Question

If a compound has an R_f of 0.25 when a 1:1 mixture of hexane and ethyl acetate is used as the solvent on a silica gel TLC plate, will the R_{f value be larger or smaller if a 4:1 mixture of hexane and ethyl acetate were used as the solvent? How would the} R_{f value change if pure ethyl acetate were used? Explain in terms of the intermolecular interactions how a change in the solvent mixture results in a change in the} R_{f value.}

Answer 1

The solvent ethyl acetate is more solvent while hexane is non-polar solvent. The 1:1 mixture of hexane and ethyl acetate is more polar than 4:1 mixture of hexane and ethyl acetate. This is because ratio of non-polar solvent is more than the plar solvent. Thus, the Rf value in  4:1 mixture of hexane and ethyl acetate solvent system is less than the 0.25 because of less polarity of solvent system.

If pure ethyl acetate is used, the solvent system becomes more polar and the Rf value becomes very high than the 0.25. We know that polar solvent i.e. ethyl acetate makes hydrogen bonding interactions with the compound and silica gel. Thus compound will move along with the polar solvent on the TLC plate. Hence, on increasing the polarity, stronger hydrogen bondings are formed and complound will move higher and higher i.e. the Rf value would be higher.