Separating a mixture of Biphenyl, Benzhydrol and Benzophenone by Thin- Layer Chromatography

Biphenyl, Benzhydrol and Benzophenone

Thin- Layer Chromatography (TLC) Simple and Inexpensive technique that can quickly and efficiently separate quantities of less than 10 micrograms. Also used for the rapid analysis of reagent and product purity. Also to determine the number of compounds in a mixture. Also used to compare known standards to tentatively identify unknown mixtures.

Chromatography A term used to identify various techniques for analyzing, identifying or separating mixtures of compounds. All chromatographic techniques consist of two parts: A). Mobile Phase----The liquid or gas phase B). Stationary Phase---- A solid support, which contains an adsorbent or another liquid.

TLC In TLC , the different compounds in the sample mixture move through the stationary phase at different rates due to the different attractions for the mobile and stationary phases. Individual compounds in the mixture separate as they move through the stationary phase.

TLC In the experiment our mobile phase will be a liquid solvent : Ethyl acetate, Hexane, Methanol, Dichloromethane and Toluene. Stationary phase will be Silica coated gel plate. Eluent Front : A point approximately 10 mm from the top of the plate. The Eluent must NEVER reach the top of the plate. Developing Chamber: A covered container with a shallow layer of the mobile phase

Developing chamber

TLC process During the TLC process , the solid stationary phase, also known as the absorbent, absorbs the mixture compounds. The mobile phase, called the Eluent, travels up the absorbent and the compounds within the mixture move at different rates.

Attraction Compounds with less attraction for the absorbent move rapidly with the eluent Compounds with more attraction for the absorbent move slowly with the eluent. TLC typically use very polar absorbents, the more polar compound in the mixture, the more strongly it adheres to the adsorbent and the more slowly it moves.

Attraction Intermolecular attractions between the eluent and the compounds determine the solubility of the compounds in the mobile phase. The more polar the eluent, the faster a given compound moves. Polar compounds that are strongly attached to the absorbent, require polar eluents to attract them away from the absorbent.

TLC plates

Retention Factor ( Rf) The ratio of the distance that a compound moves to the distance that the eluent front moves.

Retention factor The Rf can be used for identification purposes. If the compound is strongly attracted to the absorbent and does not travel far from the origin, the Rf will be small An increase in eluent polarity will increase the Rf. Identical Rf’s for a known and unknown compound on the same TLC plate will indicate that the samples are the same.

Rf, continued However, the different compounds can have the same Rf in a different eluent. Additional evidence is needed to identify the unknown compound…. This can be done by comparing the known and unknown mobility's in several different eluents.

Additional Info Silica gel (SiO2  x H2O) Polarity of eluents used in Chromatography: