Consider the following errors that could be made when running TLC. Indicate what should be done to correct the error.
a) a two-component mixture containing 1-octene and 1,4-dimethylbenzene gave only one spot with an Rf value of .95. The solvent uses was acetone.
b) a two-component mixture containing a dicarboxylic acid and tricarboxylic acid gave only one spot with an Rf value of .05. The solvent used was hexane.
c) when a TLC plate was developed, the solvent front ran off the top of the plate.
thanks in advance for all the help... i have been stuck on this question for a while!
http://siggy.chem.ucla.edu/VOH/136/TLC.pdf
The above site explains a little about why some analytes move fast and some slow.
Check my thinking on these.
a. Aren't the Hs at the double bond slightly acidic? But I would expect 1-4 dimethylbenzene to be less polar. The fact that the Rf factor was so high means that the solvent used was much more polar than either of the components. I would try a different mobile phase; i.e., one less polar.
b. The Rf value of only 0.05 means the components hardly moved at all; i.e., they were strongly attracted to the TLC plate surface. Hexane is a non-polar solvent; the solution is to use a more polar solvent.
c. When the solvent moves up past the top of the plate it means you let it develop too long.
a) In this case, the error is that only one spot with an Rf value of .95 was observed instead of two spots. The solvent used, acetone, might not be a suitable solvent for separating 1-octene and 1,4-dimethylbenzene. To correct this error, you should try using a different solvent that has a higher polarity, such as methanol or ethyl acetate. This should help in separating the two components and result in the formation of two distinct spots on the TLC plate.
b) The error in this case is that only one spot with an Rf value of .05 was observed instead of two spots. The solvent used, hexane, is a nonpolar solvent and may not effectively separate the dicarboxylic acid and tricarboxylic acid. To correct this error, you should try using a more polar solvent, such as a mixture of hexane and ethyl acetate or methanol, to increase the separation between the two components and allow for the formation of two distinct spots on the TLC plate.
c) The error here is that the solvent front ran off the top of the plate, indicating that the TLC plate was not properly set up. This could be due to improper placement of the TLC plate in the developing chamber, causing the solvent to flow unevenly or too quickly. To correct this error, you should ensure that the TLC plate is properly placed in the chamber, with the bottom edge of the plate submerged in the solvent, but not touching the solvent directly. This will allow for more controlled and even development of the spots on the plate. It's also important to check that the TLC plate is level to ensure proper solvent flow. If needed, adjust the chamber or the position of the plate to prevent the solvent front from running off the top.
a) In TLC, the Rf value of a compound is influenced by both the nature of the compound and the solvent used. In this case, the fact that both 1-octene and 1,4-dimethylbenzene gave only one spot with an Rf value of 0.95 suggests that the solvent used (acetone) was too polar for these compounds. To correct the error, a less polar solvent should be used, such as a mixture of hexane and ethyl acetate.
b) Similarly, the fact that both the dicarboxylic acid and tricarboxylic acid gave only one spot with an Rf value of 0.05 suggests that the solvent used (hexane) was too non-polar for these compounds. To correct the error, a more polar solvent should be used, such as a mixture of dichloromethane and methanol.
c) When the solvent front runs off the top of the TLC plate, it indicates that the TLC chamber was not properly sealed. This error can be corrected by ensuring that the TLC plate is placed in a closed chamber with a tight seal to prevent evaporation of the solvent. The chamber can be sealed with a lid or covered with a piece of aluminum foil to minimize solvent loss.
a) In TLC, the Rf value represents the ratio of the distance traveled by a compound to the distance traveled by the solvent front. In this case, if a two-component mixture gave only one spot with an Rf value of .95, it suggests that both 1-octene and 1,4-dimethylbenzene have similar affinities for the stationary phase (the TLC plate) and are not effectively separated.
To correct this error, you can try changing the solvent used from acetone to a different solvent system. Choose a solvent or a mixture of solvents that will provide better separation between the two components. For instance, you can try using a more polar solvent such as ethyl acetate, toluene, or a mixture of these solvents to improve the separation. It is important to perform a small-scale trial using different solvent systems to find the optimal one for the separation of the components.
b) Similar to the previous case, if a two-component mixture gave only one spot with an Rf value of .05, it indicates poor separation between the dicarboxylic acid and tricarboxylic acid using the hexane solvent.
To correct this error, you can try changing the solvent used from hexane to a more polar solvent system. Since carboxylic acids are typically polar compounds, the use of a more polar solvent could enhance the separation. For example, you can try a mixture of hexane and ethyl acetate or a more polar solvent such as chloroform or dichloromethane. Perform a small-scale trial using different solvent systems to optimize the separation.
c) If the solvent front ran off the top of the TLC plate, it means that the solvent used was too polar and had a high affinity for the stationary phase. This can lead to a loss of separation and incorrect Rf values.
To correct this error, you should use a less polar solvent. Select a solvent system that has a lower affinity for the stationary phase, such as a less polar organic solvent like hexane or ethyl acetate. It is crucial to choose a solvent that strikes a balance between good separation and proper travel distance of the solvent front. Experiment with different solvent systems to find the optimal one for your specific compounds and TLC plate.