Separating a mixture of
Biphenyl, Benzhydrol
and Benzophenone by
Thin- Layer
Chromatography
Biphenyl, Benzhydrol and
Benzophenone
OH
Biphenyl
O
C
Benzophenone
Benzhydrol
Thin- Layer Chromatography
(TLC)
o Simple and Inexpensive technique that can
quickly and efficiently separate quantities of
less than 10 micrograms.
o Also used for the rapid analysis of reagent and
product purity.
o Also to determine the number of compounds
in a mixture.
o Also used to compare known standards to
tentatively identify unknown mixtures.
Chromatography
O A term used to identify various techniques for
analyzing, identifying or separating mixtures
of compounds.
O All chromatographic techniques consist of two
parts:
O A). Mobile Phase----The liquid or gas phase
O B). Stationary Phase---- A solid support, which
contains an adsorbent or another liquid.
TLC
O 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.
O Individual compounds in the mixture separate
as they move through the stationary phase.
TLC
O In the experiment our mobile phase will be a liquid
solvent : Ethyl acetate,
Dichloromethane and Toluene.
Hexane,
Methanol,
O Stationary phase will be Silica coated gel plate.
O Eluent Front : A point approximately 10 mm from the
top of the plate. The Eluent must NEVER reach the
top of the plate.
O Developing Chamber: A covered container with a
shallow layer of the mobile phase
Developing chamber
TLC process
O During the TLC process , the solid stationary
phase, also known as the absorbent, absorbs
the mixture compounds.
O The mobile phase, called the Eluent, travels
up the absorbent and the compounds within
the mixture move at different rates.
Attraction
O Compounds
with less attraction for the
absorbent move rapidly with the eluent
O Compounds with more attraction for the
absorbent move slowly with the eluent.
O 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
O Intermolecular attractions between the eluent
and the compounds determine the solubility of
the compounds in the mobile phase.
O The more polar the eluent, the faster a given
compound moves.
O Polar compounds that are strongly attached to
the absorbent, require polar eluents to attract
them away from the absorbent.
TLC plates
Retention Factor ( Rf)
O
The ratio of the distance that a compound
moves to the distance that the eluent front
.
moves
Retention factor
O The Rf can be used for identification purposes.
O If the compound is strongly attracted to the
absorbent and does not travel far from the
origin, the Rf will be small
O An increase in eluent polarity will increase the
Rf.
O Identical Rf’s for a known and unknown
compound on the same TLC plate will indicate
that the samples are the same.
Rf, continued
O However, the different compounds can have
the same Rf in a different eluent.
O Additional evidence is needed to identify the
unknown compound….
O This can be done by comparing the known and
unknown mobility's in several different
eluents.
Additional Info
O Silica gel (SiO2  x H2O)
O Polarity of eluents used in Chromatography: