I) A mixture of benzaldehyde, benzoic acid, and toluene is spotted on a silica g
ID: 706918 • Letter: I
Question
I) A mixture of benzaldehyde, benzoic acid, and toluene is spotted on a silica gel TLC plate and developed in a mixture of l:1 Petroleum Ether-Ethyl Acetate as the solvent. Predict the sequence of relative Ry values for the three components in the sample (fastest to slowest). 2) What would happen to the Ry values of the compounds in question 1 if you were to increase the relative concentration of the Petroleum Ether in the solvent mixture? 3) Which compound will elute first when a mixture of chlorobenzene and 1,4-dichlorobenzene is loaded on a silica gel column and hexane is used as the eluent?Explanation / Answer
Answer No-(1)
Rf (Retardation factor/retention factor)- it is ratio of distance travelled by given component with that of distance tarvelled by solvent or mixture of solvents in fix ratio or velocity of solute wrt to velocity of solvent.
So , mathematical form Rf =( distance travelled by particular component )/ (distance travelled by solvent or mixture of solvents in specific ratio)
In thin Layer charomatography(TLC) which is based on adsorption principle, there are two systems one is particular component and another is solvent system.TLC is aluminium plate loaded with silica oxide which might be acidic, basic or neutral depends on requirements.
silica contains -O-Si-O- linkage which interact with more polar compounds easily than non polar, so more polar components moves at low speed thus cover low distance whereas less polar or non polar components donot bind with silica so covers more distance.
on the basis of decreasing polarity order ( Benzoic acid > Benzaldehyde > toluene )
(Rf) Toluene > (Rf) Benzaldeyhde >( Rf) Benzoic acid
Answer-2
If we are increasing the concetration of petroleum ether it means the overall polarity of the solvent system decreases as we know polarity of Ethyl acetate is more than that of petroleum ether so distance travelled by the the components as well as solvent system get reduced but Rf values will remain same because ratio remain same so orderof Rf
(Rf) Toluene > (Rf) Benzaldeyhde >( Rf) Benzoic acid
Answer -3
In mixture of 1,4-dichlorobenzene and chlorobenzene ,polarity of pevious one is less than later one so based on concept in Answer (1)
1,4-dichlorobenzene will elute first when mixture of chloro and dichlorobenzene is loaded on a silica gel column (hexane is eluent)