with the stationary phase and move ____

weakly, quickly

polar solvents will/will not interact with the polar surfaces of the starionary phase, causing compounds to move ___

will, faster!

increasing the solvent polarity will cause

all compounds to adsorb weaker and travel faster along the stationary phase

in Thin layer chromotragraphy,

the stationary phase is affixed in a thin layer to a plate, which is placed in solvent reserviorm and the solvent travels up via capillary action

in column chromotraphy,

the stationary phase is packed into a tube and the mobile phase is passed over the stationary phase using gravity or pressure

what happens when the eluent reaches the sample in TLC

the components in the sample mixture dissolve and are carried along the silica by the ascending solvent

marks should only be made in

pencil

the spots on the tlc plate are analyzed

directly, or using UV light, or if colorless exposed to iodine

retention factor equation

(distance spot traveled)/(distance solvent traveled) measured from baseline

why Always hold a TLC plate by the edges and avoid touching the silica

Oils from your fingers can be adsorbed to the plate, confusing the analysis.

why Add filter paper as a wick in TLC

to equalize the solvent vapor in the chamber and prevent eluent from evaporating off the plate.

Column chromatography starts with packing a column with the adsorbent stationary phase

SiO2or Al2O3

expose a greater surface area and slows compound travel and the eluent flow-- Column Chrom

smaller particles

why a cotton or glass wool plug is placed at the bottom in GC

to allow solvent passage but prevent the silica from escaping

why add sand to GC?

sand is added to give a flat surface(

In dry packing GC

he adsorbent is added directly to the column.

Column chrom why The top of the column should be flat

to ensure compounds travel as parallel bands (not spots) down the column

Dry loadingis used when

the compound/s are not very soluble in the eluent. The compound/s are dissolved in a different solvent and some silica gel is added. The solvent is completely evaporated to adsorb the compounds onto the stationary phase(any residual solvent can ruin the separation). The dry silica gel is then carefully loaded onto the column.

how to analyze Colum CHrom

A TLC plate is spotted with each fraction (including aspotofthe crude mixture) and the TLC is developed

A typical gradient starts with a ____ in column chromatography

a nonpolar solvent (i.e., hexane) and as the column is progressing, an increasing amount of a polar solvent (for example, ethyl acetate) is added.

why cant you use too polar of solvet?

it dissolves silica gel

too polar solvents

water, ethanol/methanol, acetoe

non polar solvents

toluene, hexane, pet. ether

med polarity solvent,

ethyl acetate, diethyl ether, dichloromethane

we need to ____ solvents to

get different ratios of solvents to have ideal separation

what happens to the rf factor if we change solvent?

different Rf

what is a good Rf separation?

delta Rf is greater than 0.2

good Rf

about 0.3

Using a less polar solvent when running a TLC will result in polar compounds migrating a ______ distance up the plate because of _____ interactions between the polar compound and the stationary phase and ______ interactions between the polar compound and the mobile phase.

lesser, increased, decreased

The stationary phase in TLC is the Silica gel which is ____Therefore, the___ component of a mixture will travel a shorter distance up the plate, resulting in a _____ Rf value for the component.

polar, more polar, smaller

Which of the following statements best describes what would generally happen to the Rf of all compounds if you left the TLC plate in the chamber for too long after the solvent reached the top?

Spots may be clustered near the solvent front; Rf values will be artificially high and close in value

The para isomer has a larger dipole moment and will have a __ Pf

smaller

Select the statement that best explains which compound in the previous question above will elute last.

The compound that elutes last is relatively polar and strongly interacts with the stationary phase through dipole-dipole and hydrogen bonding interactions

predict what Elle may see in the TLC analysis and briefly explain how bubbles in the silica gel can affect column chromatography results.

Because of the bubbles, Elle may see that the compounds did not separate very much or that they did not travel very far, making the Rf much lower and closer together. This is because the bubbles in the silica gel can let the solvent with the compounds escape into the bubbles and since there is no silica gel in the bubbles, the compound will not interact with the silica gel and therefore will not separate.

What will most likely elute in fraction 1 (using TLC) if Kim swapped the labels on Kourtney's petroleum ether bottle and the bottle containing the petroleum ether/MTBE mixture?

All of the least polar compound will elute along with some of the medium polar compound